/mip8

Fantasy 8-bit computer

Primary LanguageCMIT LicenseMIT

MIP-8 Emulator

Dependencies
TODO
  • Add other bus devices (RAM, NVRAM, Timer, Debug port and others)
  • Fix tests
  • Combine mip8vm and mip8asm into one program.
  • Make better error messages in assembler.

Overview

Minimal instruction processor, MIP for short, is a fantasy 8-bit stack-based CPU.

Charachteristics

  • 8-bit address and data buses.
  • Up to 32 Kb of RAM.
  • Up to 16 Kb of NVRAM.

CPU Specification

  • Registers:
    • A: 8-bit wide address register.
    • F: status register (Z - last calculated value was zero, O - T overflow happened).
    • IP: 8-bit wide instruction pointer. Points to the instruction in program memory that is being executed.
    • IRA: 8-bit wide interrupt return address. Points to the instruction that was being executed when the interrupt happened.
    • DS: 32-byte long data stack. Top of the DS is referenced down below as T.
    • RS: 8-byte long return address stack.
    • DSS: 5-bit wide data stack size register.
    • RSS: 3-bit wide return stack size register.

Both DS and RS grow upward. Program memory is not connected to the main memory, addressable by A.

Instructions

  • ALU instructions:
    • AND - pop two bytes from DS, logically multiply them and push the result back.
    • OR - pop a and b; push a|b.
    • XOR - pop a, b; push a^b.
    • ADD - pop a, b; push a+b.
  • Stack manipulations:
    • PUSH - push byte from main memory at A.
    • PUSHI - push memory[A], A++.
    • PUSHP - push progmem[PC], PC++.
    • PUSHA - push A to DS.
    • POP - pop T and write it to memory at address A.
    • POPA - pop T into A.
    • POPI - pop T into A and increase A.
    • DUP - push T.
    • OVER - push DS[DSP-2].
    • DROP - decrease DSP.
  • Branching:
    • JUMP - pop T into IP.
    • CMP - compare two values on the stack.
    • JZR - set IP to next byte if Z is set.
    • JNZ - set IP to next byte if Z is not set.
    • JOV - set IP to next byte if O is set.
  • Calling subroutines:
    • CALL - push IP to RS, set IP to the next byte in memory.
    • RET - pop RS into IP, add 2 to IP.
    • RETI - return from an interrupt.
    • SEI - Enable interrupts.
    • CLI - Disable interrupts.
  • Other:
    • NOP - do nothing.
    • HALT - stop the execution.

Main Memory Layout

  • 00:7F (128) - switchable RAM page.
  • 80 - RAM page index.
  • 81:C0 (64) - NVRAM page.
  • C1 - NVRAM page index.
  • C2 - Timer Period.
  • C3 - Timer Counter.
  • C4 - Timer overflow interrupt address.
  • C5:FD (58) - Unused.
  • FE:FF - Debug port (FE - ASCII character in/out, FF - number in/out).