Loop????

[wossname]

In my book it says that LOOPZ works like this...

ECX = ECX - 1
if ECX > 0 and ZF = 1 then jump <label>

Eh? How is it possible for that condition to be true?? if ECX is greater than 0 then surely its not possible for ZF to be 1, thats like saying...

if ECX > 0 and ECX = 0 then jump <label>



What am I missing here?

[Andy_P]

Do these links help any?

http://enrico.phys.cmu.edu/QCDcluste...ence/vc162.htm

http://faydoc.tripod.com/cpu/loopz.htm

http://docs.sun.com/app/docs/doc/802...oo?l=fr&a=view


Looks like th ZF flag is not affected by the change in ECX, but from other things within the loop.

Hope that helps.

[Harsh Gupta]

In my book it says that LOOPZ works like this...

ECX = ECX - 1
if ECX > 0 and ZF = 1 then jump <label>

Eh? How is it possible for that condition to be true?? if ECX is greater than 0 then surely its not possible for ZF to be 1, thats like saying...

if ECX > 0 and ECX = 0 then jump <label>



What am I missing here?

AFAIK, it means that while ECX > 0 and ZF = 1 then execute the THEN statement, if either one of them fails, i.e. IF ECX > 0 but ZF = 0 (or not equals 1) then execute the ELSE statement or come out of the IF statement.

but i may be wrong, it's been a long time since i touched Assembly language.

[chemicalNova]

I think Andy is closer. If I can remember that entire book on x86 assembly, it said that the CX register didn't directly change the ZF flag value.

chem

[Harsh Gupta]

read this, else look at the links provided by Andy for details:

Some forms of the loop instruction (LOOPcc) also accept the ZF flag as a condition for terminating the loop before the count reaches zero. With these forms of the instruction, a condition code (cc) is associated with each instruction to indicate the condition being tested for. Here, the LOOPcc instruction itself does not affect the state of the ZF flag; the ZF flag is changed by other instructions in the loop.

[wossname]

Wow this is an old thread. I worked this out days ago

It occured to me that the loop instructions don't affect ZF.

[Raedwulf]

BTW...since you are interested in performance...avoid LOOPcc ...it is slower on newer PCs (newer than 486 )