The Netwide Assembler: NASM

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Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \ 
        THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are 2 ; pi do 1.e+4000 ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. __float80m__ and __float80e__ produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and __float128l__ and __float128h__ produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

      mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into RAX. This is exactly equivalent to:

      mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens __Infinity__, __QNaN__ (or __NaN__) and __SNaN__ can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

%define Inf __Infinity__ 
%define NaN __QNaN__ 

      dq    +1.5, -Inf, NaN         ; Double-precision constants

3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with p or prefixed with 0p, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

      dt 12_345_678_901_245_678p 
      dt -12_345_678_901_245_678p 
      dt +0p33 
      dt 33p

3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 |: Bitwise OR Operator

The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.5.2 ^: Bitwise XOR Operator

^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator

& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.5.5 + and -: Addition and Subtraction Operators

The + and - operators do perfectly ordinary addition and subtraction.

3.5.6 *, /, //, % and %%: Multiplication and Division

* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, -, ~, ! and SEG

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, ! is the logical negation operator, and SEG provides the segment address of its operand (explained in more detail in section 3.6).

3.6 SEG and WRT

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

        mov     ax,seg symbol 
        mov     es,ax 
        mov     bx,symbol

will load ES:BX with a valid pointer to the symbol symbol.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do things like

        mov     ax,weird_seg        ; weird_seg is a segment base 
        mov     es,ax 
        mov     bx,symbol wrt weird_seg

to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of

        call    (seg procedure):procedure 
        call    weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

        dw      symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

3.7 STRICT: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see section 2.1.22), NASM will use size specifiers (BYTE, WORD, DWORD, QWORD, TWORD, OWORD or YWORD), but will give them the smallest possible size. The keyword STRICT can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,

        push dword 33

is encoded in three bytes 66 6A 21, whereas

        push strict dword 33

is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00.

With the optimizer off, the same code (six bytes) is generated whether the STRICT keyword was used or not.

3.8 Critical Expressions

Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called Critical Expressions.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,

        times (label-$) db 0 
label:  db      'Where am I?'

The argument to TIMES in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the TIMES line when it first sees it. It will just as firmly reject the slightly paradoxical code

        times (label-$+1) db 0 
label:  db      'NOW where am I?'

in which any value for the TIMES argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the TIMES prefix is a critical expression.

3.9 Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

label1  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret 

label2  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret

In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write

label3  ; some more code 
        ; and some more 

        jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code

label1:                         ; a non-local label 
.local:                         ; this is really label1.local 
..@foo:                         ; this is a special symbol 
label2:                         ; another non-local label 
.local:                         ; this is really label2.local 

        jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format (see section 7.4.6).

Next Chapter | Previous Chapter | Contents | Index ./usr/share/doc/nasm/html/nasmdoc4.html0000644000000000000000000025777411401452176016720 0ustar rootroot

The Netwide Assembler: NASM

Next Chapter | Previous Chapter | Contents | Index

Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \ 
        THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are 2 ; pi do 1.e+4000 ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. __float80m__ and __float80e__ produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and __float128l__ and __float128h__ produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

      mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into RAX. This is exactly equivalent to:

      mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens __Infinity__, __QNaN__ (or __NaN__) and __SNaN__ can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

%define Inf __Infinity__ 
%define NaN __QNaN__ 

      dq    +1.5, -Inf, NaN         ; Double-precision constants

3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with p or prefixed with 0p, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

      dt 12_345_678_901_245_678p 
      dt -12_345_678_901_245_678p 
      dt +0p33 
      dt 33p

3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 |: Bitwise OR Operator

The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.5.2 ^: Bitwise XOR Operator

^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator

& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.5.5 + and -: Addition and Subtraction Operators

The + and - operators do perfectly ordinary addition and subtraction.

3.5.6 *, /, //, % and %%: Multiplication and Division

* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, -, ~, ! and SEG

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, ! is the logical negation operator, and SEG provides the segment address of its operand (explained in more detail in section 3.6).

