The Netwide Assembler: NASM

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Chapter 7: 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.

7.1 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.

7.1.1 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.

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 6.2.6). 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 initialises 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 uninitialised 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.

7.1.2 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.

7.2 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.

7.2.1 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 uninitialised 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 (uninitialised 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 initialised 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.

7.2.2 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.

7.3 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.

7.4 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.

7.4.1 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.

7.4.2 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 the pointer you are given, whether a segment field is present or not; in huge model, you have to be more careful of your pointer arithmetic.
• In most memory models, there is a default data segment, whose segment address is kept in DS throughout the program. This data segment is typically the same segment as the stack, kept in SS, so that functions' local variables (which are stored on the stack) and global data items can both be accessed easily without changing DS. Particularly large data items are typically stored in other segments. However, some memory models (though not the standard ones, usually) allow the assumption that SS and DS hold the same value to be removed. Be careful about functions' local variables in this latter case.

In models with a single code segment, the segment is called _TEXT, so your code segment must also go by this name in order to be linked into the same place as the main code segment. In models with a single data segment, or with a default data segment, it is called _DATA.

7.4.3 Function Definitions and Function Calls

The C calling convention in 16-bit programs is as follows. In the following description, the words caller and callee are used to denote the function doing the calling and the function which gets called.

• The caller pushes the function's parameters on the stack, one after another, in reverse order (right to left, so that the first argument specified to the function is pushed last).
• The caller then executes a CALL instruction to pass control to the callee. This CALL is either near or far depending on the memory model.
• The callee receives control, and typically (although this is not actually necessary, in functions which do not need to access their parameters) starts by saving the value of SP in BP so as to be able to use BP as a base pointer to find its parameters on the stack. However, the caller was probably doing this too, so part of the calling convention states that BP must be preserved by any C function. Hence the callee, if it is going to set up BP as a frame pointer, must push the previous value first.
• The callee may then access its parameters relative to BP. The word at [BP] holds the previous value of BP as it was pushed; the next word, at [BP+2], holds the offset part of the return address, pushed implicitly by CALL. In a small-model (near) function, the parameters start after that, at [BP+4]; in a large-model (far) function, the segment part of the return address lives at [BP+4], and the parameters begin at [BP+6]. The leftmost parameter of the function, since it was pushed last, is accessible at this offset from BP; the others follow, at successively greater offsets. Thus, in a function such as printf which takes a variable number of parameters, the pushing of the parameters in reverse order means that the function knows where to find its first parameter, which tells it the number and type of the remaining ones.
• The callee may also wish to decrease SP further, so as to allocate space on the stack for local variables, which will then be accessible at negative offsets from BP.
• The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or DX:AX depending on the size of the value. Floating-point results are sometimes (depending on the compiler) returned in ST0.
• Once the callee has finished processing, it restores SP from BP if it had allocated local stack space, then pops the previous value of BP, and returns via RETN or RETF depending on memory model.
• When the caller regains control from the callee, the function parameters are still on the stack, so it typically adds an immediate constant to SP to remove them (instead of executing a number of slow POP instructions). Thus, if a function is accidentally called with the wrong number of parameters due to a prototype mismatch, the stack will still be returned to a sensible state since the caller, which knows how many parameters it pushed, does the removing.

It is instructive to compare this calling convention with that for Pascal programs (described in section 7.5.1). Pascal has a simpler convention, since no functions have variable numbers of parameters. Therefore the callee knows how many parameters it should have been passed, and is able to deallocate them from the stack itself by passing an immediate argument to the RET or RETF instruction, so the caller does not have to do it. Also, the parameters are pushed in left-to-right order, not right-to-left, which means that a compiler can give better guarantees about sequence points without performance suffering.

Thus, you would define a function in C style in the following way. The following example is for small model:

          global _myfunc 
_myfunc:  push bp 
          mov bp,sp 
          sub sp,0x40            ; 64 bytes of local stack space 
          mov bx,[bp+4]          ; first parameter to function 
          ; some more code 
          mov sp,bp              ; undo "sub sp,0x40" above 
          pop bp 
          ret

For a large-model function, you would replace RET by RETF, and look for the first parameter at [BP+6] instead of [BP+4]. Of course, if one of the parameters is a pointer, then the offsets of subsequent parameters will change depending on the memory model as well: far pointers take up four bytes on the stack when passed as a parameter, whereas near pointers take up two.

