LR35902

LR35902 is a custom 8-bit microprocessor designed by Sharp for Nintendo's Game Boy line of handhelds. It serves as the main CPU in the original Game Boy and the Game Boy Color, providing the processing power for games and system software. The design is commonly described as a hybrid that incorporates many features of the Zilog Z80 and the Intel 8080, but it is not fully compatible with either architecture. The instruction set is broadly Z80-like, though some instructions and features are omitted or modified.

The processor uses eight 8-bit registers: A, F, B, C, D, E, H, and L. It also exposes four 16-bit register pairs: AF, BC, DE, and HL, along with a 16-bit stack pointer (SP) and program counter (PC). The F register holds the flag bits for zero, subtraction, half-carry, and carry. A CB-prefixed instruction set provides additional bit manipulation, shifts, and rotates. The LR35902 relies on memory-mapped I/O rather than a separate I/O space and implements a straightforward interrupt system with common Game Boy sources such as V-Blank, LCD STAT, Timer, Serial, and Joypad.

Clock speed and usage: The LR35902 runs at a 4.295 MHz master clock, with most instructions completing in multiple clock cycles and timing varying by operation. Its hardware peripherals communicate with the CPU through memory-mapped registers, coordinating with the Game Boy’s LCD controller and sound hardware. Cartridges typically use a memory bank controller to switch ROM/RAM banks. In the Game Boy Color, the CPU remains compatible, while the system gains access to enhanced color palettes and faster memory access through continued integration with the same core.