# m68kemu

A very minimalistic Motorola 68000 user-mode emulation written in Kotlin,
with the actual cpu core emulation being less than 750 lines of code.

It can be used to emulate short pieces of 68000 binary code.

I used it for validation of the [Raspberry Casket Pretracker replayer](https://git.platon42.de/chrisly42/PretrackerRaspberryCasket).

I had it lying around for so many years, I thought I could just publish
it and maybe someone finds it useful.

## Features
 
- Memory access abstraction
- User-mode only (no exceptions, interrupts, supervisor-instructions etc.)
- Instructions not implemented (might not be decoded correctly!):
  - abcd, sbcd, nbcd
  - movep, cmpm, addx/subx/tas on memory
  - ori/andi/eori to CCR/SR, move from SR, move to CCR/SR, move USP
  - illegal, trap, trapv, chk, stop, reset, rte, rtr

Note: Illegal (unavailable) addressing modes may not be detected as such.
No 68020+ instructions or addressing modes are supported.

## Usage

Create an instance of M68kCpu():
```kotlin
val cpu = M68kCpu()
```

Create some memory, custom chips, etc:
```kotlin
val ram = Ram(0x0, 0x10000)
val rom = Rom(0xfc0000, kickromimage)
val customChips = CustomChips()

ram.writeWord(0x100, 0x4e75) // just an rts
```

Configure your system, registers, program counter and run `decode()` for every instruction as long as you like:
```kotlin
with(cpu) {
    memRegions.add(ram)
    memRegions.add(rom)
    memRegions.add(customChips)

    a[7] = ram.start + ram.size // setup stack register
    pushStack(OpSize.OS_LONG, 0) // write 0 return address
    pc = 0x100 // start of program

    while (pc != 0) {
        decode()
    }
}
```

The CustomChip classes are minimal stubs. There's also some Paula-Recorder class you might find useful.