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<body><h1 align=center>The Netwide Assembler: NASM</h1>

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<h2><a name="chapter-8">Chapter 8: Writing 16-bit Code (DOS, Windows 3/3.1)</a></h2>
<p>This chapter attempts to cover some of the common issues encountered
when writing 16-bit code to run under <code><nobr>MS-DOS</nobr></code> or
<code><nobr>Windows 3.x</nobr></code>. It covers how to link programs to
produce <code><nobr>.EXE</nobr></code> or <code><nobr>.COM</nobr></code>
files, how to write <code><nobr>.SYS</nobr></code> device drivers, and how
to interface assembly language code with 16-bit C compilers and with
Borland Pascal.
<h3><a name="section-8.1">8.1 Producing <code><nobr>.EXE</nobr></code> Files</a></h3>
<p>Any large program written under DOS needs to be built as a
<code><nobr>.EXE</nobr></code> file: only <code><nobr>.EXE</nobr></code>
files have the necessary internal structure required to span more than one
64K segment. Windows programs, also, have to be built as
<code><nobr>.EXE</nobr></code> files, since Windows does not support the
<code><nobr>.COM</nobr></code> format.
<p>In general, you generate <code><nobr>.EXE</nobr></code> files by using
the <code><nobr>obj</nobr></code> output format to produce one or more
<code><nobr>.OBJ</nobr></code> files, and then linking them together using
a linker. However, NASM also supports the direct generation of simple DOS
<code><nobr>.EXE</nobr></code> files using the
<code><nobr>bin</nobr></code> output format (by using
<code><nobr>DB</nobr></code> and <code><nobr>DW</nobr></code> to construct
the <code><nobr>.EXE</nobr></code> file header), and a macro package is
supplied to do this. Thanks to Yann Guidon for contributing the code for
this.
<p>NASM may also support <code><nobr>.EXE</nobr></code> natively as another
output format in future releases.
<h4><a name="section-8.1.1">8.1.1 Using the <code><nobr>obj</nobr></code> Format To Generate <code><nobr>.EXE</nobr></code> Files</a></h4>
<p>This section describes the usual method of generating
<code><nobr>.EXE</nobr></code> files by linking
<code><nobr>.OBJ</nobr></code> files together.
<p>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
<code><nobr>LZH</nobr></code> archive format from
<a href="ftp://x2ftp.oulu.fi/pub/msdos/programming/lang/"><code><nobr>x2ftp.oulu.fi</nobr></code></a>.
An LZH archiver can be found at
<a href="ftp://ftp.simtel.net/pub/simtelnet/msdos/arcers"><code><nobr>ftp.simtel.net</nobr></code></a>.
There is another `free' linker (though this one doesn't come with sources)
called FREELINK, available from
<a href="http://www.pcorner.com/tpc/old/3-101.html"><code><nobr>www.pcorner.com</nobr></code></a>.
A third, <code><nobr>djlink</nobr></code>, written by DJ Delorie, is
available at
<a href="http://www.delorie.com/djgpp/16bit/djlink/"><code><nobr>www.delorie.com</nobr></code></a>.
A fourth linker, <code><nobr>ALINK</nobr></code>, written by Anthony A.J.
Williams, is available at
<a href="http://alink.sourceforge.net"><code><nobr>alink.sourceforge.net</nobr></code></a>.
<p>When linking several <code><nobr>.OBJ</nobr></code> files into a
<code><nobr>.EXE</nobr></code> file, you should ensure that exactly one of
them has a start point defined (using the <code><nobr>..start</nobr></code>
special symbol defined by the <code><nobr>obj</nobr></code> format: see
<a href="nasmdoc7.html#section-7.4.6">section 7.4.6</a>). 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 <em>which</em> value to use.
<p>An example of a NASM source file which can be assembled to a
<code><nobr>.OBJ</nobr></code> file and linked on its own to a
<code><nobr>.EXE</nobr></code> 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
<code><nobr>test</nobr></code> subdirectory of the NASM archives, under the
name <code><nobr>objexe.asm</nobr></code>.
<p><pre>
segment code 

