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Z80/i8080 CPU Emulator in ABAP

A working Z80/i8080 CPU emulator implemented in ABAP, designed to run CP/M programs. Features transpiler-compatible code for local testing via Node.js.

Current Status (2025-10-19)

✅ 86 / ~105 core i8080 opcodes implemented (82%) ✅ 16 unit tests, all passing ✅ Local TDD workflow operational (transpile + test < 1 second) ✅ 2,175 lines of ABAP code

Quick Start

# Run tests locally
npm test

# Expected output: All 16 tests passing
# ZCL_CPU_8080_V2: running zcl_cpu_8080_test->test_init
# ZCL_CPU_8080_V2: running zcl_cpu_8080_test->test_memory
# ... (all 16 tests pass)

Documentation

📚 Start here for different purposes:

  • CONTEXT.md - Current project status, architecture decisions, session history
  • TODO.md - Implementation plan, missing opcodes (~19 remaining), test strategy
  • CLAUDE.md - Instructions for AI assistants (Claude, GPT, etc.)
  • BRAINSTORM.md - Architecture analysis (why hybrid approach)
  • TRANSPILER.md - ABAP transpiler setup and constraints
  • SESSION_NOTES.md - Debugging notes from opcode 49 bug fix

Project Structure

cpm-abap/
├── src/
│   ├── zcl_cpu_8080_v2.clas.abap              # CPU emulator (1,716 lines)
│   └── zcl_cpu_8080_v2.clas.testclasses.abap  # Unit tests (459 lines)
├── output/                                     # Transpiled JavaScript (auto-generated)
│   ├── index.mjs                               # Test runner
│   └── zcl_cpu_8080_v2.clas.mjs                # Transpiled CPU code
├── node_modules/                               # npm packages (transpiler + runtime)
├── docs/
│   ├── CONTEXT.md          # Project status and history
│   ├── TODO.md             # Implementation plan ⭐
│   ├── CLAUDE.md           # AI assistant instructions
│   ├── BRAINSTORM.md       # Architecture analysis
│   ├── TRANSPILER.md       # Transpiler setup
│   └── SESSION_NOTES.md    # Bug fix documentation
├── package.json            # npm dependencies
├── abaplint.json           # Transpiler config
└── README.md               # This file

What's Implemented

CPU Core (Complete)

  • ✅ All registers: AF, BC, DE, HL, PC, SP
  • ✅ 64KB memory (STRING representation, transpiler-compatible)
  • ✅ Pre-computed lookup tables (parity, inc, dec, carry bits)
  • ✅ Memory access (byte/word, little-endian)
  • ✅ Flag calculations (all 6 flags)

Opcodes: 86 Implemented (12 Families)

  1. ✅ Load/Store 16-bit (4) - LD BC/DE/HL/SP,nnnn
  2. ✅ Load/Store 8-bit (7) - LD r,nn
  3. ✅ Register to Register (64) - LD r,r family (MOV in i8080)
  4. ✅ Memory Operations (8) - LD (BC),A / LD A,(nnnn) / etc.
  5. ✅ Increment/Decrement (14) - INC/DEC for all registers
  6. ✅ ALU with Register (64) - ADD/SUB/AND/OR/XOR/CP A,r
  7. ✅ ALU with Immediate (8) - ADD/SUB/AND/OR/XOR/CP A,nn
  8. ✅ Rotate (4) - RLCA/RRCA/RLA/RRA
  9. ✅ Control Flow (3) - JP/CALL/RET
  10. ✅ Conditional Flow (24) - JP/CALL/RET with conditions (NZ/Z/NC/C/PO/PE/P/M)
  11. ✅ Stack (8) - PUSH/POP BC/DE/HL/AF
  12. ✅ Miscellaneous (6) - NOP/HALT/DAA/CPL/STC/CMC

What's Missing (~19 opcodes)

See TODO.md for detailed implementation plan.

