Z80 Assembly for GameBoy Development

Z80 Assembly for Game Boy Development: A Deep Dive

The Game Boy, a handheld console that defined a generation, runs on a custom Sharp LR35902 processor, often simplified as a modified Z80. Understanding Z80 assembly language is essential for pushing the Game Boy to its limits and creating truly unique gaming experiences. This article provides an in-depth exploration of Z80 assembly programming specifically for Game Boy development, covering everything from fundamental concepts to advanced techniques.

I. Introduction to Z80 and the Game Boy

The LR35902 inherits much of its architecture from the Z80 but lacks some instructions and adds a few Game Boy specific features. It’s an 8-bit processor with a 16-bit address space, meaning it can access 64KB of memory. This memory is divided into different regions, each serving specific purposes: ROM (where the game code resides), RAM (for variables and temporary data), and specialized hardware registers controlling the screen, sound, and input.

II. Core Concepts of Z80 Assembly

• Registers: The Z80 has a set of registers, small storage locations within the CPU, used for performing calculations and manipulating data. Key registers include:

  • A (Accumulator): The primary register for arithmetic and logic operations.
  • B, C, D, E, H, L: General-purpose registers, often used in pairs (BC, DE, HL) for 16-bit operations and as memory pointers.
  • SP (Stack Pointer): Points to the top of the stack, a LIFO (Last-In, First-Out) data structure used for subroutine calls and temporary data storage.
  • PC (Program Counter): Holds the address of the next instruction to be executed.
  • Flag Registers (F): Contains flags that indicate the results of operations (Zero, Carry, Half-Carry, Subtract, etc.).

• Instructions: Z80 instructions are mnemonics that represent specific operations. They operate on registers, memory locations, or immediate values. Common instructions include:

  • LD (Load): Copies data between registers or between a register and a memory location (e.g., LD A, B, LD (HL), A).
  • ADD, SUB, INC, DEC: Arithmetic operations (addition, subtraction, increment, decrement).
  • AND, OR, XOR, CPL: Logical operations (bitwise AND, OR, XOR, complement).
  • JP (Jump): Changes the program counter to a specific address, effectively branching to a different part of the code.
  • CALL, RET: Subroutine calls and returns. CALL pushes the current PC onto the stack and jumps to the subroutine address. RET pops the return address from the stack and resumes execution from there.
  • PUSH, POP: Push data onto the stack and pop data from the stack.

• Addressing Modes: Z80 supports several addressing modes, specifying how operands are accessed:

  • Register: Operate directly on registers (e.g., ADD A, B).
  • Immediate: Use a constant value embedded in the instruction (e.g., LD A, #0x10).
  • Direct: Access memory at a specific address (e.g., LD A, (0xC000)).
  • Indirect: Access memory using a register pair as a pointer (e.g., LD A, (HL)).
  • Indexed: Access memory using a register pair plus a displacement (e.g., LD A, (IX+5)).

III. Game Boy Specifics

• Memory Map: Understanding the Game Boy’s memory map is crucial. Key areas include ROM banks, RAM banks, video RAM (VRAM), and hardware registers.
• Interrupts: The Game Boy uses interrupts for handling hardware events like the vertical blank interrupt (VBlank), timer overflow, and input changes.
• Hardware Registers: These control various aspects of the Game Boy hardware, including the LCD controller, sound chip, and input buttons. Learning how to manipulate these registers is essential for creating graphics, sound, and handling user input.

IV. Developing a Simple Game Boy ROM

Let’s illustrate these concepts with a simple example: displaying a single tile on the screen.

“`assembly
; Include necessary header files
INCLUDE “gbhw.inc”

SECTION “ROM0”,ROM0[$0000]

; Entry point
_start:
; Disable interrupts
di

; Set video mode to tile mode
ld a, LCDC_TILE_MODE | LCDC_BG_DISPLAY
ld (rLCDC), a

; Load tile data into VRAM
ld hl, _tile_data
ld de, _VRAM8000
ld bc, 16 ; Tile size is 16 bytes
call memcpy

; Load tile map into VRAM
ld hl, _tile_map
ld de, _VRAM9800
ld bc, 1
call memcpy

; Enable interrupts
ei

; Main loop

.loop:
halt ; Wait for interrupt
jr .loop

; Tile data (8×8 pixels)
_tile_data:
DB %11111111, %00000000, %11111111, %00000000
DB %11111111, %00000000, %11111111, %00000000
DB %11111111, %00000000, %11111111, %00000000
DB %11111111, %00000000, %11111111, %00000000

; Tile map (single tile)
_tile_map:
DB 0

; Memory copy subroutine
memcpy:
ld a, (hl+)
ld (de+), a
dec bc
ld a, b
or c
jr nz, memcpy
ret
“`

V. Advanced Techniques

• Banking: The Game Boy’s 64KB address space limits ROM size. Banking techniques allow switching between different ROM banks, effectively expanding the available memory for code and data.
• Optimized Code: Writing efficient assembly code is critical for performance. Techniques like loop unrolling, using faster instructions, and minimizing memory access can significantly improve game speed.
• Sound Programming: The Game Boy’s sound hardware offers four channels: two square wave channels, a wave channel, and a noise channel. Manipulating registers controlling these channels allows creating diverse sound effects and music.
• DMA Transfers: Direct Memory Access (DMA) allows transferring data between memory regions without CPU intervention, which is particularly useful for updating VRAM quickly.

VI. Tools and Resources

Several tools and resources can assist in Game Boy development with Z80 assembly:

• RGBDS: A popular assembler for compiling Game Boy ROMs.
• BGB: A powerful Game Boy emulator with debugging capabilities.
• Game Boy Programming Manual: The official documentation provides detailed information about the hardware and its registers.
• Online Communities: Numerous online forums and communities dedicated to Game Boy development offer valuable support and resources.

VII. Conclusion

Mastering Z80 assembly programming opens up a world of possibilities for Game Boy development. By understanding the core concepts, Game Boy specifics, and leveraging advanced techniques, you can create complex and engaging games that push the limits of this iconic handheld console. This article provides a solid foundation for embarking on your Game Boy development journey, encouraging further exploration and experimentation. Remember that practice is key to honing your skills and unleashing the full potential of Z80 assembly. Don’t be afraid to experiment, explore, and push the boundaries of what’s possible on the Game Boy!