Day 18 - About The BIOS
Yes, BIOS. BIOS stands for Basic Input Output System, I don't know why they
called what the GBA has a BIOS, it doesn't do any real screen I/O. The GBA BIOS is more
like a small utility library with several useful functions that can be called.
Patater GBAGuy Mirror
What the BIOS Has.
The BIOS has several functions that we are interested in, they are:
Sqrt - Square Root Function, puts Square Root of r0 in r0.
Div - Divide Function, divides r0 by r1 and puts the answer in r0.
Also returns other things in r1 and r2, we'll get to those in Day 20.
SoftReset - I suppose we could try doing this one tomorrow, maybe.
To call a BIOS function, you just fill certain CPU registers and then call the
function with an instruction that we'll discuss tomorrow.
Where is the BIOS?
The BIOS starts at 0x0000000 in memory and goes until 0x1000000 (I think).
The BIOS also contains several decompression routines and some kind of routines
to set rotation/scaling properties for both sprites and BGs.
Day In Review
That's it for today, tomorrow we'll actually call a function.
-Mike H a.k.a GbaGuy
Intro - Day 19