该指令在汇编中起什么作用？

Question

So I don't know what this instruction does as I am starting just assembly.
What does this instruction do?

 cmp byte ptr [edi],00 add [eax],al

Answer 1

In fact,Those are two seprate instructions :

cmp byte ptr [edi],00 
add [eax],al

The first one goes to memory address pointed by edi register & fetches the first byte starting from that address , then comparing it to 00=0 .

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The second instruction throws away the result of the first instruction by overwriting all the flags without depending on them.

It moves the content of al (8-bit register) to memory location pointed by EAX , but since al is the lowest byte of eax , its just like we are copying the lowest byte in the address pointed by eax to memory location pointed by that address (which i think is meaningless).

Before execution: Suppose eax=0x00405060

--------------------
Address    |  Content (1byte)
--------------------
0x00405060 | 00

After Execution: eax=0x00405060 (doesn't change)

--------------------
Address    |  Content (1byte)
--------------------
0x00405060 | 60

If you notice after the instruction execution, we have copied the lowest byte of the address (60) to memory location pointed by that address.