..start: 
        mov     ax,data 
        mov     ds,ax 
        mov     ax,stack 
        mov     ss,ax 
        mov     sp,stacktop
</pre>
<p>This initial piece of code sets up <code><nobr>DS</nobr></code> to point
to the data segment, and initializes <code><nobr>SS</nobr></code> and
<code><nobr>SP</nobr></code> to point to the top of the provided stack.
Notice that interrupts are implicitly disabled for one instruction after a
move into <code><nobr>SS</nobr></code>, precisely for this situation, so
that there's no chance of an interrupt occurring between the loads of
<code><nobr>SS</nobr></code> and <code><nobr>SP</nobr></code> and not
having a stack to execute on.
<p>Note also that the special symbol <code><nobr>..start</nobr></code> is
defined at the beginning of this code, which means that will be the entry
point into the resulting executable file.
<p><pre>
        mov     dx,hello 
        mov     ah,9 
        int     0x21
</pre>
<p>The above is the main program: load <code><nobr>DS:DX</nobr></code> with
a pointer to the greeting message (<code><nobr>hello</nobr></code> is
implicitly relative to the segment <code><nobr>data</nobr></code>, which
was loaded into <code><nobr>DS</nobr></code> in the setup code, so the full
pointer is valid), and call the DOS print-string function.
<p><pre>
        mov     ax,0x4c00 
        int     0x21
</pre>
<p>This terminates the program using another DOS system call.
<p><pre>
segment data 

hello:  db      'hello, world', 13, 10, '$'
</pre>
<p>The data segment contains the string we want to display.
<p><pre>
segment stack stack 
        resb 64 
stacktop:
</pre>
<p>The above code declares a stack segment containing 64 bytes of
uninitialized stack space, and points <code><nobr>stacktop</nobr></code> at
the top of it. The directive <code><nobr>segment stack stack</nobr></code>
defines a segment <em>called</em> <code><nobr>stack</nobr></code>, and also
of <em>type</em> <code><nobr>STACK</nobr></code>. 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
<code><nobr>STACK</nobr></code>.
<p>The above file, when assembled into a <code><nobr>.OBJ</nobr></code>
file, will link on its own to a valid <code><nobr>.EXE</nobr></code> file,
which when run will print `hello, world' and then exit.
<h4><a name="section-8.1.2">8.1.2 Using the <code><nobr>bin</nobr></code> Format To Generate <code><nobr>.EXE</nobr></code> Files</a></h4>
<p>The <code><nobr>.EXE</nobr></code> file format is simple enough that
it's possible to build a <code><nobr>.EXE</nobr></code> 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
<code><nobr>DB</nobr></code> and <code><nobr>DW</nobr></code> commands by
NASM itself, so that you can use the <code><nobr>bin</nobr></code> output
format to directly generate <code><nobr>.EXE</nobr></code> files.
<p>Included in the NASM archives, in the <code><nobr>misc</nobr></code>
subdirectory, is a file <code><nobr>exebin.mac</nobr></code> of macros. It
defines three macros: <code><nobr>EXE_begin</nobr></code>,
<code><nobr>EXE_stack</nobr></code> and <code><nobr>EXE_end</nobr></code>.
<p>To produce a <code><nobr>.EXE</nobr></code> file using this method, you
should start by using <code><nobr>%include</nobr></code> to load the
<code><nobr>exebin.mac</nobr></code> macro package into your source file.
You should then issue the <code><nobr>EXE_begin</nobr></code> macro call
(which takes no arguments) to generate the file header data. Then write
code as normal for the <code><nobr>bin</nobr></code> format - you can use
all three standard sections <code><nobr>.text</nobr></code>,
<code><nobr>.data</nobr></code> and <code><nobr>.bss</nobr></code>. At the
end of the file you should call the <code><nobr>EXE_end</nobr></code> macro
(again, no arguments), which defines some symbols to mark section sizes,
and these symbols are referred to in the header code generated by
<code><nobr>EXE_begin</nobr></code>.
<p>In this model, the code you end up writing starts at
<code><nobr>0x100</nobr></code>, just like a <code><nobr>.COM</nobr></code>
file - in fact, if you strip off the 32-byte header from the resulting
<code><nobr>.EXE</nobr></code> file, you will have a valid
<code><nobr>.COM</nobr></code> program. All the segment bases are the same,
so you are limited to a 64K program, again just like a
<code><nobr>.COM</nobr></code> file. Note that an
<code><nobr>ORG</nobr></code> directive is issued by the
<code><nobr>EXE_begin</nobr></code> macro, so you should not explicitly
issue one of your own.
<p>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 <code><nobr>CS</nobr></code> instead.
<p>On entry to your <code><nobr>.EXE</nobr></code> file,
<code><nobr>SS:SP</nobr></code> 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
<code><nobr>EXE_stack</nobr></code> macro. For example, to change the stack
size of your program to 64 bytes, you would call
<code><nobr>EXE_stack 64</nobr></code>.
<p>A sample program which generates a <code><nobr>.EXE</nobr></code> file
in this way is given in the <code><nobr>test</nobr></code> subdirectory of
the NASM archive, as <code><nobr>binexe.asm</nobr></code>.
<h3><a name="section-8.2">8.2 Producing <code><nobr>.COM</nobr></code> Files</a></h3>
<p>While large DOS programs must be written as
<code><nobr>.EXE</nobr></code> files, small ones are often better written
as <code><nobr>.COM</nobr></code> files. <code><nobr>.COM</nobr></code>
files are pure binary, and therefore most easily produced using the
<code><nobr>bin</nobr></code> output format.
<h4><a name="section-8.2.1">8.2.1 Using the <code><nobr>bin</nobr></code> Format To Generate <code><nobr>.COM</nobr></code> Files</a></h4>
<p><code><nobr>.COM</nobr></code> files expect to be loaded at offset
<code><nobr>100h</nobr></code> into their segment (though the segment may
change). Execution then begins at <code><nobr>100h</nobr></code>, i.e.
right at the start of the program. So to write a
<code><nobr>.COM</nobr></code> program, you would create a source file
looking like
<p><pre>
        org 100h 

