x86 汇编中凯撒密码的问题
我有以下说明作为凯撒密码程序的一部分。
overFlow:
sub bl, 1Ah
ret
underFlow:
add bl, 1Ah
ret
correctFlow:
cmp bl, 7Ah
jg overFlow
cmp bl, 61h
jl underFlow
ret
enc_byte:
add bl, encOffset
call correctFlow
ret
将 ASCII 小写字母放入 BL 中，并在调用 enc_byte 后，将字母移动 encOffset 个字母并纠正溢出。
但由于某种原因，正确流程中的比较无法正常工作。当 CorrectFlow中BL=8Dh时，jg overFlow指令不跳转，而是jl underFlow指令在第二个cmp。为什么会发生这种情况？ 8Dh 明显大于 7Ah，那为什么它没有按预期跳跃呢？
我知道回报很奇怪。 overFlow 和 underFlow 标签是返回对 CorrectFlow 的调用的标签。这是故意的，据我所知，与该问题无关。
I have the following instructions as part of a Caesar cipher program.
overFlow:
sub bl, 1Ah
ret
underFlow:
add bl, 1Ah
ret
correctFlow:
cmp bl, 7Ah
jg overFlow
cmp bl, 61h
jl underFlow
ret
enc_byte:
add bl, encOffset
call correctFlow
ret
An ASCII lowercase letter is put into BL and after enc_byte is called, it shifts the letter by encOffset letters and corrects for an overflow.
But for some reason the compare in correctFlow doesn't work correctly. When BL=8Dh in correctFlow, the jg overFlow instruction does not jump, and instead jl underFlow jumps after the second cmp. Why is this happening? 8Dh is clearly greater than 7Ah, so why doesn't it jump as expected?
I know the returns are weird. The overFlow and underFlow labels are the ones that return the call to correctFlow. This is intentional and as far as I know, doesn't have anything to do with the issue.
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发生这种情况是因为
jg和jl将cmp的结果视为两个操作数是有符号数字。7Ah和8Dh分别表示有符号数 +122 和 -115。显然，后者是最小的。您需要的是无符号比较。请改用指令
ja和jb。This happens because
jgandjltreat the outcome ofcmpas if the two operands were signed numbers.7Ahand8Dhrepresent signed numbers +122 and -115, respectively. Obviously, the latter is the smallest.What you need is unsigned comparison. Use instructions
jaandjbinstead.