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ToyVM

A simple virtual machine written in C. (Assembler available here.)

Instruction set specification

Just like Intel-based computers, ToyVM is little-endian.

Registers

ToyVM features four 32-bit registers: REG1, REG2, REG3, REG4. Each of them is represented as a single byte with values 0x0, 0x1, 0x2, 0x3, respectively.

Data types

ToyVM has only one data type: 32-bit signed integers.

Instruction format

The first byte is always an opcode specifying the instruction. It is immediately followed by arguments. If the instruction has both registers and immediate data as arguments, registers are specified before the immediate data.

Arithmetic

  • 0x01: ADD REGi REGj - adds the integer in REGi to REGj.
  • 0x02: NEG REGi - negates the integer stored in REGi.
  • 0x03: MUL REGi REGj - copies the product of REGi and REGj to REGj.
  • 0x04: DIV REGi REGj - copies the ratio of REGi and REGj to REGj
  • 0x05: MOD REGi REGj - divides REGi by REGj and stores the remainder in REGj.

Conditionals

  • 0x10: CMP REGi REGj - compares the value in REGi to the value in REGj.
  • 0x11: JA ADDRESS - jumps to address ADDRESS only if the vm.cpu.status has flag COMPARISON_ABOVE on.
  • 0x12: JE ADDRESS - jumps to address ADDRESS only if the vm.cpu.status has flag COMPARISON_EQUAL on.
  • 0x13: JB ADDRESS - jumps to address ADDRESS only if the vm.cpu.status has flag COMPARISON_BELOW on.
  • 0x14: JMP ADDRESS - jumps unconditionally to address ADDRESS.

The three comparison flags above are mutually exclusive: at most only one of them is set on.

Subroutines

  • 0x20: CALL ADDRESS - pushes the return address (i.e., ADDRESS + 5) into the stack and jumps to the instruction at the specified address.
  • 0x21: RET - pops the return address and sets it to the program counter, hence returning from a subroutine.

Memory and data

  • 0x30: LOAD REGi ADDRESS - stores the value at address ADDRESS to the register REGi.
  • 0x31: STORE REGi ADDRESS - stores the value in REGi to the memory at address ADDRESS.
  • 0x32: CONST REGi DATA - stores the value DATA in the register REGi.
  • 0x33: RLOAD REGi REGj - loads the word at the address in the register REGi to the register REGj.
  • 0x34: RSTORE REGi REGj - stores the contents of register REGi to the address at REGj.

Auxiliary

  • 0x40: HALT - halts the virtual machine.
  • 0x41: INT INTERRUPT_NUMBER - issues an interrupt with number INTERRUPT_NUMBER.
  • 0x42: NOP - a no-op instruction, does nothing else but increase the program counter towards the next instruction.

Stack

  • 0x50: PUSH REGi - pushes the contents of register REGi to the stack.
  • 0x51: PUSH_ALL - pushes all four registers to the stack.
  • 0x52: POP REGi - pops the stack into register REGi.
  • 0x53: POP_ALL - pops all values of registers from the stack back to the registers.
  • 0x54: LSP REGi - loads the value of the stack pointer to the register REGi.