Tutorial 1: The Basics
This tutorial assume that the reader knows how to use MASM. If you're not familiar with
MASM, download win32asm.exe
and study the text inside the package before going on with the tutorial. Good. You're now
ready. Let's go!
Win32 programs run in protected mode which is available since 80286. But 80286 is now
history. So we only have to concern ourselves with 80386 and its descendants. Windows run
each Win32 program in separated virtual space. That means each
Win32 program will have its own 4 GB address space. Each
program is alone in its address space. This is in contrast to the situation in Win16. All
Win16 programs can *see* each other. Not so in Win32. This feature helps reduce the chance
of one program writing over other program's code/data.
Memory model is also drastically different from the old days
of the 16-bit world. In Win32, we need not be concerned with memory model or segment
anymore! There's only one memory model: Flat memory model. There's no more 64K segments. The memory is a large continuous space of 4 GB. That also means you don't have to play with segment registers. You can
use any segment register to address any point in the memory space. That's a GREAT help to programmers. This is what makes
Win32 assembly programming as easy as C.
Here's the skeleton program. If you don't understand some of the codes, don't panic.
I'll explain each of them later.
.MODEL Flat, STDCALL
<Your initialized data>
<Your uninitialized data>
That's all! Let's analyze this skeleton program.
This is an assembler directive, telling the assembler to use
80386 instruction set. You can also use .486, .586 but the safest bet is to stick to .386.
.MODEL FLAT, STDCALL
.MODEL is an assembler directive
that specifies memory model of your program. Under Win32, there's only on model, FLAT model.
STDCALL tells MASM about parameter
passing convention. Parameter passing convention specifies the order of parameter
passing, left-to-right or right-to-left, and also who will balance the stack frame after
the function call.
In Win16, there's two types of calling convention, C and PASCAL
C calling convention passes
parameters from right to left, that is , the rightmost parameter is pushed first. The
caller is responsible for balancing the stack frame after the call. For example, in order
to call a function named foo(int first_param, int second_param, int third_param) in C
calling convention the asm codes will look like this:
; Push the third parameter
Followed by the second
; And the first
; The caller balances the stack frame
PASCAL calling convention is the reverse of C calling
convention. It passes parameters from left to right and the callee is responsible for the
stack balancing after the call.
Win16 adopts PASCAL convention because it produces smaller codes. C convention is useful when
you don't know how many parameters will be passed to the function as in the case of
wsprintf(). In the case of wsprintf(), the function has no way to determine beforehand how
many parameters will be pushed on the stack, so it cannot do the stack balancing.
STDCALL is the hybrid of C and
PASCAL convention. It passes parameter from right to left but the callee is responsible
for stack balancing after the call.Win32 platform use STDCALL exclusively. Except in one case: wsprintf(). You must use C calling
convention with wsprintf().
All four directives are what's called section. You don't have
segments in Win32, remember? But you can divide your entire address space into logical
sections. The start of one section denotes the end of the previous section. There'are two
groups of section: data and code. Data sections are divided into 3 categories:
- .DATA This
section contains initialized data of your program.
- .DATA? This section contains
uninitialized data of your program. Sometimes you just want to preallocate some memory but
doesn't want to initialize it. This section is for that purpose.
- .CONST This section contains
declaration of constants used by your program. Constants in this section can never be
modified in your program. They are just *constant*.
You don't have to use all three sections in your program. Declare only the section(s)
you want to use.
There's only one section for code: .CODE. This is where your codes reside.
where <label> is any arbitrary label is used to specify the
extent of your code. Both labels must be identical. All your codes must reside
between <label> and end <label>