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.
.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. 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
add sp, 12 ; The caller balances the stack frame
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.
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>