section .text 

start: 
        ; put your code here 

section .data 

        ; put data items here 

section .bss 

        ; put uninitialized data here
</pre>
<p>The <code><nobr>bin</nobr></code> format puts the
<code><nobr>.text</nobr></code> 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.
<p>The BSS (uninitialized data) section does not take up space in the
<code><nobr>.COM</nobr></code> 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.
<p>To assemble the above program, you should use a command line like
<p><pre>
nasm myprog.asm -fbin -o myprog.com
</pre>
<p>The <code><nobr>bin</nobr></code> format would produce a file called
<code><nobr>myprog</nobr></code> if no explicit output file name were
specified, so you have to override it and give the desired file name.
<h4><a name="section-8.2.2">8.2.2 Using the <code><nobr>obj</nobr></code> Format To Generate <code><nobr>.COM</nobr></code> Files</a></h4>
<p>If you are writing a <code><nobr>.COM</nobr></code> program as more than
one module, you may wish to assemble several <code><nobr>.OBJ</nobr></code>
files and link them together into a <code><nobr>.COM</nobr></code> program.
You can do this, provided you have a linker capable of outputting
<code><nobr>.COM</nobr></code> files directly (TLINK does this), or
alternatively a converter program such as <code><nobr>EXE2BIN</nobr></code>
to transform the <code><nobr>.EXE</nobr></code> file output from the linker
into a <code><nobr>.COM</nobr></code> file.
<p>If you do this, you need to take care of several things:
<ul>
<li>The first object file containing code should start its code segment
with a line like <code><nobr>RESB 100h</nobr></code>. This is to ensure
that the code begins at offset <code><nobr>100h</nobr></code> 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 <code><nobr>.COM</nobr></code> file. Other assemblers use an
<code><nobr>ORG</nobr></code> directive for this purpose, but
<code><nobr>ORG</nobr></code> in NASM is a format-specific directive to the
<code><nobr>bin</nobr></code> output format, and does not mean the same
thing as it does in MASM-compatible assemblers.
<li>You don't need to define a stack segment.
<li>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 <code><nobr>.COM</nobr></code>
file is loaded, all the segment registers contain the same value.
</ul>
<h3><a name="section-8.3">8.3 Producing <code><nobr>.SYS</nobr></code> Files</a></h3>
<p>MS-DOS device drivers - <code><nobr>.SYS</nobr></code> files - are pure
binary files, similar to <code><nobr>.COM</nobr></code> files, except that
they start at origin zero rather than <code><nobr>100h</nobr></code>.
Therefore, if you are writing a device driver using the
<code><nobr>bin</nobr></code> format, you do not need the
<code><nobr>ORG</nobr></code> directive, since the default origin for
<code><nobr>bin</nobr></code> is zero. Similarly, if you are using
<code><nobr>obj</nobr></code>, you do not need the
<code><nobr>RESB 100h</nobr></code> at the start of your code segment.
<p><code><nobr>.SYS</nobr></code> 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.
<p>For more information on the format of <code><nobr>.SYS</nobr></code>
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
<a href="news:comp.os.msdos.programmer"><code><nobr>comp.os.msdos.programmer</nobr></code></a>.
<h3><a name="section-8.4">8.4 Interfacing to 16-bit C Programs</a></h3>
<p>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 <code><nobr>.OBJ</nobr></code> file, and link it with
your C modules to produce a mixed-language program.
<h4><a name="section-8.4.1">8.4.1 External Symbol Names</a></h4>
<p>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 <code><nobr>printf</nobr></code> appears to an
assembly language programmer as <code><nobr>_printf</nobr></code>. 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.
<p>If you find the underscores inconvenient, you can define macros to
replace the <code><nobr>GLOBAL</nobr></code> and
<code><nobr>EXTERN</nobr></code> directives as follows:
<p><pre>
%macro  cglobal 1 

