Building a Z80 disassembler involves parsing raw binary machine code and translating it back into human-readable Z80 assembly language. The Z80 is an 8-bit microprocessor with a variable-length instruction set, making structural planning critical for accurate decoding. 🏛️ 1. Understand Z80 Architecture
Before writing code, you must understand how the Z80 structures its instructions. Variable Length: Instructions range from 1 to 4 bytes.
Instruction Types: Divided into primary opcodes, prefix bytes, and immediate data/dispositions.
Prefix Bytes: The Z80 uses prefixes to alter the meaning of subsequent bytes: \(CB</code>: Bit instructions (Rotate, Shift, Set, Res, Bit). <code>\)DD: Shifts execution to the IX index register. \(FD</code>: Shifts execution to the <code>IY</code> index register.</p> <p><code>\)DD \(CB</code> / <code>\)FD \(CB</code>: Indexed bit operations (4-byte instructions). 🛠️ 2. Set Up the Data Structures</p> <p>Do not use giant <code>switch</code> or <code>if/else</code> chains for all 256 core opcodes. Instead, build a look-up table (LUT) structure.</p> <p><code>typedef struct { uint8_t length; // Total instruction size in bytes const charmnemonic; // Assembly string template (e.g., "LD A, %02Xh") uint8_t operands; // Enum flag for immediate data, relative offsets, etc. } Z80Opcode; </code> Use code with caution.</p> <p>Create four separate lookup tables to handle the structural shifts caused by prefixes: <strong>Main LUT</strong>: For standard opcodes ( <strong>CB LUT</strong>: For bitwise operations.</p> <p><strong>DD/FD LUTs</strong>: For index register swaps (modifying standard <code>HL</code> instructions to use <code>IX</code> or <code>IY</code>). 🔄 3. Implement the Core Decoding Loop</p> <p>Your disassembler operates sequentially through a buffer of binary data, acting as a stream reader.</p> <p><code># Conceptual Decoding Steps 1. Read byte at Program Counter (PC). 2. Is it a prefix (\)CB, \(DD, \)FD)? -> YES: Set flags, consume next byte, switch to target LUT. -> NO: Fetch entry from Main LUT. 3. Read additional bytes based on the instruction’s payload requirements (e.g., immediate 8-bit, 16-bit address). 4. Format the output string using the template and arguments. 5. Advance PC by the total bytes consumed. Use code with caution. ⚠️ 4. Handle Complex Edge Cases
Relative Jumps (JR / DJNZ): The offset byte is a signed 8-bit integer (\(-!128\) to \(+!127\)). Calculate the absolute target address using:$\( ext{Target} = ext{Current PC} + 2 + ext{offset}\)\(</p> <p><strong>Displacement Bytes</strong>: Index instructions like <code>LD A, (IX+d)</code> place the displacement byte <code>(d) *before* the opcode if combined with a </code>\)CB` prefix.
Invalid Opcodes: Ensure your disassembler gracefully outputs DB ??h (Define Byte) for undocumented or invalid bytes to prevent crashing. 📊 Z80 Instruction Decoding Blueprint Instruction Class Example Byte Sequence Interpreted Assembly Bytes Consumed No Argument \(00</code> <code>NOP</code> <strong>Immediate 8-bit</strong> <code>\)3E \(4F</code> <code>LD A, 4Fh</code> <strong>Immediate 16-bit</strong> <code>\)21 \(00 \)20 LD HL, 2000h 3 (Little-Endian) Bit / Shift Prefix \(CB \)07 RLC A Index + Displacement \(DD \)7E \(05</code> <code>LD A, (IX+05h)</code> 🚀 5. Advanced Refinements</p> <p>To elevate your disassembler into a professional reverse-engineering tool, implement these tracking mechanisms:</p> <p><strong>Two-Pass Assembly</strong>: Scan the file on Pass 1 to log jump targets. On Pass 2, replace raw hexadecimal addresses with readable labels (e.g., <code>LABEL_0100:</code>).</p> <p><strong>Symbolic Mapping</strong>: Allow loading of hardware vectors (e.g., mapping address <code>\)0005 to BDOS for CP/M software). If you want to build this tool, let me know: Your programming language of choice (C, Python, Rust, etc.)
The target platform (Game Boy, ZX Spectrum, ColecoVision, or generic Z80)
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