I came through the following statement while going through a binary search program code
l1: mov si,low_ cmp si,high_ Why do we need to store low_ in si and then compare with high_ ? Can't we directly write cmp low_,high_
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- 5Only up to one memory operand is supported by most x86 instructions.fuz– fuz2021-08-04 16:48:26 +00:00Commented Aug 4, 2021 at 16:48
- Did you try it? Your assembler would have told you "too many memory operands".Peter Cordes– Peter Cordes2021-08-05 00:14:10 +00:00Commented Aug 5, 2021 at 0:14
- Try it first, then ask here..Erik Eidt– Erik Eidt2021-08-27 22:20:23 +00:00Commented Aug 27, 2021 at 22:20
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1 Answer
You cannot write cmp low_, high_.
For such questions, always refer to the official instruction description, e.g. at https://www.felixcloutier.com/x86/cmp. Note that cmp has forms cmp r/m16, r16, for which the first operand can be either register or memory, and the second must be a register; as well as cmp r16, r/m16 which is the reverse. But there is nothing like cmp r/m16, r/m16 or cmp m16, m16. So cmp low_, high_ would be attempting to assemble an instruction which does not exist, and your assembler will reject it.
This is the case for most x86 arithmetic instructions: one of the operands may be a memory reference, but not both. To operate on two values from memory, you must load one of them into a register first, as this code does.
