MIP-8 Emulator

Dependencies

• GNU Make for Windows
• Tiny C compiler
• Hex Editor (optional)

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).