The processor inside the GBA is a custom CPU created by Nintendo. Many people call it an ARM7tdmi. That's mostly correct, but the ARM7 core is just that, a core. There are several other things that share the same chip. Another interesting thing is that ARM (the company) doesn't make the chips, they just liscence out the design. But for all purposes (we don't have any intent ;) ), it's a 32bit ARM7tdmi in there.
The CPU has 2 instruction sets, called ARM and THUMB. (Note: this is just an overview, don't worry if you don't understand all of it.)
The ARM instruction set's instructions are multifunctional, and most can incorporate a shift
instruction within, so that means stuff like r3 = r2 + r1<<4 is possible in one instruction
(but won't look like that). Opcode length is also fixed at 32bits, which means if we want to load a 32bit
number, we have to do a memory access which can be slow, I'll show a way (or two, maybe) that can be faster.
And get this, they can all be executed conditionally! Many instructions let you choose whether or not to update
the condition codes (e.g. so you can subtract without messing your carry flag). Since most instructions can
use the shifter, there are no stand-alone shift instructions.
The THUMB instruction set's instructions are less multifunctional, infact, almost all are one function. THUMB opcode size is fixed at 16bits, which means the same code (most instructions have a similar counterpart in each instruction set) is faster when run from ROM (since ROM only has a 16bit bus). Only branches can be conditional, and you can't turn condition code setting off with these instructions (e.g. ADD always sets carry, overflow, zero, sign).
The ARM7 processor has 16 32bit registers (even though only the first 7 (r0-r6) are easily available while in THUMB mode):
r0-r12: GPRs (General Purpose Registers) r13: Typically used as a stack pointer, if you don't use the stack (small demo or something, then this can be considered a GPR). r14: Link register, can be used as a GPR, but its main purpose is to hold return addresses for the Branch and Link Instruction (a sub-routine call). r15: PC register, holds the address +8 of the current instruction (ARM7 has a 3 stage pipeline, so if you read PC, it will have the address of 2 instructions a head of the one you used to read it).
There are pros and cons to each instruction set, they are (Any that could go either way are just my opinion):
| ARM | THUMB |
|---|---|
| Pros: | Pros: |
| Multifunctional | Way faster than ARM in ROM. |
| All opcodes are conditional | Smaller opcode size |
| Choose to update conditional codes | |
| Cons: | Cons: |
| Slower than THUMB in ROM. | Opcodes that can, always set condition codes |
| Only branches can be conditional | |
| Most instructions can only use r0-r6 |