At the other end of the process, to call a C function from your assembly code, you would do something like this:

          extern _printf 
          ; and then, further down... 
          push word [myint]      ; one of my integer variables 
          push word mystring     ; pointer into my data segment 
          call _printf 
          add sp,byte 4          ; `byte' saves space 
          ; then those data items... 
          segment _DATA 
myint     dw 1234 
mystring  db 'This number -> %d <- should be 1234',10,0

This piece of code is the small-model assembly equivalent of the C code

    int myint = 1234; 
    printf("This number -> %d <- should be 1234\n", myint);

In large model, the function-call code might look more like this. In this example, it is assumed that DS already holds the segment base of the segment _DATA. If not, you would have to initialise it first.

          push word [myint] 
          push word seg mystring ; Now push the segment, and... 
          push word mystring     ; ... offset of "mystring" 
          call far _printf 
          add sp,byte 6

The integer value still takes up one word on the stack, since large model does not affect the size of the int data type. The first argument (pushed last) to printf, however, is a data pointer, and therefore has to contain a segment and offset part. The segment should be stored second in memory, and therefore must be pushed first. (Of course, PUSH DS would have been a shorter instruction than PUSH WORD SEG mystring, if DS was set up as the above example assumed.) Then the actual call becomes a far call, since functions expect far calls in large model; and SP has to be increased by 6 rather than 4 afterwards to make up for the extra word of parameters.

7.4.4 Accessing Data Items

To get at the contents of C variables, or to declare variables which C can access, you need only declare the names as GLOBAL or EXTERN. (Again, the names require leading underscores, as stated in section 7.4.1.) Thus, a C variable declared as int i can be accessed from assembler as

          extern _i 
          mov ax,[_i]

And to declare your own integer variable which C programs can access as extern int j, you do this (making sure you are assembling in the _DATA segment, if necessary):

          global _j 
_j        dw 0

To access a C array, you need to know the size of the components of the array. For example, int variables are two bytes long, so if a C program declares an array as int a[10], you can access a[3] by coding mov ax,[_a+6]. (The byte offset 6 is obtained by multiplying the desired array index, 3, by the size of the array element, 2.) The sizes of the C base types in 16-bit compilers are: 1 for char, 2 for short and int, 4 for long and float, and 8 for double.

To access a C data structure, you need to know the offset from the base of the structure to the field you are interested in. You can either do this by converting the C structure definition into a NASM structure definition (using STRUC), or by calculating the one offset and using just that.

To do either of these, you should read your C compiler's manual to find out how it organises data structures. NASM gives no special alignment to structure members in its own STRUC macro, so you have to specify alignment yourself if the C compiler generates it. Typically, you might find that a structure like

struct { 
    char c; 
    int i; 
} foo;

might be four bytes long rather than three, since the int field would be aligned to a two-byte boundary. However, this sort of feature tends to be a configurable option in the C compiler, either using command-line options or #pragma lines, so you have to find out how your own compiler does it.

7.4.5 c16.mac: Helper Macros for the 16-bit C Interface

Included in the NASM archives, in the misc directory, is a file c16.mac of macros. It defines three macros: proc, arg and endproc. These are intended to be used for C-style procedure definitions, and they automate a lot of the work involved in keeping track of the calling convention.

An example of an assembly function using the macro set is given here:

          proc _nearproc 
%$i       arg 
%$j       arg 
          mov ax,[bp + %$i] 
          mov bx,[bp + %$j] 
          add ax,[bx] 
          endproc

This defines _nearproc to be a procedure taking two arguments, the first (i) an integer and the second (j) a pointer to an integer. It returns i + *j.

Note that the arg macro has an EQU as the first line of its expansion, and since the label before the macro call gets prepended to the first line of the expanded macro, the EQU works, defining %$i to be an offset from BP. A context-local variable is used, local to the context pushed by the proc macro and popped by the endproc macro, so that the same argument name can be used in later procedures. Of course, you don't have to do that.

The macro set produces code for near functions (tiny, small and compact-model code) by default. You can have it generate far functions (medium, large and huge-model code) by means of coding %define FARCODE. This changes the kind of return instruction generated by endproc, and also changes the starting point for the argument offsets. The macro set contains no intrinsic dependency on whether data pointers are far or not.

arg can take an optional parameter, giving the size of the argument. If no size is given, 2 is assumed, since it is likely that many function parameters will be of type int.

The large-model equivalent of the above function would look like this:

%define FARCODE 
          proc _farproc 
%$i       arg 
%$j       arg 4 
          mov ax,[bp + %$i] 
          mov bx,[bp + %$j] 
          mov es,[bp + %$j + 2] 
          add ax,[bx] 
          endproc

This makes use of the argument to the arg macro to define a parameter of size 4, because j is now a far pointer. When we load from j, we must load a segment and an offset.

7.5 Interfacing to Borland Pascal Programs

Interfacing to Borland Pascal programs is similar in concept to interfacing to 16-bit C programs. The differences are:

• The leading underscore required for interfacing to C programs is not required for Pascal.
• The memory model is always large: functions are far, data pointers are far, and no data item can be more than 64K long. (Actually, some functions are near, but only those functions that are local to a Pascal unit and never called from outside it. All assembly functions that Pascal calls, and all Pascal functions that assembly routines are able to call, are far.) However, all static data declared in a Pascal program goes into the default data segment, which is the one whose segment address will be in DS when control is passed to your assembly code. The only things that do not live in the default data segment are local variables (they live in the stack segment) and dynamically allocated variables. All data pointers, however, are far.
• The function calling convention is different - described below.
• Some data types, such as strings, are stored differently.
• There are restrictions on the segment names you are allowed to use - Borland Pascal will ignore code or data declared in a segment it doesn't like the name of. The restrictions are described below.