  global  _%1 
  %define %1 _%1 

%endmacro 

%macro  cextern 1 

  extern  _%1 
  %define %1 _%1 

%endmacro
</pre>
<p>(These forms of the macros only take one argument at a time; a
<code><nobr>%rep</nobr></code> construct could solve this.)
<p>If you then declare an external like this:
<p><pre>
cextern printf
</pre>
<p>then the macro will expand it as
<p><pre>
extern  _printf 
%define printf _printf
</pre>
<p>Thereafter, you can reference <code><nobr>printf</nobr></code> as if it
was a symbol, and the preprocessor will put the leading underscore on where
necessary.
<p>The <code><nobr>cglobal</nobr></code> macro works similarly. You must
use <code><nobr>cglobal</nobr></code> before defining the symbol in
question, but you would have had to do that anyway if you used
<code><nobr>GLOBAL</nobr></code>.
<p>Also see <a href="nasmdoc2.html#section-2.1.27">section 2.1.27</a>.
<h4><a name="section-8.4.2">8.4.2 Memory Models</a></h4>
<p>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:
<ul>
<li>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 <code><nobr>CS</nobr></code> register
never changes its value, and always gives the segment part of the full
function address), and that functions are called using ordinary near
<code><nobr>CALL</nobr></code> instructions and return using
<code><nobr>RETN</nobr></code> (which, in NASM, is synonymous with
<code><nobr>RET</nobr></code> anyway). This means both that you should
write your own routines to return with <code><nobr>RETN</nobr></code>, and
that you should call external C routines with near
<code><nobr>CALL</nobr></code> instructions.
<li>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 <code><nobr>CALL FAR</nobr></code> (or
<code><nobr>CALL seg:offset</nobr></code>) and return using
<code><nobr>RETF</nobr></code>. Again, you should therefore write your own
routines to return with <code><nobr>RETF</nobr></code> and use
<code><nobr>CALL FAR</nobr></code> to call external routines.
<li>In models using a single data segment (tiny, small and medium), data
pointers are 16 bits long, containing only an offset field (the
<code><nobr>DS</nobr></code> register doesn't change its value, and always
gives the segment part of the full data item address).
<li>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
<code><nobr>DS</nobr></code> in your routines without restoring it
afterwards, but <code><nobr>ES</nobr></code> is free for you to use to
access the contents of 32-bit data pointers you are passed.
<li>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.
<li>In most memory models, there is a <em>default</em> data segment, whose
segment address is kept in <code><nobr>DS</nobr></code> throughout the
program. This data segment is typically the same segment as the stack, kept
in <code><nobr>SS</nobr></code>, so that functions' local variables (which
are stored on the stack) and global data items can both be accessed easily
without changing <code><nobr>DS</nobr></code>. Particularly large data
items are typically stored in other segments. However, some memory models
(though not the standard ones, usually) allow the assumption that
<code><nobr>SS</nobr></code> and <code><nobr>DS</nobr></code> hold the same
value to be removed. Be careful about functions' local variables in this
latter case.
</ul>
<p>In models with a single code segment, the segment is called
<code><nobr>_TEXT</nobr></code>, 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 <code><nobr>_DATA</nobr></code>.
<h4><a name="section-8.4.3">8.4.3 Function Definitions and Function Calls</a></h4>
<p>The C calling convention in 16-bit programs is as follows. In the
following description, the words <em>caller</em> and <em>callee</em> are
used to denote the function doing the calling and the function which gets
called.
<ul>
<li>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).
<li>The caller then executes a <code><nobr>CALL</nobr></code> instruction
to pass control to the callee. This <code><nobr>CALL</nobr></code> is
either near or far depending on the memory model.
<li>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 <code><nobr>SP</nobr></code> in
<code><nobr>BP</nobr></code> so as to be able to use
<code><nobr>BP</nobr></code> 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 <code><nobr>BP</nobr></code> must be
preserved by any C function. Hence the callee, if it is going to set up
<code><nobr>BP</nobr></code> as a <em>frame pointer</em>, must push the
previous value first.
<li>The callee may then access its parameters relative to
<code><nobr>BP</nobr></code>. The word at <code><nobr>[BP]</nobr></code>
holds the previous value of <code><nobr>BP</nobr></code> as it was pushed;