High Priority (10 opcodes):

  • ADD HL,rr (4 opcodes) - 16-bit arithmetic
  • Exchange operations (4 opcodes) - EX DE,HL / JP (HL) / etc.
  • DI/EI (2 opcodes) - Interrupt control

Medium Priority (8 opcodes):

  • RST instructions (8 opcodes) - System calls

Low Priority (2 opcodes):

  • I/O operations (2 opcodes) - IN/OUT

Architecture: Hybrid Approach

Based on analysis of RunCPM, this uses a hybrid architecture:

Code-driven dispatch - Direct CASE statement for opcodes

CASE iv_opcode.
  WHEN 1.  " LD BC,nnnn (Z80) / LXI B (i8080)
    mv_bc = read_word_pp( CHANGING cv_addr = mv_pc ).
    rv_cycles = 10.

Table-driven flags - Pre-computed lookup tables

" INC uses pre-computed flag table (257 entries)
lv_offset = lv_temp * 2.
lv_hex = mv_inc_table+lv_offset(2).
lv_flags = hex_to_byte( lv_hex ).

Why Hybrid?

✅ Fast - Direct execution, no indirection ✅ Debuggable - Step through actual logic, not metadata ✅ Simple - No dynamic method calls ✅ Efficient - CASE optimized by ABAP kernel ✅ Accurate - Pre-computed flags eliminate calculation errors

See BRAINSTORM.md for detailed analysis.

Key Design Decisions

1. Memory as STRING (Transpiler-Compatible)

" 64KB memory = 131,072 hex characters (2 per byte)
DATA: mv_memory TYPE string.

" Read byte at address 0x1000:
lv_offset = 4096 * 2.              " Address × 2
lv_hex = mv_memory+8192(2).        " Extract 2 hex chars
rv_val = hex_to_byte( lv_hex ).    " Convert to integer

Why? Transpiler doesn't support internal table operations. STRING works perfectly.

2. Registers as 32-bit Integers

" Store 16-bit pairs as 32-bit integers
DATA: mv_af TYPE i.  " A=high byte (bits 8-15), F=low byte (bits 0-7)

" Access via arithmetic (transpiler-compatible)
METHOD get_high_byte.
  rv_val = iv_pair DIV 256.
  rv_val = rv_val MOD 256.  " Ensure 8-bit
ENDMETHOD.

Why? Simpler 16-bit operations, no bit operations needed.

3. Dual i8080/Z80 Naming

All opcodes documented with both architectures:

WHEN 1.   " LD BC,nnnn (Z80) / LXI B (i8080)
WHEN 6.   " LD B,nn (Z80) / MVI B,nn (i8080)
WHEN 195. " JP nnnn (Z80) / JMP nnnn (i8080)

Why? Prepares for Z80 extension, helps readers familiar with either CPU.

Local Development Workflow

Prerequisites

npm install  # Installs @abaplint/transpiler-cli and runtime

Test-Driven Development Cycle

# 1. Edit ABAP code in your favorite editor
vim src/zcl_cpu_8080_v2.clas.abap

# 2. Run tests (< 1 second!)
npm test

# 3. All tests pass ✓
# 4. Commit
git add src/*.abap
git commit -m "Added feature X"
git push

Benefits:

  • ⚡ Instant feedback - 100x faster than SAP upload
  • 💻 Work offline - No SAP server required
  • 🔄 CI/CD ready - Can run in GitHub Actions
  • 🛠️ Modern tools - VS Code, git workflows

See TRANSPILER.md for setup details.

Testing

16 Comprehensive Tests (All Passing)

  1. test_init - CPU initialization
  2. test_memory - Memory read/write operations
  3. test_nop - NOP instruction
  4. test_ld_bc - 16-bit register load
  5. test_inc_bc - 16-bit increment with flags
  6. test_halt - CPU halt state
  7. test_jump - Unconditional jump (JP)
  8. test_call_ret - Subroutine calls and returns
  9. test_program - Complex multi-instruction program
  10. test_ld_r_r - Register-to-register moves
  11. test_alu_add - Addition with flags
  12. test_alu_sub - Subtraction with flags
  13. test_alu_and - Bitwise AND with flags
  14. test_alu_or - Bitwise OR with flags
  15. test_alu_xor - Bitwise XOR with flags
  16. (More tests as opcodes are added)

Industry Standard Test Suite (Coming Soon)

8080 Exerciser by Ian Bartholomew - Gold standard for i8080 validation

  • Tests all instructions systematically
  • Computes CRC checksums
  • Prints "PASS" or shows which instruction failed
  • See TODO.md for test suite integration plan

Implementation Plan

See TODO.md for complete implementation plan.

