Instructions
Mnemonic
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Purpose
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Examples
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mov dest,src
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Move data between registers, load immediate data into registers, move data between registers and memory.
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mov eax,4 ; Load constant into eax
mov ebx,eax ; Copy eax into ebx
mov ebx,[123] ; Copy ebx to memory address 123
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push src
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Insert a value onto the stack. Useful for passing arguments, saving registers, etc.
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push ebp
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pop dest
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Remove topmost value from the stack. Equivalent to "mov dest,[esp] add esp,4"
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pop ebp
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call func
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Push the address of the next instruction and start executing func.
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call print_int
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ret
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Pop the return program counter, and jump there. Ends a subroutine.
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ret
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add src,dest
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dest=dest+src
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add eax,ebx # Add ebx to eax
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mul src
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Multiply eax and src as unsigned integers, and put the result in eax. High 32 bits of product go into eax.
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mul ebx #Multiply eax by ebx
|
| jmp label |
Goto the instruction label:. Skips anything else in the way. |
jmp post_mem
...
post_mem:
|
cmp a,b
|
Compare two values. Sets
flags that are used by the conditional jumps (below).
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cmp eax,10
|
|
jl label |
Goto label
if previous comparison came out as less-than. Other conditionals
available are: jle (<=), je (==), jge (>=), jg (>), jne (!=),
and many others. |
jl loop_start ; Jump if eax<10
|
|
Stack Frame
(example without ebp or local variables)
Contents
|
off esp
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caller's variables
|
[esp+12]
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Argument 2
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[esp+8]
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Argument 1
|
[esp+4]
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Caller Return Address
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[esp]
|
my_sub: # Returns first argument
mov eax,[esp+4]
ret
(example when using ebp and two local variables)
Contents
|
off ebp
|
off esp
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caller's variables
|
[ebp+16]
|
[esp+24]
|
Argument 2
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[ebp+12]
|
[esp+20]
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Argument 1
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[ebp+8]
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[esp+16]
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Caller Return Address
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[ebp+4]
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[esp+12]
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Saved ebp
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[ebp]
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[esp+8]
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Local variable 1
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[ebp-4]
|
[esp+4]
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Local variable 2
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[ebp-8] |
[esp]
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my_sub2: # Returns first argument
push ebp # Prologue
mov ebp, esp
mov eax, [ebp+8]
mov esp, ebp # Epilogue
pop ebp
ret
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Constants, Registers, Memory
"12" means decimal 12;
"0xF0" is hex. "some_function" is the address of the first
instruction of a label.
Memory access (use register as pointer): "[esp]". Same as C "*esp".
Memory access with offset (use register + offset as pointer): "[esp+4]". Same as C
"*(esp+4)".
Memory access with scaled index (register + another register * scale): "[eax + 4*ebx]".
Same as C "*(eax+ebx*4)".
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Registersesp is the stack pointer
ebp is the stack frame pointer
Return value in eax
Arguments are on the stack
Free for use (no save needed):
eax, ebx, ecx, edx
Must be saved:
esi, edi, ebp, esp
8 bit: ah (high 8 bits) and al (low 8 bits)
16 bit: ax
32 bit: eax
64 bit: rax
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