3.6 SEG and WRT

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

        mov     ax,seg symbol 
        mov     es,ax 
        mov     bx,symbol

will load ES:BX with a valid pointer to the symbol symbol.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do things like

        mov     ax,weird_seg        ; weird_seg is a segment base 
        mov     es,ax 
        mov     bx,symbol wrt weird_seg

to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of

        call    (seg procedure):procedure 
        call    weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

        dw      symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

3.7 STRICT: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see section 2.1.22), NASM will use size specifiers (BYTE, WORD, DWORD, QWORD, TWORD, OWORD or YWORD), but will give them the smallest possible size. The keyword STRICT can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,

        push dword 33

is encoded in three bytes 66 6A 21, whereas

        push strict dword 33

is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00.

With the optimizer off, the same code (six bytes) is generated whether the STRICT keyword was used or not.

3.8 Critical Expressions

Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called Critical Expressions.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,

        times (label-$) db 0 
label:  db      'Where am I?'

The argument to TIMES in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the TIMES line when it first sees it. It will just as firmly reject the slightly paradoxical code

        times (label-$+1) db 0 
label:  db      'NOW where am I?'

in which any value for the TIMES argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the TIMES prefix is a critical expression.

3.9 Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

label1  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret 

label2  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret

In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write

label3  ; some more code 
        ; and some more 

        jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code

label1:                         ; a non-local label 
.local:                         ; this is really label1.local 
..@foo:                         ; this is a special symbol 
label2:                         ; another non-local label 
.local:                         ; this is really label2.local 

        jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format (see section 7.4.6).

Next Chapter | Previous Chapter | Contents | Index ./usr/share/doc/nasm/html/nasmdoc4.html0000644000000000000000000025777411401452176016720 0ustar rootroot

The Netwide Assembler: NASM

Next Chapter | Previous Chapter | Contents | Index

Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \ 
        THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are 2 ; pi do 1.e+4000 ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. __float80m__ and __float80e__ produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and __float128l__ and __float128h__ produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

      mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into RAX. This is exactly equivalent to:

      mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens __Infinity__, __QNaN__ (or __NaN__) and __SNaN__ can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

%define Inf __Infinity__ 
%define NaN __QNaN__ 

      dq    +1.5, -Inf, NaN         ; Double-precision constants

3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with p or prefixed with 0p, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

      dt 12_345_678_901_245_678p 
      dt -12_345_678_901_245_678p 
      dt +0p33 
      dt 33p

3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 |: Bitwise OR Operator

The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.5.2 ^: Bitwise XOR Operator

^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator

& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.5.5 + and -: Addition and Subtraction Operators

The + and - operators do perfectly ordinary addition and subtraction.

3.5.6 *, /, //, % and %%: Multiplication and Division

* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, -, ~, ! and SEG

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, ! is the logical negation operator, and SEG provides the segment address of its operand (explained in more detail in section 3.6).

3.6 SEG and WRT

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

        mov     ax,seg symbol 
        mov     es,ax 
        mov     bx,symbol

will load ES:BX with a valid pointer to the symbol symbol.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do things like

        mov     ax,weird_seg        ; weird_seg is a segment base 
        mov     es,ax 
        mov     bx,symbol wrt weird_seg

to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of

        call    (seg procedure):procedure 
        call    weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

        dw      symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

3.7 STRICT: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see section 2.1.22), NASM will use size specifiers (BYTE, WORD, DWORD, QWORD, TWORD, OWORD or YWORD), but will give them the smallest possible size. The keyword STRICT can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,

        push dword 33

is encoded in three bytes 66 6A 21, whereas

        push strict dword 33

is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00.

With the optimizer off, the same code (six bytes) is generated whether the STRICT keyword was used or not.

3.8 Critical Expressions

Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called Critical Expressions.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,

        times (label-$) db 0 
label:  db      'Where am I?'

