string_reversal.asm

; Reverse a null-terminated string in-place
; using the stack.

.text 0
    mov ax, 48
    mov si, ax      ; Move our si to start of string
    push 0          ; Push 0 as a marker for stack rewind
read:
    mov ax, [si]    ; Get a letter into ax
    shl ax, 8
    shr ax, 8
    jnz collect     ; Check if we hit '\0'
    jmp rewind
collect:
    push ax         ; Push current char onto the stack
    mov ax, si
    add ax, 1
    mov si, ax      ; Advance our si pointer
    jmp read

rewind:
    mov ax, 48
    mov si, ax      ; Reload si to start of string
write:
    pop ax          ; Pop off last letter
    shr ax, 8
    mov [si], ax    ; Write it into the string
    push ax         ; Store it so we can jnz
    mov ax, si
    add ax, 1
    mov si, ax      ; Advance si
    pop ax
    jnz write       ; Repeat if ax isn't zero
    jmp exit        ;
exit:
                    ; We're done

.data 48
"Hello world."