the next word, at <code><nobr>[BP+2]</nobr></code>, holds the offset part
of the return address, pushed implicitly by <code><nobr>CALL</nobr></code>.
In a small-model (near) function, the parameters start after that, at
<code><nobr>[BP+4]</nobr></code>; in a large-model (far) function, the
segment part of the return address lives at
<code><nobr>[BP+4]</nobr></code>, and the parameters begin at
<code><nobr>[BP+6]</nobr></code>. The leftmost parameter of the function,
since it was pushed last, is accessible at this offset from
<code><nobr>BP</nobr></code>; the others follow, at successively greater
offsets. Thus, in a function such as <code><nobr>printf</nobr></code> 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.
<li>The callee may also wish to decrease <code><nobr>SP</nobr></code>
further, so as to allocate space on the stack for local variables, which
will then be accessible at negative offsets from
<code><nobr>BP</nobr></code>.
<li>The callee, if it wishes to return a value to the caller, should leave
the value in <code><nobr>AL</nobr></code>, <code><nobr>AX</nobr></code> or
<code><nobr>DX:AX</nobr></code> depending on the size of the value.
Floating-point results are sometimes (depending on the compiler) returned
in <code><nobr>ST0</nobr></code>.
<li>Once the callee has finished processing, it restores
<code><nobr>SP</nobr></code> from <code><nobr>BP</nobr></code> if it had
allocated local stack space, then pops the previous value of
<code><nobr>BP</nobr></code>, and returns via
<code><nobr>RETN</nobr></code> or <code><nobr>RETF</nobr></code> depending
on memory model.
<li>When the caller regains control from the callee, the function
parameters are still on the stack, so it typically adds an immediate
constant to <code><nobr>SP</nobr></code> to remove them (instead of
executing a number of slow <code><nobr>POP</nobr></code> 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 <em>knows</em> how many parameters
it pushed, does the removing.
</ul>
<p>It is instructive to compare this calling convention with that for
Pascal programs (described in <a href="#section-8.5.1">section 8.5.1</a>).
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 <code><nobr>RET</nobr></code> or
<code><nobr>RETF</nobr></code> 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.
<p>Thus, you would define a function in C style in the following way. The
following example is for small model:
<p><pre>
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
</pre>
<p>For a large-model function, you would replace
<code><nobr>RET</nobr></code> by <code><nobr>RETF</nobr></code>, and look
for the first parameter at <code><nobr>[BP+6]</nobr></code> instead of
<code><nobr>[BP+4]</nobr></code>. Of course, if one of the parameters is a
pointer, then the offsets of <em>subsequent</em> 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.
<p>At the other end of the process, to call a C function from your assembly
code, you would do something like this:
<p><pre>
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 -&gt; %d &lt;- should be 1234',10,0
</pre>
<p>This piece of code is the small-model assembly equivalent of the C code
<p><pre>
    int myint = 1234; 
    printf("This number -&gt; %d &lt;- should be 1234\n", myint);
</pre>
<p>In large model, the function-call code might look more like this. In
this example, it is assumed that <code><nobr>DS</nobr></code> already holds
the segment base of the segment <code><nobr>_DATA</nobr></code>. If not,
you would have to initialize it first.
<p><pre>
      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
</pre>
<p>The integer value still takes up one word on the stack, since large
model does not affect the size of the <code><nobr>int</nobr></code> data
type. The first argument (pushed last) to <code><nobr>printf</nobr></code>,
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, <code><nobr>PUSH DS</nobr></code> would
have been a shorter instruction than
<code><nobr>PUSH WORD SEG mystring</nobr></code>, if
<code><nobr>DS</nobr></code> 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 <code><nobr>SP</nobr></code> has to be increased by 6
rather than 4 afterwards to make up for the extra word of parameters.
<h4><a name="section-8.4.4">8.4.4 Accessing Data Items</a></h4>
<p>To get at the contents of C variables, or to declare variables which C
can access, you need only declare the names as
<code><nobr>GLOBAL</nobr></code> or <code><nobr>EXTERN</nobr></code>.
(Again, the names require leading underscores, as stated in
<a href="#section-8.4.1">section 8.4.1</a>.) Thus, a C variable declared as
<code><nobr>int i</nobr></code> can be accessed from assembler as
<p><pre>
extern _i 

