大会 - JG / JNLE / JL / CMP后JNGE [英] Assembly - JG/JNLE/JL/JNGE after CMP
问题描述
我不明白的 JG / JNLE / JL / JNGE 指令，来CMP后。
I don't understand the JG/JNLE/JL/JNGE instructions, which come after CMP.
例如，如果我有：
CMP al,dl
jg label1
在人= 101; DL = 200 。
在我们问 JG ？它是关于人＆GT; DL ？或 AL-DL＆GT; 0
On what we ask the jg? Is it on al>dl? or al-dl>0?
在接下来的code相同prolbem：
Same prolbem on the next code:
test al,dl
jg label1
我不明白我们比较，以及我们问什么是 JG
在换句话说，我不明白，当我们将跳转到LABEL1，当我们不肯。
In other words, I don't understand when we would jump to label1, and when we wouldn't.
感谢。
推荐答案
当你做了 CMP A，B 的标志，如果你算过设置 A - b
When you do a cmp a,b, the flags are set as if you had calculated a - b.
然后 JMP 型指令检查这些标志，看是否跳应。
Then the jmp-type instructions check those flags to see if the jump should be made.
在换句话说，$ C $的第一个C座有（添加了我的意见）：
In other words, the first block of code you have (with my comments added):
cmp al,dl ; set flags based on the comparison
jg label1 ; then jump based on the flags
会跳转到卷标1 当且仅当人比 DL <更大/ code>。
would jump to label1 if and only if al was greater than dl.
您是它可能会更好过思想为人＆GT; DL 但你有两个选择有数学上等价：
You're probably better off thinking of it as al > dl but the two choices you have there are mathematically equivalent:
al > dl
al - dl > dl - dl (subtract dl from both sides)
al - dl > 0 (cancel the terms on the right hand side)
您需要使用 JG 因为它假设你的价值观签署时要小心。这样，如果比较的字节的101（以二的补101）（以二的补-56）200，前者实际上将越大。如果这不是什么需要，你应使用等效的无符号比较。
You need to be careful when using jg inasmuch as it assumes your values were signed. So, if you compare the bytes 101 (101 in two's complement) with 200 (-56 in two's complement), the former will actually be greater. If that's not what was desired, you should use the equivalent unsigned comparison.
请参阅这里上跳选择的更多细节，完整性转载如下。首先，那些在这里签署岬是不恰当的：
See here for more detail on jump selection, reproduced below for completeness. First the ones where signed-ness is not appropriate:
+--------+------------------------------+-------------+--------------------+
|Instr | Description | signed-ness | Flags |
+--------+------------------------------+-------------+--------------------+
| JO | Jump if overflow | | OF = 1 |
+--------+------------------------------+-------------+--------------------+
| JNO | Jump if not overflow | | OF = 0 |
+--------+------------------------------+-------------+--------------------+
| JS | Jump if sign | | SF = 1 |
+--------+------------------------------+-------------+--------------------+
| JNS | Jump if not sign | | SF = 0 |
+--------+------------------------------+-------------+--------------------+
| JE/ | Jump if equal | | ZF = 1 |
| JZ | Jump if zero | | |
+--------+------------------------------+-------------+--------------------+
| JNE/ | Jump if not equal | | ZF = 0 |
| JNZ | Jump if not zero | | |
+--------+------------------------------+-------------+--------------------+
| JP/ | Jump if parity | | PF = 1 |
| JPE | Jump if parity even | | |
+--------+------------------------------+-------------+--------------------+
| JNP/ | Jump if no parity | | PF = 0 |
| JPO | Jump if parity odd | | |
+--------+------------------------------+-------------+--------------------+
| JCXZ/ | Jump if CX is zero | | CX = 0 |
| JECXZ | Jump if ECX is zero | | ECX = 0 |
+--------+------------------------------+-------------+--------------------+
然后无符号的：
+--------+------------------------------+-------------+--------------------+
|Instr | Description | signed-ness | Flags |
+--------+------------------------------+-------------+--------------------+
| JB/ | Jump if below | unsigned | CF = 1 |
| JNAE/ | Jump if not above or equal | | |
| JC | Jump if carry | | |
+--------+------------------------------+-------------+--------------------+
| JNB/ | Jump if not below | unsigned | CF = 0 |
| JAE/ | Jump if above or equal | | |
| JNC | Jump if not carry | | |
+--------+------------------------------+-------------+--------------------+
| JBE/ | Jump if below or equal | unsigned | CF = 1 or ZF = 1 |
| JNA | Jump if not above | | |
+--------+------------------------------+-------------+--------------------+
| JA/ | Jump if above | unsigned | CF = 0 and ZF = 0 |
| JNBE | Jump if not below or equal | | |
+--------+------------------------------+-------------+--------------------+
和，最后，签名的：
+--------+------------------------------+-------------+--------------------+
|Instr | Description | signed-ness | Flags |
+--------+------------------------------+-------------+--------------------+
| JL/ | Jump if less | signed | SF <> OF |
| JNGE | Jump if not greater or equal | | |
+--------+------------------------------+-------------+--------------------+
| JGE/ | Jump if greater or equal | signed | SF = OF |
| JNL | Jump if not less | | |
+--------+------------------------------+-------------+--------------------+
| JLE/ | Jump if less or equal | signed | ZF = 1 or SF <> OF |
| JNG | Jump if not greater | | |
+--------+------------------------------+-------------+--------------------+
| JG/ | Jump if greater | signed | ZF = 0 and SF = OF |
| JNLE | Jump if not less or equal | | |
+--------+------------------------------+-------------+--------------------+
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