跳回1000线 [英] Jumping back 1000 lines

问题描述

我试图让一个code，当你在最后，它会问你，如果你想再试一次。如果preSS'Y'，那么它会跳回到1000行，就在节目的开头。

I was trying to make a code, that when you're at the very end, it will ask you if you want to try again. If you press 'y', then it will jump back a 1000 lines, right at the beginning of the program.

很明显嘛，就没有工作了，因为我的错误跳相对越界了。所以我做了跳跃每50行，总共具有20跳，像

Well obviously, it didn't work out, as I got the error "jump relative out of range". So I made jumps every 50 lines, having a total of 20 jumps, like

start:
.
s20: jmp start
.
.
.
s2: jmp s3
.
s1: jmp s2
.
jmp s1

现在这样做之后，我跑的程序，当我pressed'Y'，TASM样冻结了。这只是显示最后一个屏幕，与Y的输入，和闪烁_。我不能preSS字符了。

Now after doing that, I ran the program, and when I pressed 'y', TASM kind of froze. It was just displaying the last screen, with the 'y' input, and a blinking _. I couldn't press a character anymore.

感谢。

推荐答案

在x86的你并不需要跳跃的层叠顺序，因为 JMP 可以跳较全分割。就像 JNE 条件跳转有一个有限的范围内。所以，你可以改变一个errorneous条件跳转到一个无条件的近跳转和有条件短跳转的组合：

In x86 you don't need a cascading sequence of jumps, since jmp can jump over the whole segment. Just a conditional jump like jne has a limited range. So you can change an errorneous conditional jump to a combination of an unconditional near jump and a conditional short jump:

作为一个例子，改变

.MODEL small
.STACK 1000h

.CODE
main:

top:
    mov ax, 1
    jmp bottom


ORG 1000h               ; A big block between top and bottom

bottom:
    cmp ax, 0

    je top              ; **Error** Relative jump out of range by 0F85h bytes

    mov ax, 4C00h       ; Return 0
    int 21h

END main

到

.MODEL small
.STACK 1000h

.CODE
main:

top:
    mov ax, 1
    jmp bottom


ORG 1000h               ; A big block between top and bottom

bottom:
    cmp ax, 0

    jne skip            ; Short conditional jump
    jmp top             ; Near unconditional jump
    skip:

    mov ax, 4C00h       ; Return 0
    int 21h

END main

TASM能为你做自动的。放置一个跳开头（或者在你需要它）的文件：

TASM can do that automagically for you. Place a "JUMPS" at the beginning (or where you need it) of the file:

JUMPS

.MODEL small
.STACK 1000h

.CODE
main:

top:
    mov ax, 1
    jmp bottom


ORG 1000h               ; A big block between top and bottom

bottom:
    cmp ax, 0

    je top              ; TASM will change this line to a JNE-JMP combination

    mov ax, 4C00h       ; Return 0
    int 21h

END main

80386指令集（ISA）有近条件跳转指令。如果你的模拟器支持80386 ISA（DOSBox中一样），你可以告诉TASM使用它。插入 .386 指令：

.MODEL small
.386                    ; Use 80386 instruction set
.STACK 1000h

.CODE
main:

top:
    mov ax, 1
    jmp bottom


ORG 1000h               ; A huge block between top and bottom

bottom:
    cmp ax, 0

    je top              ; Correct jump because of '.386'

    mov ax, 4C00h       ; Return 0
    int 21h

END main

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