Trying to figure out the syntax of MASM
I have done some assembly programming on Linux and now I am trying to do this on Windows using MASM. However, I am facing several problems.
(Here I am trying to implement the strlen () function. I know the functional logic / instructions are not optimal, but I'm just trying to pick up something dirty so I can continue implementing other C library functions.)
.386
.model flat, stdcall
option casemap:none
include \masm32\include\kernel32.inc
includelib \masm32\lib\kernel32.lib
include \masm32\include\masm32.inc
includelib \masm32\lib\masm32.lib
.data
testString db "test string", 0 ; 0 -> terminator
.code
my_strlen proc
mov ebp, esp ; function prologue
push esp
push ebx
push ecx
xor ecx, ecx ; set count to 0
mov bl, byte ptr [ebp + 8] ; set low of b to 1st character
repeat:
cmp bl, 0 ; if null terminator, return
jz done
inc ecx ; increase count
mov bl, byte ptr [ebp + 8 + ecx] ; increase *ebx
jmp repeat ; repeat
done:
mov eax, ecx ; return count
pop ecx ; function epilogue
pop ebx
pop esp
ret
my_strlen endp
main:
push offset testString ; result = my_strlen(testString)
call my_strlen
push eax ; StdOut(result)
call StdOut
push 0 ; ExitProcess(0)
call ExitProcess
end main
When I try to compile it doesn't look like my jmp shortcuts, throwing unmatched macro folding, etc. What is the correct way to do this? postscript I try to avoid using MASM macros where possible, preferring to type instructions myself.
Can someone please compile this program? Once I see it done right, I'll be on my fun-filled path, I can. Yes, I have looked for resources and still since this question came up.
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Can anyone compile this program?
Voilà (explanation in comments):
.386
.model flat, stdcall
option casemap:none
include \masm32\include\kernel32.inc
includelib \masm32\lib\kernel32.lib
include \masm32\include\masm32.inc
includelib \masm32\lib\masm32.lib
.data
testString db "test string", 0
.code
my_strlen proc
; mov ebp, esp ; function prologue
; push esp
push ebp ; this is a correct prologue
mov ebp, esp
push ebx
push ecx
push esi
mov esi, [ebp+8] ; in [EBP+8] is a pointer
xor ecx, ecx
mov bl, byte ptr [esi]
repea: ; "repeat" is a keyword
cmp bl, 0
jz done
inc ecx
mov bl, byte ptr [esi + ecx] ; increase *esi
jmp repea ; repeat
done:
mov eax, ecx
pop esi
pop ecx
pop ebx
leave ; this is a correct epilogue
ret
my_strlen endp
main proc ; this is better
push offset testString
call my_strlen
; push eax ; StdOut(result)
push offset testString ; The MASM32-function StdOut needs only an ASCIZ-string
call StdOut
push 0
call ExitProcess
main endp
end main
I suspect you want to output the result of my_strlen. This is not possible with StdOut
, because StdOut
a pointer to a string is required... You have to create a function to convert EAX
to string.
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This is a macro solution. The length of the string is finally stored in EAX.
The first macro checks an integer DWORD from memory and is faster than regular read and check. Good for big strings.
;//-----------------------------------------------------------------------
strlenFast MACRO stringPtr:REQ
;//-----------------------------------------------------------------------
local STRLEN_LOOP
mov eax, stringPtr
mov esi, 0
STRLEN_LOOP:
mov ebx, [eax+esi]
mov ecx, ebx
inc esi
and ebx, 0FFh
jz EOS
mov ebx, ecx
inc esi
and ebx, 0FF00h
jz EOS
mov ebx, ecx
inc esi
and ebx, 0FF0000h
jz EOS
mov ebx, ecx
inc esi
and ebx, 0FF000000h
jnz short STRLEN_LOOP
EOS:
dec esi
mov eax, esi
ENDM
Short version with check:
;//-----------------------------------------------------------------------
strlenShort MACRO stringPtr:REQ
;//-----------------------------------------------------------------------
local STRLEN_LOOP
mov eax, stringPtr
mov esi, -1
STRLEN_LOOP:
inc esi
mov ebx, [eax+esi]
and ebx, 0FFh
jnz STRLEN_LOOP
mov eax, esi
ENDM
Basically proc:
.data
anyString byte "Hello World", 0
.code
main PROC
strlenShort OFFSET anyString ;// => result in EAX
strlenFast OFFSET anyString ;// => result in EAX
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