The point of test %eax %eax
assembly, att, x86
Solution
`CMP` subtracts the operands and sets the flags. Namely, it sets the zero flag if the difference is zero (operands are equal).
`TEST` sets the zero flag, `ZF`, when the result of the AND operation is zero. If two operands are equal, their bitwise AND is zero when both are zero. `TEST` also sets the sign flag, `SF`, when the most significant bit is set in the result, and the parity flag, `PF`, when the number of set bits is even.
`JE` [Jump if Equals] tests the zero flag and jumps if the flag is set. `JE` is an alias of `JZ` [Jump if Zero] so the disassembler cannot select one based on the opcode. `JE` is named such because the zero flag is set if the arguments to `CMP` are equal.
So,
TEST %eax, %eax
JE 400e77 <phase_1+0x23>
jumps if the `%eax` is zero.
Problem
Possible Duplicate: x86 Assembly - ‘testl’ eax against eax? I'm very very new to assembly language programming, and I'm currently trying to read the assembly language generated from a binary. I've run across ``` test %eax,%eax ``` or `test %rdi, %rdi`, etc. etc. I'm very confused as to what this does. Isn't the values in `%eax, %eax` the same? What is it testing? I read somewhere that it is doing the `AND` operation.....but since they are the same value, wouldn't it just return `%eax`? The following is just one instance where I found this usage: ``` 400e6e: 85 c0 test %eax,%eax 400e70: 74 05 je 400e77 <phase_1+0x23> ``` I thought `je` jumps if the two values being compared are equal......well, because `%eax` is well, itself, in what situation would we NOT jump? I'm a beginner to programming in general, so I'd appreciate it very much if someone could explain this to me. Thanks!