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Thursday February 23, 2017
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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.

    <Your initialized data>
   <Your uninitialized data>
   <Your constants>
    <Your code>
    end <label>
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 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  [third_param]               ; Push the third parameter
push  [second_param]            ; Followed by the second
push  [first_param]                ; And the first
call    foo
add    sp, 12                                ; 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.
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>

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