Question

I’m just beginning to toy with masm. I don’t understand why this code isn’t working.

.data 
MsgBoxCaption  db "Iczelion Tutorial No.2",0 
MsgBoxText       db "Win32 Assembly is Great!",0
savedAddr DWORD ?

.code 
start: 
mov eax, 10
mov savedAddr, OFFSET MsgBoxText

lab:
inc MsgBoxText
MOV MsgBoxText, 'm'
cmp eax, 0
dec eax
jnz lab

invoke MessageBox, NULL, savedAddr, addr MsgBoxCaption, MB_OK  
invoke ExitProcess, NULL
end start

Edit: I expect to see the first 10 characters in MsgBoxText be ‘m’s. Instead, only the first letter is an ‘m’. I assume that inc MsgBoxText increments a pointer.

Answer 1

At first glance it seems like this code should do nothing except change the ‘W’ of Win32 to an ‘a’ and then to a single ‘m’.

You are incrementing the word in memory at MsgBoxText, the same word, in each loop iteration.

To clobber the string using the ‘m’ characters, a better strategy would be to load the address of the string into a register, start storing ‘m’ bytes, and then increment the value in the register, as well as decrementing the counter.

Update: Ok, to answer the question in the comment, change the loop to:

  lea esi, MsgBoxText
  mov bl, 'm'
lab:
  mov [esi], bl
  inc esi
  cmp eax, 0
  dec eax
  jnz lab