Copetti.org has an excellent page on the hardware Architecture of the Game Boy Advance, it is also a great introduction to the system:
Game Boy Advance Architecture - A Practical Analysis
Check out this excellent introduction to the GBA hardware
The GameBoy Advance uses a 32-bit RISC CPU (ARM7TDMI) running at 16.78 MHz known as the AGS-CPU-11, for really low level detail for how this CPU works at a hardware level check out the Schematics:
gb-schematics/AGS-CPU-11.pdf at master · Gekkio/gb-schematics · GitHub
Game Boy Advance (AGB) Development Kit Hardware
For more information on the GBA Development Kit hardware check out this post.
M4A Music Library for Game Boy Advance (GBA)
For more information on the GBA Music SDK check out this post.
Game Boy Advance SDK Demos
For more information on the GBA SDK Demo games check out this post.
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