Tutorial 1: The Basics
This tutorial assumes 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 runs each Win32 program in separated virtual space. That means each
Win32 program will have its own 4 GB address space. However, this doesn't mean
every win32 program has 4GB of physical memory, only that the program can address
any address in that range. Windows will do anything necessary to make the memory
the program references valid. Of course, the program must adhere to the rules
set by Windows, else it will cause the dreaded General Protection Fault. 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 under 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. Under Win32, we need not be concerned with memory model or segments 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.
When you program under Win32, you must know some important rules. One such rule
is that, Windows uses esi, edi, ebp and ebx internally and it doesn't expect
the values in those registers to change. So remember this rule first: if you
use any of those four registers in your callback function, don't ever forget
to restore them before returning control to Windows. A callback function is
your own function which is called by Windows. The obvious example is the windows
procedure. This doesn't mean that you cannot use those four registers, you can.
Just be sure to restore them back before passing control back to Windows.
Here's the skeleton program. If you don't understand some of the codes, don't panic. I'll explain each of them later.
.386
.MODEL Flat, STDCALL
.DATA
<Your initialized data>
......
.DATA?
<Your uninitialized data>
......
.CONST
<Your constants>
......
.CODE
<label>
<Your code>
.....
end <label>
That's all! Let's analyze this skeleton program.
.386
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. There
are actually two nearly identical forms for each CPU model. .386/.386p, .486/.486p.
Those "p" versions are necessary only when your program uses privileged
instructions. Privileged instructions are the instructions reserved by the CPU/operating
system when in protected mode. They can only be used by privileged code, such
as the virtual device drivers. Most of the time, your program will work in non-privileged
mode so it's safe to use non-p versions.
.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.
Under Win16, there are 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 [third_param] ; Push the third parameterPASCAL 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.
push [second_param] ; Followed by the second
push [first_param] ; And the first
call foo
add sp, 12 ; The caller balances the stack frame
.DATA
.DATA?
.CONST
.CODE
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:
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.
<label>
end <label>
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>