The Netwide Assembler: NASM

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Chapter 8: Writing 16-bit Code (DOS, Windows 3/3.1)

This chapter attempts to cover some of the common issues encountered when writing 16-bit code to run under MS-DOS or Windows 3.x. It covers how to link programs to produce .EXE or .COM files, how to write .SYS device drivers, and how to interface assembly language code with 16-bit C compilers and with Borland Pascal.

8.1 Producing `.EXE` Files

Any large program written under DOS needs to be built as a .EXE file: only .EXE files have the necessary internal structure required to span more than one 64K segment. Windows programs, also, have to be built as .EXE files, since Windows does not support the .COM format.

In general, you generate .EXE files by using the obj output format to produce one or more .OBJ files, and then linking them together using a linker. However, NASM also supports the direct generation of simple DOS .EXE files using the bin output format (by using DB and DW to construct the .EXE file header), and a macro package is supplied to do this. Thanks to Yann Guidon for contributing the code for this.

NASM may also support .EXE natively as another output format in future releases.

8.1.1 Using the `obj` Format To Generate `.EXE` Files

This section describes the usual method of generating .EXE files by linking .OBJ files together.

Most 16-bit programming language packages come with a suitable linker; if you have none of these, there is a free linker called VAL, available in LZH archive format from x2ftp.oulu.fi. An LZH archiver can be found at ftp.simtel.net. There is another `free' linker (though this one doesn't come with sources) called FREELINK, available from www.pcorner.com. A third, djlink, written by DJ Delorie, is available at www.delorie.com. A fourth linker, ALINK, written by Anthony A.J. Williams, is available at alink.sourceforge.net.

When linking several .OBJ files into a .EXE file, you should ensure that exactly one of them has a start point defined (using the ..start special symbol defined by the obj format: see section 7.4.6). If no module defines a start point, the linker will not know what value to give the entry-point field in the output file header; if more than one defines a start point, the linker will not know which value to use.

An example of a NASM source file which can be assembled to a .OBJ file and linked on its own to a .EXE is given here. It demonstrates the basic principles of defining a stack, initialising the segment registers, and declaring a start point. This file is also provided in the test subdirectory of the NASM archives, under the name objexe.asm.

segment code 

..start: 
        mov     ax,data 
        mov     ds,ax 
        mov     ax,stack 
        mov     ss,ax 
        mov     sp,stacktop

This initial piece of code sets up DS to point to the data segment, and initializes SS and SP to point to the top of the provided stack. Notice that interrupts are implicitly disabled for one instruction after a move into SS, precisely for this situation, so that there's no chance of an interrupt occurring between the loads of SS and SP and not having a stack to execute on.

Note also that the special symbol ..start is defined at the beginning of this code, which means that will be the entry point into the resulting executable file.

        mov     dx,hello 
        mov     ah,9 
        int     0x21

The above is the main program: load DS:DX with a pointer to the greeting message (hello is implicitly relative to the segment data, which was loaded into DS in the setup code, so the full pointer is valid), and call the DOS print-string function.

        mov     ax,0x4c00 
        int     0x21

This terminates the program using another DOS system call.

segment data 

hello:  db      'hello, world', 13, 10, '$'

The data segment contains the string we want to display.

segment stack stack 
        resb 64 
stacktop:

The above code declares a stack segment containing 64 bytes of uninitialized stack space, and points stacktop at the top of it. The directive segment stack stack defines a segment called stack, and also of type STACK. The latter is not necessary to the correct running of the program, but linkers are likely to issue warnings or errors if your program has no segment of type STACK.

The above file, when assembled into a .OBJ file, will link on its own to a valid .EXE file, which when run will print `hello, world' and then exit.

8.1.2 Using the `bin` Format To Generate `.EXE` Files

The .EXE file format is simple enough that it's possible to build a .EXE file by writing a pure-binary program and sticking a 32-byte header on the front. This header is simple enough that it can be generated using DB and DW commands by NASM itself, so that you can use the bin output format to directly generate .EXE files.

Included in the NASM archives, in the misc subdirectory, is a file exebin.mac of macros. It defines three macros: EXE_begin, EXE_stack and EXE_end.

To produce a .EXE file using this method, you should start by using %include to load the exebin.mac macro package into your source file. You should then issue the EXE_begin macro call (which takes no arguments) to generate the file header data. Then write code as normal for the bin format - you can use all three standard sections .text, .data and .bss. At the end of the file you should call the EXE_end macro (again, no arguments), which defines some symbols to mark section sizes, and these symbols are referred to in the header code generated by EXE_begin.

In this model, the code you end up writing starts at 0x100, just like a .COM file - in fact, if you strip off the 32-byte header from the resulting .EXE file, you will have a valid .COM program. All the segment bases are the same, so you are limited to a 64K program, again just like a .COM file. Note that an ORG directive is issued by the EXE_begin macro, so you should not explicitly issue one of your own.

You can't directly refer to your segment base value, unfortunately, since this would require a relocation in the header, and things would get a lot more complicated. So you should get your segment base by copying it out of CS instead.

On entry to your .EXE file, SS:SP are already set up to point to the top of a 2Kb stack. You can adjust the default stack size of 2Kb by calling the EXE_stack macro. For example, to change the stack size of your program to 64 bytes, you would call EXE_stack 64.

A sample program which generates a .EXE file in this way is given in the test subdirectory of the NASM archive, as binexe.asm.

8.2 Producing `.COM` Files

While large DOS programs must be written as .EXE files, small ones are often better written as .COM files. .COM files are pure binary, and therefore most easily produced using the bin output format.