Short Term (4-6 hours)

✅ Implement remaining 19 i8080 opcodes

  • ADD HL,rr family (4 opcodes)
  • Exchange operations (4 opcodes)
  • RST instructions (8 opcodes)
  • I/O operations (2 opcodes)
  • DI/EI (2 opcodes)

Medium Term (1-2 weeks)

⏳ CP/M BDOS Emulation

  • Console I/O (functions 1, 2, 9, 10, 11)
  • File I/O (functions 15, 16, 20-26)
  • .COM file loader
  • Run real CP/M programs!

Long Term (2-3 weeks, optional)

⏳ Z80 Extensions

  • CB prefix - Bit operations
  • ED prefix - Block operations
  • DD/FD prefix - IX/IY registers
  • Alternate register set

Usage Example

" Create CPU instance
DATA(lo_cpu) = NEW zcl_cpu_8080_v2( ).

" Write a simple program to memory
" Program: LD BC,0x1234; INC BC; HALT
lo_cpu->write_byte( iv_addr = 256 iv_val = 1 ).    " LD BC,nnnn
lo_cpu->write_byte( iv_addr = 257 iv_val = 52 ).   " 0x34
lo_cpu->write_byte( iv_addr = 258 iv_val = 18 ).   " 0x12
lo_cpu->write_byte( iv_addr = 259 iv_val = 3 ).    " INC BC
lo_cpu->write_byte( iv_addr = 260 iv_val = 118 ).  " HALT

" Execute until HALT
DATA(lv_count) = lo_cpu->execute_until_halt( iv_max_instructions = 1000 ).

" Inspect results
WRITE: / 'Executed', lv_count, 'instructions'.
WRITE: / 'BC =', lo_cpu->get_bc( ).  " Should be 0x1235

Performance Estimates

JavaScript (Current - via transpiler)

  • ~5-10M instructions/second
  • V8 engine optimization
  • Perfect for development/testing

SAP System (When deployed)

  • Modern (2020+): ~500K-1M instructions/second
  • Older (2010): ~100K-200K instructions/second
  • Original 8080 @ 2MHz: 500K instructions/second
  • Can emulate at original speed on modern systems!

For AI Assistants

Working on this project with Claude, GPT, or another AI?

👉 Read CLAUDE.md first!

It contains:

  • Current implementation status
  • Transpiler constraints and patterns
  • Code style guidelines
  • Common tasks and examples
  • What NOT to do

Resources

Code & Tools

Documentation

Test Suites

Contributing

This is an educational/experimental project. Contributions welcome!

Before contributing:

  1. Read CONTEXT.md for project status
  2. Check TODO.md for what needs doing
  3. Review CLAUDE.md for code patterns
  4. Run npm test to ensure all tests pass

License

Educational/experimental project. RunCPM reference implementation is MIT licensed.

Next Milestones

🎯 Immediate: Complete i8080 instruction set (~19 opcodes, 4-6 hours) 🎯 Short term: Run 8080 Exerciser test suite (validate all instructions) 🎯 Medium term: CP/M BDOS emulation (console I/O, file I/O) 🎯 Long term: Run real CP/M programs (ZORK, Turbo Pascal, MBASIC)

See TODO.md for detailed plan and timeline.

Built with ABAP, validated by industry-standard test suites, documented for the future 🚀

Last updated: 2025-10-19

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Z80/i8080 CPU Emulator in ABAP - Hybrid switch/case + lookup tables architecture

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