        mov ax,[_i]
</pre>
<p>And to declare your own integer variable which C programs can access as
<code><nobr>extern int j</nobr></code>, you do this (making sure you are
assembling in the <code><nobr>_DATA</nobr></code> segment, if necessary):
<p><pre>
global  _j 

_j      dw      0
</pre>
<p>To access a C array, you need to know the size of the components of the
array. For example, <code><nobr>int</nobr></code> variables are two bytes
long, so if a C program declares an array as
<code><nobr>int a[10]</nobr></code>, you can access
<code><nobr>a[3]</nobr></code> by coding
<code><nobr>mov ax,[_a+6]</nobr></code>. (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
<code><nobr>char</nobr></code>, 2 for <code><nobr>short</nobr></code> and
<code><nobr>int</nobr></code>, 4 for <code><nobr>long</nobr></code> and
<code><nobr>float</nobr></code>, and 8 for
<code><nobr>double</nobr></code>.
<p>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 <code><nobr>STRUC</nobr></code>), or by calculating the one offset
and using just that.
<p>To do either of these, you should read your C compiler's manual to find
out how it organizes data structures. NASM gives no special alignment to
structure members in its own <code><nobr>STRUC</nobr></code> macro, so you
have to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like
<p><pre>
struct { 
    char c; 
    int i; 
} foo;
</pre>
<p>might be four bytes long rather than three, since the
<code><nobr>int</nobr></code> 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
<code><nobr>#pragma</nobr></code> lines, so you have to find out how your
own compiler does it.
<h4><a name="section-8.4.5">8.4.5 <code><nobr>c16.mac</nobr></code>: Helper Macros for the 16-bit C Interface</a></h4>
<p>Included in the NASM archives, in the <code><nobr>misc</nobr></code>
directory, is a file <code><nobr>c16.mac</nobr></code> of macros. It
defines three macros: <code><nobr>proc</nobr></code>,
<code><nobr>arg</nobr></code> and <code><nobr>endproc</nobr></code>. 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.
<p>(An alternative, TASM compatible form of <code><nobr>arg</nobr></code>
is also now built into NASM's preprocessor. See
<a href="nasmdoc4.html#section-4.8">section 4.8</a> for details.)
<p>An example of an assembly function using the macro set is given here:
<p><pre>
proc    _nearproc 