8.2.1 Using the `bin` Format To Generate `.COM` Files

.COM files expect to be loaded at offset 100h into their segment (though the segment may change). Execution then begins at 100h, i.e. right at the start of the program. So to write a .COM program, you would create a source file looking like

        org 100h 

section .text 

start: 
        ; put your code here 

section .data 

        ; put data items here 

section .bss 

        ; put uninitialized data here

The bin format puts the .text section first in the file, so you can declare data or BSS items before beginning to write code if you want to and the code will still end up at the front of the file where it belongs.

The BSS (uninitialized data) section does not take up space in the .COM file itself: instead, addresses of BSS items are resolved to point at space beyond the end of the file, on the grounds that this will be free memory when the program is run. Therefore you should not rely on your BSS being initialized to all zeros when you run.

To assemble the above program, you should use a command line like

nasm myprog.asm -fbin -o myprog.com

The bin format would produce a file called myprog if no explicit output file name were specified, so you have to override it and give the desired file name.

8.2.2 Using the `obj` Format To Generate `.COM` Files

If you are writing a .COM program as more than one module, you may wish to assemble several .OBJ files and link them together into a .COM program. You can do this, provided you have a linker capable of outputting .COM files directly (TLINK does this), or alternatively a converter program such as EXE2BIN to transform the .EXE file output from the linker into a .COM file.

If you do this, you need to take care of several things:

The first object file containing code should start its code segment with a line like RESB 100h. This is to ensure that the code begins at offset 100h relative to the beginning of the code segment, so that the linker or converter program does not have to adjust address references within the file when generating the .COM file. Other assemblers use an ORG directive for this purpose, but ORG in NASM is a format-specific directive to the bin output format, and does not mean the same thing as it does in MASM-compatible assemblers.
You don't need to define a stack segment.
All your segments should be in the same group, so that every time your code or data references a symbol offset, all offsets are relative to the same segment base. This is because, when a .COM file is loaded, all the segment registers contain the same value.

8.3 Producing `.SYS` Files

MS-DOS device drivers - .SYS files - are pure binary files, similar to .COM files, except that they start at origin zero rather than 100h. Therefore, if you are writing a device driver using the bin format, you do not need the ORG directive, since the default origin for bin is zero. Similarly, if you are using obj, you do not need the RESB 100h at the start of your code segment.

.SYS files start with a header structure, containing pointers to the various routines inside the driver which do the work. This structure should be defined at the start of the code segment, even though it is not actually code.

For more information on the format of .SYS files, and the data which has to go in the header structure, a list of books is given in the Frequently Asked Questions list for the newsgroup comp.os.msdos.programmer.

8.4 Interfacing to 16-bit C Programs

This section covers the basics of writing assembly routines that call, or are called from, C programs. To do this, you would typically write an assembly module as a .OBJ file, and link it with your C modules to produce a mixed-language program.

8.4.1 External Symbol Names

C compilers have the convention that the names of all global symbols (functions or data) they define are formed by prefixing an underscore to the name as it appears in the C program. So, for example, the function a C programmer thinks of as printf appears to an assembly language programmer as _printf. This means that in your assembly programs, you can define symbols without a leading underscore, and not have to worry about name clashes with C symbols.

If you find the underscores inconvenient, you can define macros to replace the GLOBAL and EXTERN directives as follows:

%macro  cglobal 1 

  global  _%1 
  %define %1 _%1 

%endmacro 

%macro  cextern 1 

  extern  _%1 
  %define %1 _%1 

%endmacro

(These forms of the macros only take one argument at a time; a %rep construct could solve this.)

If you then declare an external like this:

cextern printf

then the macro will expand it as

extern  _printf 
%define printf _printf

Thereafter, you can reference printf as if it was a symbol, and the preprocessor will put the leading underscore on where necessary.

The cglobal macro works similarly. You must use cglobal before defining the symbol in question, but you would have had to do that anyway if you used GLOBAL.

Also see section 2.1.27.

8.4.2 Memory Models

NASM contains no mechanism to support the various C memory models directly; you have to keep track yourself of which one you are writing for. This means you have to keep track of the following things:

In models using a single code segment (tiny, small and compact), functions are near. This means that function pointers, when stored in data segments or pushed on the stack as function arguments, are 16 bits long and contain only an offset field (the CS register never changes its value, and always gives the segment part of the full function address), and that functions are called using ordinary near CALL instructions and return using RETN (which, in NASM, is synonymous with RET anyway). This means both that you should write your own routines to return with RETN, and that you should call external C routines with near CALL instructions.
In models using more than one code segment (medium, large and huge), functions are far. This means that function pointers are 32 bits long (consisting of a 16-bit offset followed by a 16-bit segment), and that functions are called using CALL FAR (or CALL seg:offset) and return using RETF. Again, you should therefore write your own routines to return with RETF and use CALL FAR to call external routines.
In models using a single data segment (tiny, small and medium), data pointers are 16 bits long, containing only an offset field (the DS register doesn't change its value, and always gives the segment part of the full data item address).
In models using more than one data segment (compact, large and huge), data pointers are 32 bits long, consisting of a 16-bit offset followed by a 16-bit segment. You should still be careful not to modify DS in your routines without restoring it afterwards, but ES is free for you to use to access the contents of 32-bit data pointers you are passed.
The huge memory model allows single data items to exceed 64K in size. In all other memory models, you can access the whole of a data item just by doing arithmetic on the offset field of t