Core8080 - Intel 8080 Emulator & CP/M System for iOS

An iPad/iPhone app for creating, editing, assembling, and running Intel 8080 assembly code - including a working CP/M 2.2 operating system!

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🎉 Current Status (January 2026)

✅ What Works

8080 Assembler

• ✅ Complete instruction set support
• ✅ Directives: ORG, DB, DS, DW, EQU, END
• ✅ String literals in DB: db 'Hello World', 0Dh, 0Ah, '$'
• ✅ Character literals: cpi 'D', mvi a, '?'
• ✅ Labels and forward references
• ✅ Multi-value DB directives
• ✅ Hex, octal, binary, and decimal number formats
• ✅ Regex-based tokenization with label protection

8080 CPU Emulator

• ✅ Full 8080 instruction set
• ✅ All flags (carry, zero, sign, parity, aux carry)
• ✅ Stack operations
• ✅ Interrupts
• ✅ I/O ports (IN/OUT instructions)
• ✅ Visual step-through debugging
• ✅ Register inspection

CP/M 2.2 Operating System

• ✅ BDOS (Basic Disk Operating System) - Fully implemented in C
  • Console I/O (functions 1, 2, 9, 10, 11)
  • Disk operations (functions 13, 14, 25, 26)
  • File operations (functions 15-23)
    • Open, Close, Make, Delete, Rename
    • Sequential Read/Write
    • Search First/Next (directory listing)
• ✅ Disk Emulation
  • 2 disk drives (A: and B:)
  • 256KB each (77 tracks × 26 sectors × 128 bytes)
  • Real CP/M directory structure
  • Sample files pre-loaded (WELCOME.TXT, HELP.TXT, README.TXT)
• ✅ CP/M Terminal Interface
  • Full-screen interactive terminal
  • Keyboard input with CP/M control characters
  • Character echo
  • Scrolling output
• ✅ BIOS Support (via I/O ports 0xF0-0xFA)
  • Console I/O
  • Disk I/O
  • Ready for real CP/M binaries

Sample Programs

1. Kill the Bit - Classic Altair 8800 game
2. Echo Test - CP/M console I/O test
3. Disk Test - CP/M disk read/write test
4. File Operations - Create, write, read files
5. Directory Operations - Search, rename, delete
6. Command Processor Demo - Full DIR, TYPE, ERA, REN commands
7. CP/M Boot Loader - Interactive command prompt with DIR and EXIT

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🚀 Recent Accomplishments

Assembler Improvements (Latest Session)

• ✅ Added string literal support in DB directives
  • Preserves strings during tokenization
  • Handles quoted strings with spaces
  • Mixed numeric and string values: db 'Text', 0Dh, 0Ah, 24h
• ✅ Added character literal support for instructions
  • cpi 'D' automatically converts to cpi 44h (ASCII value)
  • Works in all instructions that take immediate values
• ✅ Fixed tokenizer to preserve quoted strings
  • No longer splits 'Hello World' into separate tokens

UI Enhancements

• ✅ Added "Run" button to execute assembled programs in CP/M Terminal
• ✅ Sample cycling includes new CP/M Boot program
• ✅ Automatic assembly on sample load

Documentation

• ✅ CPM_IMPLEMENTATION_GUIDE.md - Complete CP/M implementation details
• ✅ CPM_QUICKSTART_GUIDE.md - Getting started with CP/M
• ✅ CPM_BOOT_READY.md - Testing the CP/M boot program
• ✅ Assembly source files: cpm_bios.asm, cpm_ccp.asm, cpm_boot.asm

