What does 'test al, al' mean?

Question

I'm trying to reverse engineer a program and i found the following instruction:

test al, al
je label

what does these two instruction do ? Can someone explain please ?

test equals and but does not place the result in the target operand. However, like and it manipulates the zero flag (ZF). — 0xC0000022L, Commented Jun 1, 2020 at 19:36
This is a standard idiom to test if the 8-bit value stored in AL (the register that is the low 8 bits of EAX) is zero. In other words, it's checking whether a Boolean is true or false, and branching accordingly. — Cody Gray, Commented Jun 2, 2020 at 3:17

Yotamz · Accepted Answer · 2020-06-01 18:22:06Z

13

In x86 assembly, al is the least significant byte of eax register, which is typically used to return values from function calls.

The test al,al is a bitwise AND operation between al and itself.

If al & al == 0, the zero flag will be set to 1.

je (or jz) instruction will jump to the address of label, if the zero flag is 1. Otherwise, the je will do nothing.

In short: If eax == ######00 jump to label.

answered Jun 1, 2020 at 18:22

Yotamz

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