.model small
.stack 100h
.data
    x dd 0ABCDEFh
    y dd 0111111h
    z dd ?
    s db 0
.code
START:
    mov ax,@Data
    mov ds,ax
    xor ax,ax
    xor dx,dx

    mov ax, word ptr x
    cmp ax, word ptr y
    jge all_ok

    mov s, 1
    mov ax, word ptr y
    sub ax, word ptr x
    mov word ptr z, ax
    jmp c
all_ok:
    sub ax, word ptr y
    mov word ptr z, ax

c:
    mov ax, word ptr x + 2
    sub ax, word ptr y + 2

    cmp ax,0
    jge positive

;negative
    neg ax
    mov word ptr z + 2, ax

    neg z ; <--- ошибка

    mov ah,4Ch
    int 21h
positive:
    xor cx,cx
    mov cl,s
    cmp s,0
    je was_pos
    dec ax
    mov dx,0FFFFh
    sub dx,word ptr y
    add dx,word ptr x

was_pos:
    mov word ptr z, dx
    mov word ptr z + 2, ax

    mov ah,4Ch
    int 21h
end START