A MOS 6502 CPU emulator written in ABAP. Run 6502 machine code on SAP systems.
1975: MOS 6502 CPU designed
1977: Microsoft BASIC released
2025: Running on SAP via ABAP emulator, vibecoded with Claude Code
Working! All 44 unit tests pass. The emulator correctly executes 6502 machine code including:
- All arithmetic operations (ADC, SBC) with proper carry/overflow
- Logical operations (AND, ORA, EOR)
- Shifts and rotates (ASL, LSR, ROL, ROR)
- Comparisons (CMP, CPX, CPY)
- Branches (BEQ, BNE, BCS, BCC, BMI, BPL, BVS, BVC)
- Jumps (JMP absolute/indirect, JSR/RTS)
- Stack operations (PHA, PLA, PHP, PLP)
- All addressing modes (immediate, zero page, absolute, indexed, indirect)
Microsoft BASIC (1977) runs on this emulator!
MEMORY SIZE?
TERMINAL WIDTH?
1279 BYTES FREE
COPYRIGHT 1977 BY MICROSOFT CO.
OK
PRINT "HELLO WORLD"
HELLO WORLD
OK
PRINT 2+2
4
OK
10 PRINT "TEST"
20 PRINT 3*4
RUN
TEST
12
OK
A Python 6502 emulator (test_basic.py) is included for local testing:
# Batch mode - run pre-defined commands
python3 test_basic.py --batch
# Interactive mode - terminal session
python3 test_basic.pyInteractive mode uses raw terminal input with cursor position queries to seamlessly integrate with BASIC's line editing - type naturally, and BASIC echoes your input as it processes each character.
The emulator supports two levels of interception that work together:
PC Hooks - Trigger when PC reaches specific addresses:
- Halt CPU at function entry (e.g., RDKEY at
$28CE) - Perfect for ABAP PBO/PAI integration (halt → get line → resume)
Port Hooks - Trigger on memory-mapped I/O reads/writes:
- Character I/O at
$FFF0-$FFF3 - Custom code reading ports directly still works
Hook actions:
| Action | Description |
|---|---|
| CONTINUE | Continue normal execution |
| HALT | Stop CPU, return to caller |
| SKIP | Skip read/write operation |
| OVERRIDE | Return custom value (reads) |
| Address | Name | Purpose |
|---|---|---|
$FFF0 |
CHAROUT | Write character to output |
$FFF1 |
CHARIN | Read character from input (consuming) |
$FFF2 |
STATUS | I/O status (bit 0 = char available) |
$FFF3 |
PEEK | Peek next char without consuming |
| Address | Name | Purpose |
|---|---|---|
$2730 |
COLD_START | Cold boot entry |
$28CA |
COUT | Character output routine |
$28CE |
RDKEY | Read key routine (halt here for line input) |
The ABAP emulator can intercept these addresses to connect BASIC to SAP I/O.
The MOS 6502 is the legendary 8-bit CPU that powered the Apple II, Commodore 64, NES, Atari 2600, and many other classic systems. This ABAP implementation emulates the 6502 instruction set, allowing you to run 6502 programs inside SAP.
This project was vibecoded using Claude Code - Anthropic's AI coding assistant.
Vibecoded in ABAP, directly in the SAP system. No additional tools needed on SAP - just the vibing-steampunk MCP server running locally, which gives Claude direct access to SAP ADT APIs.
zcl_cpu_00_cpu - Main CPU emulator (registers, fetch-decode-execute)
zcl_cpu_00_bus_simple - Simple bus implementation (64KB RAM)
zcl_cpu_00_test - Unit tests (44 tests covering all operations)
zcl_cpu_00_speedrun - Automated test execution
zif_cpu_00_bus - Bus interface for memory/IO
| Register | Size | Description |
|---|---|---|
| A | 8-bit | Accumulator |
| X | 8-bit | Index register X |
| Y | 8-bit | Index register Y |
| SP | 8-bit | Stack pointer (page $01) |
| PC | 16-bit | Program counter |
| P | 8-bit | Status flags (N V - B D I Z C) |
- Immediate (
#$nn) - Zero Page (
$nn) - Zero Page,X (
$nn,X) - Zero Page,Y (
$nn,Y) - Absolute (
$nnnn) - Absolute,X (
$nnnn,X) - Absolute,Y (
$nnnn,Y) - Indirect (
($nnnn)) - Indexed Indirect (
($nn,X)) - Indirect Indexed (
($nn),Y)
Run the full test suite via MCP:
RunUnitTests(object_url="/sap/bc/adt/oo/classes/ZCL_CPU_00_TEST")
Or in SAP GUI: SE38 -> ZCPU6502_SPEEDRUN
SE38 -> ZCPU6502_CONSOLE
Load a ROM file and step through execution.
- Create package
$CPU6502in your SAP system - Import this repo via abapGit
- Activate all objects
- Run unit tests to verify
src/cpu6502/ - ABAP 6502 emulator classes
bin/msbasic.bin - Compiled MS-BASIC (30KB)
test_basic.py - Python test harness
msbasic/ - MS-BASIC source (gitignored, from mist64/msbasic)
This project uses vibing-steampunk for AI-assisted development:
{
"mcpServers": {
"a4h-abap-adt": {
"command": "/path/to/vsp",
"env": {
"SAP_URL": "http://your-sap-host:50000",
"SAP_USER": "your-user",
"SAP_PASSWORD": "your-password"
}
}
}
}See CLAUDE.md for detailed development guidelines.
ABAP's / operator performs decimal division, not integer division. All bit manipulation operations use DIV instead:
" LSR - Logical Shift Right
lv_result = mv_a DIV 2. " NOT mv_a / 2!
" ROR - Rotate Right
lv_result = lv_value DIV 2 + lv_carry * 128.- 6502.org - The 6502 information hub
- Easy 6502 - Interactive 6502 tutorial
- 6502 Instruction Set - Complete opcode reference
- mist64/msbasic - MS-BASIC for 6502 (ca65 compatible)
- Microsoft BASIC History - The story of MS-BASIC
- vibing-steampunk - MCP server for SAP ADT
- Claude Code - AI coding assistant
MIT License - see LICENSE file.
- MOS Technology - Original 6502 design (1975)
- Microsoft - BASIC interpreter (1975-1977)
- Anthropic - Claude Code, the AI that wrote this
- vibing-steampunk - MCP server bridging Claude to SAP ADT
Special thanks to Scott Hanselman and everyone involved in the effort to preserve and open-source historically significant code. The release of the original Microsoft BASIC source code transforms what was once locked away in archives into living history that can be studied, learned from, and—as this project demonstrates—brought back to life on platforms its creators never imagined. This act of digital preservation ensures that the foundations of personal computing remain accessible to future generations of developers, historians, and enthusiasts.
"The 6502 - powering dreams since 1975, now in your SAP system."
"READY." - Microsoft BASIC, running on ABAP, 2024