cratonica/micro_vm

A small virtual machine written in C++, complete with an assembler and a debugger

micro_vm

A small virtual machine written in C++, complete with an assembler and a debugger.

Overview

This is a virtual machine that simulates execution of a nonexistent 16-bit CPU. It includes tools for assembling machine code from assembly code, a dissasembler, and a graphical debugger.

Building

micro_vm uses SCons to build. Use the scons command to build, and scons -c to clean up. This is also wrapped up in a Makefile, but you need to have SCons installed in order to build.

You will also need to install the ncurses development headers; on Debian-based Linux distros this can be found in the ncurses-dev package.

Running

Once you have built the VM, you can execute the test program by running ./cpu out.bin. It will present a blank prompt. Enter 2 numbers and the VM will add them together:

./cpu out.bin
2
4
6

More test programs can be found in the progs/ directory

Datasheet

CPU

The CPU is a 16-bit little-endian RISC processor with a few high-level instructions for interacting with console input and output.

Registers

• A (8 bit)
• X = I[High]
• Y = I[Low]
• FP (16-bit)
• SP (8-bit offset from FP)
• IP (16-bit)
• F (flags)
  • Zero
  • Carry
  • Sign

SP points to next available element in stack (uninitialized). Therefore, it should never be accessed directly by program code.

Instruction Set

The instruction set is pretty standard and should look familiar to most assembly programmers. The specification listed beside the instruction is C syntax, with a * indicating a memory address dereference (so for example, *I refers to the memory location at the address stored in register I).

A numeric suffix indicates the byte size an operand literal.

• Operations
  • BAD: Invalid
  • NOP: Nothing
  • ADD: A = A + *X:Y
  • ADD1: A = A + Literal
  • AND: A = A & *X:Y
  • AND1: A = A & Literal
  • OR: A = A | *X:Y
  • ORL: A = A | Literal
  • CMPL: A = ~A
  • INCA: ++A
  • INCX: ++X
  • INCY: ++Y
  • INCI: ++I
  • DECA: --A
  • DECX: --X
  • DECY: --Y
  • DECI: --I
  • DIV: I = I / A; A = I % A
  • MUL: I = A * Y (8-bit operands, 16-bit result)
• Compare
  • CMP: Flags = A Compared to *X:Y
  • CMP1: Flags = A Compared to Literal
• Memory
  • LDA: A = *X:Y
  • LDA1: A = Literal
  • LDX1: X = Literal
  • LDY1: Y = Literal
  • DREF: I = *I
  • LAS1: A = SP[Literal]
  • LIS1: I = SP[Literal]
  • STOR: *I = A
  • TXA: A = X
  • TYA: A = Y
  • TAX: X = A
  • TAY: Y = A
  • TSPI: I = FP + SP
  • LDI2: I = Literal
• Stack manipulation
  • PSHA: *SP = A; ++SP
  • PSHX: *SP = X; ++SP
  • PSHY: *SP = Y; ++SP
  • PSH1: *SP = Lit8; ++SP
  • PSH2: *SP = Lit16; SP += 2
  • POPA: --SP; A = *(FP + SP)
  • POPX: --SP; X = *(FP + SP)
  • POPY: --SP; Y = *(FP + SP)
  • POP1: SP -= Lit
  • POP: --SP
  • LAL1: A = FP[Literal] (Load A from local)
  • LAP1: A = FP[-5 - Literal] (Load A from parameter)
  • LIP1: I = FP -5 - Literal (Load I from parameter address)
  • LIL1: I = FP + Literal (Load I from local address)
  • STL1: FP[Literal] = A
• Flow Control
  • JMP2: IP = Literal
  • JGE2: If F[Zero] || !F[Sign], IP = Literal
  • JG2: If !F[Sign], IP = Literal
  • JL2: If F[Sign], IP = Literal
  • JLE2: If F[Zero] || F[Sign], IP = Literal
  • JE2: If F[Zero], IP = Literal
  • JNE2: If !F[Zero], IP = Literal
  • JC2: If F[Carry], IP = Literal
  • CAL2: *(FP+SP)=FP; *(FP+SP+2)=SP; *(FP+SP+3)=IP; FP+=SP+5; SP = 0; IP = Literal (Call subroutine)
  • RET: IP = FP[-2]; SP = FP[-3]; FP = FP[-5] (Return from subroutine)
• I/O
  • PRNT: Print A as ASCII byte
  • KEYB: Read key into A. \0 is end of input
• Flags
  • CC: Clear Carry
• Debug
  • BRK: Debug break (NOP if no debugger)

Tools

Assembler

Assembles input assembly code into a machine code file (out.bin) and a debug metadata file (out.dbg)

Usage: ./asm input.asm

Disassembler

Disassembles machine code into human-readable assembly code and outputs it to stdout.

Usage: ./disasm out.bin

Graphical Debugger

Runs a program interactively on a debugger GUI (written in ncurses).

Usage: ./debug out (Notice no '.bin')

The enter key steps the program forward, and the 'r' steps the program backwards.

Example Program: Fibonocci

main:
    PSH1 13
    CAL2 fib
    POP
    PSHA
    CAL2 printint8
    POP
    PSH2 endl
    CAL2 puts
    POP1 2
    RET

fib:
    LAP1 1
    CMP1 1
    JG2 .recur
    RET
.recur:
    CC
    DECA
    PSHA
    CAL2 fib
    POPX
    PSHA
    TXA
    DECA
    PSHA
    CAL2 fib
    POP
    TSPI
    DECI
    ADD
    POP1 1
    RET

%file lib.asm