The argument to TIMES in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the TIMES line when it first sees it. It will just as firmly reject the slightly paradoxical code

        times (label-$+1) db 0 
label:  db      'NOW where am I?'

in which any value for the TIMES argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the TIMES prefix is a critical expression.

3.9 Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

label1  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret 

label2  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret

In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write

label3  ; some more code 
        ; and some more 

        jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code

label1:                         ; a non-local label 
.local:                         ; this is really label1.local 
..@foo:                         ; this is a special symbol 
label2:                         ; another non-local label 
.local:                         ; this is really label2.local 

        jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format (see section 7.4.6).

Next Chapter | Previous Chapter | Contents | Index ./usr/share/doc/nasm/html/nasmdoc4.html0000644000000000000000000025777411401452176016720 0ustar rootroot

The Netwide Assembler: NASM

Next Chapter | Previous Chapter | Contents | Index

Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \ 
        THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are 2 ; pi do 1.e+4000 ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. __float80m__ and __float80e__ produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and __float128l__ and __float128h__ produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

      mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into RAX. This is exactly equivalent to:

      mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens __Infinity__, __QNaN__ (or __NaN__) and __SNaN__ can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

%define Inf __Infinity__ 
%define NaN __QNaN__ 

      dq    +1.5, -Inf, NaN         ; Double-precision constants

3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with p or prefixed with 0p, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

      dt 12_345_678_901_245_678p 
      dt -12_345_678_901_245_678p 
      dt +0p33 
      dt 33p

3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 |: Bitwise OR Operator

The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.5.2 ^: Bitwise XOR Operator

^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator

& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.5.5 + and -: Addition and Subtraction Operators

The + and - operators do perfectly ordinary addition and subtraction.

3.5.6 *, /, //, % and %%: Multiplication and Division

* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, -, ~, ! and SEG

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, ! is the logical negation operator, and SEG provides the segment address of its operand (explained in more detail in section 3.6).

3.6 SEG and WRT

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

        mov     ax,seg symbol 
        mov     es,ax 
        mov     bx,symbol

will load ES:BX with a valid pointer to the symbol symbol.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do things like

        mov     ax,weird_seg        ; weird_seg is a segment base 
        mov     es,ax 
        mov     bx,symbol wrt weird_seg

to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of

        call    (seg procedure):procedure 
        call    weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

        dw      symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

3.7 STRICT: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see section 2.1.22), NASM will use size specifiers (BYTE, WORD, DWORD, QWORD, TWORD, OWORD or YWORD), but will give them the smallest possible size. The keyword STRICT can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,

        push dword 33

is encoded in three bytes 66 6A 21, whereas

        push strict dword 33

is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00.

With the optimizer off, the same code (six bytes) is generated whether the STRICT keyword was used or not.

3.8 Critical Expressions

Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called Critical Expressions.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,

        times (label-$) db 0 
label:  db      'Where am I?'

The argument to TIMES in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the TIMES line when it first sees it. It will just as firmly reject the slightly paradoxical code

        times (label-$+1) db 0 
label:  db      'NOW where am I?'

in which any value for the TIMES argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the TIMES prefix is a critical expression.

3.9 Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

label1  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret 

label2  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret

In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write

label3  ; some more code 
        ; and some more 

        jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code

label1:                         ; a non-local label 
.local:                         ; this is really label1.local 
..@foo:                         ; this is a special symbol 
label2:                         ; another non-local label 
.local:                         ; this is really label2.local 

        jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format (see section 7.4.6).

