Any idea what this assembly code is doing?

[StackMasher]

I'm tired of failing all my programming projects and want to try something new. So I decided to try some reverse engineering. Currently I'm working on the linux version of saints row 4, and I'm stuck on this part (parts cut out, ask if more info needed):

1000a3b0:   e8 0b cf ff ff           call   100072c0 <SteamAPI_RestartAppIfNecessary@plt> #Checks if steam is open, launches it if not and returns 1 indicating that the app should be closed as steam will reopen it. Returns 0 if the app can continue
1000a3b5:   84 c0                   test   %al,%al # ???
1000a3b7:   0f 85 81 01 00 00       jne    1000a53e <__isinff@plt+0x197e>

From looking around, it seems like this code tests if %al is 0, and jumps if not. But why %al? I'm not super familiar with 32bit x86 calling conventions but according to wikipedia, return codes are passed in %eax

 

[tetrabrik]

yes it does seem it's testing the al part of the eax register. this would be the lower 8 bits of the eax. eax is composed of: ax (16 bits) ah (8 bits) and al (8 bits). they can be used as one register of 32 bits (eax) or just parts of it (as in this case, al only 8 bits).
my guess is that you're correct, the value represents a boolean inside a conditional statement.
without more context not sure where that value's coming from.

 

[Mikemk]

Extending what tetrabrik said:

All general registers and several other follow the same pattern.

In the early days of 8-bit computers, you had registers a, b, c, and d. When 16 bit came around, it kept 8 bit access to each half to make porting old code easier.
32-bit and 64-bit CPUs are backwards compatible, so they still have access. r*x (where * is a/b/c/d) is the full 64-bit register, eax is the lower 32 bits, ax the lower 16 bits. al and ah are the lower and higher half of ax.

 

[StackMasher]

I figured this out ages ago but thanks

 

[Mikemk]

Also, you're probably seeing a compiler optimization, it's doubtful that the init code was written in assembly.

--------------------- MERGED ---------------------------

Oh, sorry, didn't notice the date

 

[wolfmankurd]

... tests if %al is 0, and jumps if not. But why %al? ... return codes are passed in %eax

You just answered your own question.