c - GDB / GNU assembly: test %esi,%esi returns not equal? -


i'm working on homework assignment. we're given pre-compiled binary , have use gdb assembly dumps, traverse data structures, view values stored in memory, etc. in order puzzle out binary does. here few lines of disassembler dump function call:

0x08048e14 <+21>:   test   %esi,%esi 0x08048e16 <+23>:   jne    0x8048e4b <fun6+76> 0x08048e18 <+25>:   jmp    0x8048e5d <fun6+94>

i assumed test %esi,%esi return result of "equals" (or, rather, equivalent statement expressed using register flags, believe zf set?), , jne instruction never execute, , instead program execute instruction @ line <+25>. however, after stepping through these instructions, program jumps line <+76>! why happen? baffled.

in case helps explain answer, here register flags after test instruction @ line <+21> (zf isn't set?)(i still don't know how interpret flags):

eflags         0x202    [ if ]

the test instruction performs bitwise and, not store result; sets flags.

and jne "jump if zf not equal 0", here it's testing if esi zero.

also see how `test` instruction work? , what `test` instruction do?


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