Next Chapter | Previous Chapter | Contents | Index ./usr/share/doc/nasm/html/nasmdoc4.html0000644000000000000000000025777411401452176016720 0ustar rootroot

The Netwide Assembler: NASM

Next Chapter | Previous Chapter | Contents | Index

Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \ 
        THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are 2 ; pi do 1.e+4000 ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. __float80m__ and __float80e__ produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and __float128l__ and __float128h__ produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

      mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into RAX. This is exactly equivalent to:

      mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens __Infinity__, __QNaN__ (or __NaN__) and __SNaN__ can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

%define Inf __Infinity__ 
%define NaN __QNaN__ 

      dq    +1.5, -Inf, NaN         ; Double-precision constants

3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with p or prefixed with 0p, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

      dt 12_345_678_901_245_678p 
      dt -12_345_678_901_245_678p 
      dt +0p33 
      dt 33p

3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 |: Bitwise OR Operator

The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.5.2 ^: Bitwise XOR Operator

^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator

& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.5.5 + and -: Addition and Subtraction Operators

The + and - operators do perfectly ordinary addition and subtraction.

3.5.6 *, /, //, % and %%: Multiplication and Division

* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, -, ~, ! and SEG

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, ! is the logical negation operator, and SEG provides the segment address of its operand (explained in more detail in section 3.6).

3.6 SEG and WRT

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

        mov     ax,seg symbol 
        mov     es,ax 
        mov     bx,symbol

will load ES:BX with a valid pointer to the symbol symbol.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do things like

        mov     ax,weird_seg        ; weird_seg is a segment base 
        mov     es,ax 
        mov     bx,symbol wrt weird_seg

to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of

        call    (seg procedure):procedure 
        call    weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

        dw      symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

3.7 STRICT: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see section 2.1.22), NASM will use size specifiers (BYTE, WORD, DWORD, QWORD, TWORD, OWORD or YWORD), but will give them the smallest possible size. The keyword STRICT can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,

        push dword 33

is encoded in three bytes 66 6A 21, whereas

        push strict dword 33

is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00.

With the optimizer off, the same code (six bytes) is generated whether the STRICT keyword was used or not.

3.8 Critical Expressions

Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called Critical Expressions.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,

        times (label-$) db 0 
label:  db      'Where am I?'

The argument to TIMES in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the TIMES line when it first sees it. It will just as firmly reject the slightly paradoxical code

        times (label-$+1) db 0 
label:  db      'NOW where am I?'

in which any value for the TIMES argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the TIMES prefix is a critical expression.

3.9 Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

label1  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret 

label2  ; some code 

.loop 
        ; some more code 

        jne     .loop 
        ret

In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write

label3  ; some more code 
        ; and some more 

        jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code

label1:                         ; a non-local label 
.local:                         ; this is really label1.local 
..@foo:                         ; this is a special symbol 
label2:                         ; another non-local label 
.local:                         ; this is really label2.local 

        jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format (see section 7.4.6).

Next Chapter | Previous Chapter | Contents | Index ./usr/share/doc/nasm/html/nasmdoc4.html0000644000000000000000000025777411401452176016720 0ustar rootroot

The Netwide Assembler: NASM

Next Chapter | Previous Chapter | Contents | Index

Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \ 
        THIS_VALUE

will work like a single-line macro without the backslash-newline sequence.

4.1 Single-Line Macros

4.1.1 The Normal Way: %define

Single-line macros are 2 ; pi do 1.e+4000 ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. __float80m__ and __float80e__ produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and __float128l__ and __float128h__ produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

      mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into RAX. This is exactly equivalent to:

      mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens __Infinity__, __QNaN__ (or __NaN__) and __SNaN__ can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

%define Inf __Infinity__ 
%define NaN __QNaN__ 

      dq    +1.5, -Inf, NaN         ; Double-precision constants

3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with p or prefixed with 0p, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

      dt 12_345_678_901_245_678p 
      dt -12_345_678_901_245_678p 
      dt +0p33 
      dt 33p

3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 |: Bitwise OR Operator

The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

3.5.2 ^: Bitwise XOR Operator

^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator

& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

3.5.5 + and -: Addition and Subtraction Operators

The + and - operators do perfectly ordinary addition and subtraction.

3.5.6 *, /, //, % and %%: Multiplication and Division

* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, -, ~, ! and SEG

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, ! is the logical negation operator, and SEG provides the segment address of its operand (explained in more detail in section 3.6).

3.6 SEG and WRT

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this fu