Assembly Program Strange Behavior

Question

I am attempting to write a small program in MASM that will take a string of user input, subtract four from the ASCII value of each character, then output the new character.

This is mostly successful, except when StdOut is called, it prints not only the current modified character, but also the next character.

I've been trying to figure out what is happening for several hours and still don't have a clue. Here's my code:

.486
.model flat, stdcall
option casemap :none

include \masm32\include\windows.inc
include \masm32\macros\macros.asm

include \masm32\include\masm32.inc
include \masm32\include\gdi32.inc
include \masm32\include\user32.inc
include \masm32\include\kernel32.inc

includelib \masm32\lib\masm32.lib
includelib \masm32\lib\gdi32.lib
includelib \masm32\lib\user32.lib
includelib \masm32\lib\kernel32.lib

.data?
    inputtxt dw 10000 dup(?)
    current  dd           ?

.code
start:
    call main
    exit

main proc
    Invoke StdIn, addr inputtxt, 10000
    xor esi, esi

    processLoop:
        movzx eax, inputtxt[esi]        ; Get the character at index ESI
        sub eax, 4                      ; Subtract 4 from the character's ASCII code
        mov current, eax                ; StdOut can't print a register
        Invoke StdOut, addr current     ; Print the character: the problem lies here.
        inc esi                         ; Increment the loop counter
        cmp byte ptr[inputtxt[esi]], 0  ; If the next character is NUL, we're done
        jne processLoop                 ; If it's not, loop again
        ret

main endp

end start

Here's a sample input and output:

Input: HE

Output: DEA

D and A are correct, but E is incorrect and printed in the same pass as D.

Will someone who isn't currently at wit's end please try to figure out what is going on here?

Answer 1

The assembler assumes that your movzx is supposed to convert 16-bit data to 32-bit since you haven't specified a size for the source data in the instruction.

Add a byte ptr specifier:

; Move a byte from [inputtxt+esi] to eax, zero-extended
movzx eax, byte ptr inputtxt[esi]