The Central Processor | goiabada/docs

The Central Processor

The GBA processor is an ARM7TDMI chip. A 32-bit RISC processor with a three-stage pipeline, a hardware multiplier and quite a few registers.

Two Instruction Sets

The ARM is a RISC processor design. It’s instrution set is very regular, so it’s possible to encode an instrucion in quite a few ways.
A second and denser instruction set was created by the ARM designers, the so-called Thumb instruction set
- It allows two instruction sets to work together
- The CPU converts Thumb instructions into equivalent ARM instructions on the fly
- Each Thumb instruction is 16-bits, whereas the ARM ones are 32-bits

CPU Registers

The ARM has 16 registers accessible at any time, all of them can hold up to 32-bits

Registers	Usage
R0-R3	Used for passing parameters to functions. Any parameter that don’t fit here gets passed on the stack
R4-R10	Used for register variables. Registers 9 and 10 are also used for stack manipulation
R11	Called the frame pointer. It’s tipically set and restored during the prologue or epilogue code, since it’s the pointer through all local variables are accessed
R12	Called the interlink pointer. A scratch register for most GBA code.
R13	The Stack pointer, points to the last item pushed on the stack.
R14	The link register. It holds the return address for the subroutine. Normally it’s pushed to the stack and then popped directly into the program counter for the return.
R15	The program counter. We can directly change it to execute a jump