Assembly

Flags used to test conditional jumps...

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runtogetdone runtogetdone is offline

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Flags used to test conditional jumps...

Oct 28th, 2009

Hi, I do not understand how the x86 processor chooses whether to not to jump for a conditional jump. My professor told me that conditional jumps are based on the OF, ZF, SF, and CF flags, but I do not understand which flags are checked under a given jump instruction...

For example, what flags are checked for a unsigned greater-than jump?

Thanks,
Tyler

(In other words, how do I determine the flags in the right column of this website?)
http://siyobik.info/index.php?module=x86&id=146

Last edited by runtogetdone; Oct 28th, 2009 at 12:29 am.

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wildgoose

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Oct 28th, 2009

You can download processor books but my favorite reference is the 32/64-bit 80x86 Assembly Language Architecture Book.

Your mathematical operation and/or compares will set conditional flags.
You then branch based upon those flags.

For math there is a signed result and an unsigned result.
That book pg. 292

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
              Signed               Unsigned            Signed         Unsigned
op1 > op2    OF=SF, ZF=0      ZF=0,CF=0          Greater            Above
OP1>=op2    OF=SF               CF=0            GreaterEqual   AboveEqual
op1=op2      ZF=1                  ZF=1             Equal               Equal
op1<=op2    OF<>SF, ZF=1     ZF=1,CF=1               LessEqual        BelowEqual
op1<op2       OF<>SF            CF=1              Less                 Below
              Signed               Unsigned            Signed         Unsigned
op1 > op2    OF=SF, ZF=0      ZF=0,CF=0          Greater            Above
OP1>=op2    OF=SF               CF=0            GreaterEqual   AboveEqual
op1=op2      ZF=1                  ZF=1             Equal               Equal
op1<=op2    OF<>SF, ZF=1     ZF=1,CF=1               LessEqual        BelowEqual
op1<op2       OF<>SF            CF=1              Less                 Below

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
Unsigned
JA,JNBE >            <= JBE,JNA
JAE,JNB >=          <  JB,JNAE
 
Signed
JG,JNLE  >             <= JLE,JNG
JGE,JNL  >=            < JL,JNGE
 
the others are specific
JO   Jump if Overflow                 JNO
JP,JPE   Jump if Parity                      JNP,JPO
JS         Jump if sign                   JNS
JC        Jump if carry                  JNC
JZ,JE    Jump if zero                  JNZ,JNE
Unsigned
JA,JNBE >            <= JBE,JNA
JAE,JNB >=          <  JB,JNAE

Signed
JG,JNLE  >             <= JLE,JNG
JGE,JNL  >=            < JL,JNGE

the others are specific
JO   Jump if Overflow                 JNO
JP,JPE   Jump if Parity                      JNP,JPO
JS         Jump if sign                   JNS
JC        Jump if carry                  JNC
JZ,JE    Jump if zero                  JNZ,JNE

Last edited by wildgoose; Oct 28th, 2009 at 10:20 pm.

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cyb3rl0rd1867 cyb3rl0rd1867 is offline

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Oct 28th, 2009

This also confused me at first but it's really pretty simple. In this example I will use the CMP (compare) instruction. This is the equivalent of subtracting two numbers without storing the result. This is very helpful even without storing the answer because it sets certain flags which can help you compare two numbers. For example: mov al,1
cmp al,1
In this example the result is obvious that they are the same. But remember that it's subtracting them (even though it doesn't store the result) so the answer is 0. The ZF (Zero Flag) is therefore set to 1.
So the instruction "JE wherever" will jump since the ZF is set to 1.
Similarly the OF flag is set to 1 when the MSB, Most significant digit, or leftmost bit is set or cleared. Otherwise it is set to 0. So if the OF is not set (meaning it's 0) then JO (jump if overflow=1) will not jump.
The NF is the same thing. If the first bit (MSB) was set to one in the an operation preceding it the NF is set to 1. Otherwise it is set to 0. To find out the negative of a number in "bit language" you must NOT it.

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runtogetdone runtogetdone is offline

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Oct 29th, 2009

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Originally Posted by wildgoose

You can download processor books but my favorite reference is the 32/64-bit 80x86 Assembly Language Architecture Book.

Your mathematical operation and/or compares will set conditional flags.
You then branch based upon those flags.

For math there is a signed result and an unsigned result.
That book pg. 292
Help with Code Tags

Assembly Syntax (Toggle Plain Text)
              Signed               Unsigned            Signed         Unsigned
op1 > op2    OF=SF, ZF=0      ZF=0,CF=0          Greater            Above
OP1>=op2    OF=SF               CF=0            GreaterEqual   AboveEqual
op1=op2      ZF=1                  ZF=1             Equal               Equal
op1<=op2    OF<>SF, ZF=1     ZF=1,CF=1               LessEqual        BelowEqual
op1<op2       OF<>SF            CF=1              Less                 Below
Signed Unsigned Signed Unsigned op1 > op2 OF=SF, ZF=0 ZF=0,CF=0 Greater Above OP1>=op2 OF=SF CF=0 GreaterEqual AboveEqual op1=op2 ZF=1 ZF=1 Equal Equal op1<=op2 OF<>SF, ZF=1 ZF=1,CF=1 LessEqual BelowEqual op1<op2 OF<>SF CF=1 Less Below
Help with Code Tags

Assembly Syntax (Toggle Plain Text)
Unsigned
JA,JNBE >            <= JBE,JNA
JAE,JNB >=          <  JB,JNAE
 
Signed
JG,JNLE  >             <= JLE,JNG
JGE,JNL  >=            < JL,JNGE
 
the others are specific
JO   Jump if Overflow                 JNO
JP,JPE   Jump if Parity                      JNP,JPO
JS         Jump if sign                   JNS
JC        Jump if carry                  JNC
JZ,JE    Jump if zero                  JNZ,JNE
Unsigned JA,JNBE > <= JBE,JNA JAE,JNB >= < JB,JNAE Signed JG,JNLE > <= JLE,JNG JGE,JNL >= < JL,JNGE the others are specific JO Jump if Overflow JNO JP,JPE Jump if Parity JNP,JPO JS Jump if sign JNS JC Jump if carry JNC JZ,JE Jump if zero JNZ,JNE

Thanks wildgoose, this is the most concise thing I've seen...

cyb3rl0rd1867, thanks, that too makes sense. I know that the cmp instruction does a subtraction and doesn't store a result or anything, I was just wondering as to how people remember that for op1<op2, both op's signed, how you would know that the processor checks OF<>SF. After looking it kind of makes sense, but it looks like it's just easier to memory the values.