7.5.1 The Pascal Calling Convention

The 16-bit Pascal calling convention is as follows. In the following description, the words caller and callee are used to denote the function doing the calling and the function which gets called.

• The caller pushes the function's parameters on the stack, one after another, in normal order (left to right, so that the first argument specified to the function is pushed first).
• The caller then executes a far CALL instruction to pass control to the callee.
• The callee receives control, and typically (although this is not actually necessary, in functions which do not need to access their parameters) starts by saving the value of SP in BP so as to be able to use BP as a base pointer to find its parameters on the stack. However, the caller was probably doing this too, so part of the calling convention states that BP must be preserved by any function. Hence the callee, if it is going to set up BP as a frame pointer, must push the previous value first.
• The callee may then access its parameters relative to BP. The word at [BP] holds the previous value of BP as it was pushed. The next word, at [BP+2], holds the offset part of the return address, and the next one at [BP+4] the segment part. The parameters begin at [BP+6]. The rightmost parameter of the function, since it was pushed last, is accessible at this offset from BP; the others follow, at successively greater offsets.
• The callee may also wish to decrease SP further, so as to allocate space on the stack for local variables, which will then be accessible at negative offsets from BP.
• The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or DX:AX depending on the size of the value. Floating-point results are returned in ST0. Results of type Real (Borland's own custom floating-point data type, not handled directly by the FPU) are returned in DX:BX:AX. To return a result of type String, the caller pushes a pointer to a temporary string before pushing the parameters, and the callee places the returned string value at that location. The pointer is not a parameter, and should not be removed from the stack by the RETF instruction.
• Once the callee has finished processing, it restores SP from BP if it had allocated local stack space, then pops the previous value of BP, and returns via RETF. It uses the form of RETF with an immediate parameter, giving the number of bytes taken up by the parameters on the stack. This causes the parameters to be removed from the stack as a side effect of the return instruction.
• When the caller regains control from the callee, the function parameters have already been removed from the stack, so it needs to do nothing further.

Thus, you would define a function in Pascal style, taking two Integer-type parameters, in the following way:

          global myfunc 
myfunc:   push bp 
          mov bp,sp 
          sub sp,0x40            ; 64 bytes of local stack space 
          mov bx,[bp+8]          ; first parameter to function 
          mov bx,[bp+6]          ; second parameter to function 
          ; some more code 
          mov sp,bp              ; undo "sub sp,0x40" above 
          pop bp 
          retf 4                 ; total size of params is 4

At the other end of the process, to call a Pascal function from your assembly code, you would do something like this:

          extern SomeFunc 
          ; and then, further down... 
          push word seg mystring ; Now push the segment, and... 
          push word mystring     ; ... offset of "mystring" 
          push word [myint]      ; one of my variables 
          call far SomeFunc

This is equivalent to the Pascal code

procedure SomeFunc(String: PChar; Int: Integer); 
    SomeFunc(@mystring, myint);

7.5.2 Borland Pascal Segment Name Restrictions

Since Borland Pascal's internal unit file format is completely different from OBJ, it only makes a very sketchy job of actually reading and understanding the various information contained in a real OBJ file when it links that in. Therefore an object file intended to be linked to a Pascal program must obey a number of restrictions:

• Procedures and functions must be in a segment whose name is either CODE, CSEG, or something ending in _TEXT.
• Initialised data must be in a segment whose name is either CONST or something ending in _DATA.
• Uninitialised data must be in a segment whose name is either DATA, DSEG, or something ending in _BSS.
• Any other segments in the object file are completely ignored. GROUP directives and segment attributes are also ignored.

7.5.3 Using c16.mac With Pascal Programs

The c16.mac macro package, described in section 7.4.5, can also be used to simplify writing functions to be called from Pascal programs, if you code %define PASCAL. This definition ensures that functions are far (it implies FARCODE), and also causes procedure return instructions to be generated with an operand.

Defining PASCAL does not change the code which calculates the argument offsets; you must declare your function's arguments in reverse order. For example:

%define PASCAL 
          proc _pascalproc 
%$j       arg 4 
%$i       arg 
          mov ax,[bp + %$i] 
          mov bx,[bp + %$j] 
          mov es,[bp + %$j + 2] 
          add ax,[bx] 
          endproc

This defines the same routine, conceptually, as the example in section 7.4.5: it defines a function taking two arguments, an integer and a pointer to an integer, which returns the sum of the integer and the contents of the pointer. The only difference between this code and the large-model C version is that PASCAL is defined instead of FARCODE, and that the arguments are declared in reverse order.

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