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🔧 Known Issues

Assembler

• ⚠️ DB string assembly - PARTIALLY WORKING
  • ✅ First DB string on a line assembles correctly
  • ❌ Second DB string fails (possibly comma-related)
  • Example working: db 'CP/M 2.2 CORE8080 EDITION', 0Dh, 0Ah ✅
  • Example failing: db 'Type DIR to see files, EXIT to quit', 0Dh, 0Ah, '$' ❌
  • Issue: Commas inside strings may confuse the DB handler's comma-splitting
  • Workaround: Use multiple DB lines or avoid commas in strings
• ✅ String literals now preserve spaces/colons (fixed)
• ⚠️ Label arithmetic not supported (label+1)
  • Must use pointer arithmetic instead

CP/M Terminal

• ⚠️ Need to test the CP/M Boot interactive prompt
• ⚠️ Directory listing prints extra bytes/NULs after filenames
• ⚠️ No disk persistence yet (changes lost on restart)

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📋 Next Steps

Immediate (Fix Current Issues)

1. Fix DB comma-in-string bug 🔥 CURRENT ISSUE
   • The DB handler splits on commas, breaking strings with commas
   • Line 315 in Assemble.swift: let values = code[opCounter].components(separatedBy: ",")
   • Need to split on commas ONLY outside of quoted strings
   • Solution: Write a custom comma-splitter that respects quotes
   • Test case: db 'files, EXIT' should stay as one value
2. Test CP/M Boot program
   • Once DB strings work, assemble should succeed
   • Run in terminal
   • Type DIR command
   • Verify file listing works
3. Clean up directory display
   • Remove drive byte from filename display
   • Format 8.3 names properly

Short Term (Polish)

1. Disk Persistence
   • Save disk images to iOS Documents folder
   • Load/save disk images
   • Import/export individual files
2. More CP/M Commands
   • TYPE filename (display file contents)
   • ERA filename (delete with confirmation)
   • REN oldname newname (rename files)
   • STAT (disk statistics)
3. Better Terminal UI
   • Clear screen button
   • Copy/paste support
   • Font size adjustment
   • Color schemes

Medium Term (Features)

1. Load Real CP/M Binaries
   • Download CCP.COM and BDOS.COM from Archive.org
   • Use authentic CP/M 2.2 instead of custom implementation
   • Load BIOS from cpm_bios.asm
2. Run CP/M Software
   • Load .COM files from iOS
   • WordStar, dBase, Zork, etc.
   • CP/M games and utilities
3. Complete BDOS
   • Random file access (functions 33-40)
   • User areas (0-15)
   • File attributes
   • Timestamps

Long Term (Advanced)

1. CP/M 3.0 Support
   • Banked memory
   • Date/time stamps
   • Extended BDOS functions
2. Z80 CPU Emulation
   • Add Z80 instructions
   • Run Z80 CP/M software
3. Networking
   • Serial port emulation
   • Telnet BBS access
   • File transfer (XMODEM, Kermit)

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🎮 How to Use

Running CP/M Programs

1. Open the app
2. Tap "Samples" repeatedly to cycle through programs
3. Find "CP/M Boot Loader"
4. Tap "Assemble" (should auto-assemble)
5. Tap "Run" to launch CP/M Terminal
6. Type commands:
   • DIR - List files on disk A:
   • EXIT - Exit the program

Writing Your Own Programs

1. Create a new document or open existing .s file
2. Write 8080 assembly code:

org 100h

start:
    lxi d, message
    mvi c, 09h          ; BDOS print string
    call 0005h          ; Call BDOS
    hlt

message:
    db 'Hello, CP/M!', 0Dh, 0Ah, '$'

end

3. Tap "Assemble"
4. Check for errors in the assembled code view
5. Tap "Run" to execute in CP/M Terminal
6. Or tap "Emulate" to step through instructions

Using the Emulator

1. After assembling, tap "Emulate"
2. Tap "STEP" to execute one instruction
3. Watch registers update in the display
4. See program counter highlighted in source code
5. Monitor flags (carry, zero, sign, etc.)