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

endproc
</pre>
<p>This defines <code><nobr>_nearproc</nobr></code> to be a procedure
taking two arguments, the first (<code><nobr>i</nobr></code>) an integer
and the second (<code><nobr>j</nobr></code>) a pointer to an integer. It
returns <code><nobr>i + *j</nobr></code>.
<p>Note that the <code><nobr>arg</nobr></code> macro has an
<code><nobr>EQU</nobr></code> 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 <code><nobr>EQU</nobr></code> works, defining
<code><nobr>%$i</nobr></code> to be an offset from
<code><nobr>BP</nobr></code>. A context-local variable is used, local to
the context pushed by the <code><nobr>proc</nobr></code> macro and popped
by the <code><nobr>endproc</nobr></code> macro, so that the same argument
name can be used in later procedures. Of course, you don't <em>have</em> to
do that.
<p>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
<code><nobr>%define FARCODE</nobr></code>. This changes the kind of return
instruction generated by <code><nobr>endproc</nobr></code>, 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.
<p><code><nobr>arg</nobr></code> 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
<code><nobr>int</nobr></code>.
<p>The large-model equivalent of the above function would look like this:
<p><pre>
%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
</pre>
<p>This makes use of the argument to the <code><nobr>arg</nobr></code>
macro to define a parameter of size 4, because <code><nobr>j</nobr></code>
is now a far pointer. When we load from <code><nobr>j</nobr></code>, we
must load a segment and an offset.
<h3><a name="section-8.5">8.5 Interfacing to Borland Pascal Programs</a></h3>
<p>Interfacing to Borland Pascal programs is similar in concept to
interfacing to 16-bit C programs. The differences are:
<ul>
<li>The leading underscore required for interfacing to C programs is not
required for Pascal.
<li>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
<code><nobr>DS</nobr></code> 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 <em>pointers</em>, however, are far.
<li>The function calling convention is different - described below.
<li>Some data types, such as strings, are stored differently.
<li>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.
</ul>
<h4><a name="section-8.5.1">8.5.1 The Pascal Calling Convention</a></h4>
<p>The 16-bit Pascal calling convention is as follows. In the following
description, the words <em>caller</em> and <em>callee</em> are used to
denote the function doing the calling and the function which gets called.
<ul>
<li>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).
<li>The caller then executes a far <code><nobr>CALL</nobr></code>
instruction to pass control to the callee.
<li>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 <code><nobr>SP</nobr></code> in
<code><nobr>BP</nobr></code> so as to be able to use
<code><nobr>BP</nobr></code> 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 <code><nobr>BP</nobr></code> must be
preserved by any function. Hence the callee, if it is going to set up
<code><nobr>BP</nobr></code> as a frame pointer, must push the previous
value first.
<li>The callee may then access its parameters relative to
<code><nobr>BP</nobr></code>. The word at <code><nobr>[BP]</nobr></code>
holds the previous value of <code><nobr>BP</nobr></code> as it was pushed.
The next word, at <code><nobr>[BP+2]</nobr></code>, holds the offset part
of the return address, and the next one at <code><nobr>[BP+4]</nobr></code>
the segment part. The parameters begin at <code><nobr>[BP+6]</nobr></code>.
The rightmost parameter of the function, since it was pushed last, is
accessible at this offset from <code><nobr>BP</nobr></code>; the others
follow, at successively greater offsets.
<li>The callee may also wish to decrease <code><nobr>SP</nobr></code>
further, so as to allocate space on the stack for local variables, which
