In assembly, say you have a hex string of '0x2f'. How would you convert this into a single byte of the value '0x2f'?
Example (Assumes SI equals the location of your 2-character hex string, and conv_hex2byte does the conversion, returning the result in AL):
jmp main
foo db '2f'
bar db 0
main:
mov si, foo
call conv_hex2byte
mov byte [bar], al
; bar should now have a value of '2f'
ret
conv_hex2byte:
; What would go here?
ret
Edit: I've got this attempt at conv_hex2byte...
conv_hex2byte:
push ebx
push ecx
push edx
; Clear general registers
xor eax, eax
mov bx, .tmp0
mov ecx, eax
mov edx, eax
.char:
lodsb
mov dl, al
cmp dl, '0'
jl .is_hex
cmp dl, '9'
jg .is_hex
sub al, '0' ; Make '0' = 0h
.write_val:
mov bx, .tmp0
add bx, cx
mov [bx], dl
inc cl
cmp cl, 2
jl .char
mov ah, byte [.tmp0]
shl ah, 4
mov al, byte [.tmp1]
or al, ah
xor ah, ah
;call os_dump_registers
pop edx
pop ecx
pop ebx
ret
.is_hex:
cmp dl, 'F'
jg .lower
sub dl, 55 ; Make 'A' = 0ah
jmp .write_val
.lower:
sub dl, 87 ; Make 'a' = 0ah
jmp .write_val
.tmp0 db 0
.tmp1 db 0
For some reason, it produces the wrong number... i.e.
push hex
call conv_hex2byte
hex db '20'
results in AX equaling 0x32 rather than 0x20.