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🏗️ Architecture

File Structure

Core8080Manager/
├── Document Browser/
│   ├── 8080.c                 # CPU emulator & CP/M BDOS
│   ├── 8080.h                 # CPU definitions
│   ├── Assemble.swift         # 8080 assembler
│   ├── CPMTerminalViewController.swift  # CP/M terminal UI
│   ├── TextDocumentViewController.swift # Main editor
│   └── EmulatorViewController.swift     # Step-through debugger
├── cpm_bios.asm              # CP/M BIOS (assembly source)
├── cpm_ccp.asm               # CP/M CCP (assembly source)
├── cpm_boot.asm              # Simple boot loader
└── Documentation/
    ├── CPM_IMPLEMENTATION_GUIDE.md
    ├── CPM_QUICKSTART_GUIDE.md
    └── CPM_BOOT_READY.md

CP/M Memory Map

0x0000-0x00FF : Page Zero (vectors, buffers)
0x0005        : BDOS entry point
0x0080-0x00FF : Default DMA buffer
0x0100-0xDBFF : TPA (Transient Program Area) - User programs
0xDC00-0xE3FF : CCP (Console Command Processor) - 2KB
0xE400-0xF9FF : BDOS (Basic Disk OS) - 5.5KB
0xFA00-0xFFFF : BIOS (Basic I/O System) - 1.5KB

I/O Port Map

BDOS/Legacy Ports:

• 0x00 - Console status
• 0x01 - Console data
• 0x10 - Disk select
• 0x11 - Track number
• 0x12 - Sector number
• 0x13 - DMA address low
• 0x14 - DMA address high
• 0x15 - Disk operation

BIOS Ports (0xF0-0xFA):

• 0xF0 - Console status (CONST)
• 0xF1 - Console input (CONIN)
• 0xF2 - Console output (CONOUT)
• 0xF3 - Disk select (SELDSK)
• 0xF4 - Set track (SETTRK)
• 0xF5 - Set sector (SETSEC)
• 0xF6 - DMA address low (SETDMA)
• 0xF7 - DMA address high
• 0xF8 - Read sector (READ)
• 0xF9 - Write sector (WRITE)
• 0xFA - Home disk (HOME)

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📚 Resources

CP/M Information

• CP/M 2.2 Binaries on Archive.org
• CP/M 2.2 Source Code
• Digital Research Source Code
• CPMish - Open Source CP/M

8080 Documentation

• Intel 8080 Datasheet
• 8080 Instruction Set
• CP/M BDOS Reference

Original Project

• 8080Core on GitHub

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🙏 Credits

• Original 8080Core by GrantMeStrength
• CPU Emulator based on i8080-emu by cbrooks90
• CP/M 2.2 by Digital Research (now public domain)
• Kill the Bit game by Dean McDaniel (1975)

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📝 License

See LICENSE folder for details. CP/M is public domain. Original 8080 emulator code used with attribution.

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🎯 Project Goals

This project aims to:

1. Educate - Learn 8080 assembly language and CP/M operating system concepts
2. Preserve - Keep 1970s computing history alive and accessible
3. Experiment - Explore retro computing on modern iOS devices
4. Inspire - Show how simple yet powerful early computers were

Status: Active development. CP/M system is functional but needs polish and testing.

Last Updated: January 2026

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🐛 Debugging Notes

If Assembly Fails

• Check that DB strings use single quotes: 'text' not "text"
• Character literals need quotes: cpi 'D' not cpi D
• Labels must end with colon: loop: not loop
• Hex numbers need 'h': 0Dh or 0DH
• No label arithmetic: Use inx h instead of lxi h, label+1

If CP/M Terminal Doesn't Open

• Make sure code assembled successfully (check for "Error" in output)
• Look for hex output in assembled view
• Try the "CP/M Terminal" button (loads built-in shell)
• Check Xcode console for error messages

If DIR Shows No Files

• Files should be created automatically by cpm_init()
• Check Xcode console for "CP/M System Initialized" message
• Make sure BDOS functions 17/18 (Search First/Next) are working
• Verify disk initialization in cpm_disk_init()

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Happy Retro Computing! 🖥️✨