will then be accessible at negative offsets from
<code><nobr>BP</nobr></code>.
<li>The callee, if it wishes to return a value to the caller, should leave
the value in <code><nobr>AL</nobr></code>, <code><nobr>AX</nobr></code> or
<code><nobr>DX:AX</nobr></code> depending on the size of the value.
Floating-point results are returned in <code><nobr>ST0</nobr></code>.
Results of type <code><nobr>Real</nobr></code> (Borland's own custom
floating-point data type, not handled directly by the FPU) are returned in
<code><nobr>DX:BX:AX</nobr></code>. To return a result of type
<code><nobr>String</nobr></code>, 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 <code><nobr>RETF</nobr></code>
instruction.
<li>Once the callee has finished processing, it restores
<code><nobr>SP</nobr></code> from <code><nobr>BP</nobr></code> if it had
allocated local stack space, then pops the previous value of
<code><nobr>BP</nobr></code>, and returns via
<code><nobr>RETF</nobr></code>. It uses the form of
<code><nobr>RETF</nobr></code> 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.
<li>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.
</ul>
<p>Thus, you would define a function in Pascal style, taking two
<code><nobr>Integer</nobr></code>-type parameters, in the following way:
<p><pre>
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
</pre>
<p>At the other end of the process, to call a Pascal function from your
assembly code, you would do something like this:
<p><pre>
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
</pre>
<p>This is equivalent to the Pascal code
<p><pre>
procedure SomeFunc(String: PChar; Int: Integer); 
    SomeFunc(@mystring, myint);
</pre>
<h4><a name="section-8.5.2">8.5.2 Borland Pascal Segment Name Restrictions</a></h4>
<p>Since Borland Pascal's internal unit file format is completely different
from <code><nobr>OBJ</nobr></code>, it only makes a very sketchy job of
actually reading and understanding the various information contained in a
real <code><nobr>OBJ</nobr></code> file when it links that in. Therefore an
object file intended to be linked to a Pascal program must obey a number of
restrictions:
<ul>
<li>Procedures and functions must be in a segment whose name is either
<code><nobr>CODE</nobr></code>, <code><nobr>CSEG</nobr></code>, or
something ending in <code><nobr>_TEXT</nobr></code>.
<li>initialized data must be in a segment whose name is either
<code><nobr>CONST</nobr></code> or something ending in
<code><nobr>_DATA</nobr></code>.
<li>Uninitialized data must be in a segment whose name is either
<code><nobr>DATA</nobr></code>, <code><nobr>DSEG</nobr></code>, or
something ending in <code><nobr>_BSS</nobr></code>.
<li>Any other segments in the object file are completely ignored.
<code><nobr>GROUP</nobr></code> directives and segment attributes are also
ignored.
</ul>
<h4><a name="section-8.5.3">8.5.3 Using <code><nobr>c16.mac</nobr></code> With Pascal Programs</a></h4>
<p>The <code><nobr>c16.mac</nobr></code> macro package, described in
<a href="#section-8.4.5">section 8.4.5</a>, can also be used to simplify
writing functions to be called from Pascal programs, if you code
<code><nobr>%define PASCAL</nobr></code>. This definition ensures that
functions are far (it implies <code><nobr>FARCODE</nobr></code>), and also
causes procedure return instructions to be generated with an operand.
<p>Defining <code><nobr>PASCAL</nobr></code> does not change the code which
calculates the argument offsets; you must declare your function's arguments
in reverse order. For example:
<p><pre>
%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
</pre>
<p>This defines the same routine, conceptually, as the example in
<a href="#section-8.4.5">section 8.4.5</a>: 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
<code><nobr>PASCAL</nobr></code> is defined instead of
<code><nobr>FARCODE</nobr></code>, and that the arguments are declared in
reverse order.
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