Game Boy emulation in C#, part 1: CPU, memory and registers

A new dotnetpro series walks through building a Game Boy emulator in C#. Part one covers the 16-bit address bus, the 64 KB memory map, CPU registers modeled as C# fields and properties, and the flag register. It also shows how opcodes are dispatched in a switch loop and how cycle counting keeps components in sync.

Key points

• The Game Boy has a 16-bit address bus, so its memory map spans exactly 64 KB (0 to 65535).
• CPU registers A, B, C, D, E, H and L are 8-bit byte fields; AF, BC, DE and HL are ushort properties built from them.
• The F register holds Z (zero), N (subtract), H (half-carry) and C (carry) in bits 7 to 4; bits 0 to 3 are always 0.
• An emulator loop reads the opcode at PC, dispatches it with a switch and adds each instruction's cycles to a counter.
• One Game Boy cycle equals four clock ticks; the CPU runs at 4 MHz and instructions take one to six cycles.
• DEC must update the Z, N and H flags, not just decrement the value; a plain A-- is not enough.

The Game Boy makes a good teaching target for emulation: a 16-bit address bus, a 4 MHz CPU and a compact set of 256 opcode variants. This first part lays the groundwork. It treats the 64 KB address space as one byte array and maps the 8-bit CPU registers, and their 16-bit pairs such as AF and HL, to C# fields and properties. It explains the flag bits Z, N, H and C that conditional jumps rely on.

The emulator core is a fetch-and-dispatch loop: read the opcode at PC, switch on it, consume operands and add the instruction's cycles to a counter. Cycle counting later keeps the CPU and graphics unit in sync. A small test program that decrements a register with DEC and loops with JP NZ shows why flags must be updated exactly like the hardware does. The PPU, which draws the screen, is deferred to part two.

Real-world use

• Writing a cycle-accurate emulator for a retro console to learn how the CPU, memory map and timers interact.
• Building a small virtual machine or bytecode interpreter in C# that needs a fetch-decode-execute loop.
• Studying how flags and conditional branches behave before writing a low-level parser or runtime component.

Why it matters

The series shows how hardware concepts map onto C# types such as byte and ushort, which helps when writing performance-sensitive or low-level code. Understanding flags and cycles also explains why emulators depend on precise timing.

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