Indirect indexing using variable contents instead of using a register ?

[dsouza123]

Is there a way to use the contents of a variable, (containing an address),
to access the contents pointed to by the address,
without transferring the address to a register ?

mov lparr, offset arr
add lparr, 7
...
mov byte ptr [lparr], 0 ;

versus

mov esi, lparr
mov byte ptr [esi], 0    ; this works


Also what other syntax/addressing modes can be used to change the string length.

Code Select Expand


.586
.model flat,stdcall
option casemap:none
include \masm32\include\windows.inc
include \masm32\include\kernel32.inc
include \masm32\include\user32.inc
includelib \masm32\lib\kernel32.lib
includelib \masm32\lib\user32.lib

.data
  arr   db "123456789ABCDEF0"  ; string of 16 chars
        db 48 dup 0
  lparr dd 0

.code
start:

  lea eax, arr
  mov lparr, eax

  mov lparr, offset arr  ; equivalent to preceding instructions

  mov esi, 2
  mov byte ptr [arr+esi*4+2], 0   ; make string 10 chars long

  mov esi, 9
  mov byte ptr [arr+esi], 0       ; make string 9 chars long

  mov esi, 8
  mov byte ptr arr[esi], 0        ; make string 8 spaces long
 
  add lparr, 7
  mov esi, lparr
  mov byte ptr [esi], 0           ; make string 7 spaces long

; Following doesn't work as commented.
  dec lparr
  mov byte ptr [lparr], 0                  ; make string 6 spaces long

  invoke ExitProcess, NULL
end start


[hutch--]

I think this does what you want. You can add to the array offset with either an immediate or a register.

Code Select Expand


.586
.model flat,stdcall
option casemap:none
include \masm32\include\windows.inc
include \masm32\include\masm32.inc
include \masm32\include\kernel32.inc
include \masm32\include\user32.inc
include \masm32\include\msvcrt.inc


includelib \masm32\lib\masm32.lib
includelib \masm32\lib\kernel32.lib
includelib \masm32\lib\user32.lib
includelib \masm32\lib\msvcrt.lib

include \masm32\macros\macros.asm

.data
  arr   db "123456789ABCDEF0"  ; string of 16 chars
        db 48 dup (0)
  ;; lparr dd arr

.code
start:

    mov BYTE PTR [arr+14], 0
    print str$(len(OFFSET arr)),13,10

    mov BYTE PTR [arr+10], 0
    print str$(len(OFFSET arr)),13,10

    mov eax, 6

    mov BYTE PTR [arr+eax], 0
    print str$(len(OFFSET arr)),13,10

    inkey

  invoke ExitProcess, NULL

end start