rasp/vm.md

VM

This is an outline of the VM that drives this language.

Primitives

• Numbers may be big endian (BE) or little endian (LE) at the byte level. This guide will use LE.
• Addresses point to single bytes.
• Signed numbers use two's complement.

Type	Size (bits)
Address	64
Word	64
Halfword	32
Byte	8

Registers

CPU registers are addressed by a value between 0-63 (6 bits). All registers are 64 bits wide.

• IP - Instruction pointer
• SP - Stack pointer
• FP - Frame pointer
• FLAGS - CPU flags
• (10 unused registers)
• R0-R49

CPU Flags

CPU flags are addressed by bit index, going from right to left.

• 00 - Halt flag
• 01 - Compare flag

Flag ideas

• "Trace" flag - halts the CPU when certain conditions are met that may be causing undesired behavior - for debugging
  • Overwriting a register without its value being used
  • Mixing arithmetic with bit twiddling on the same target

Instructions

Arithmetic

Arithmetic instructions store their result in the first register specified. Overflow is handled by wrapping around to 0.

• Add
  • Params: REG1, REG2
  • REG1 = REG1 + REG2
  • Unsigned addition
• Mul
  • Params: REG1, REG2
  • REG1 = REG1 * REG2
  • Unsigned multiplication
• Div
  • Params: REG1, REG2
  • REG1 = REG1 / REG2
  • Unsigned division
• Mod
  • Params: REG1, REG2
  • REG1 = REG1 % REG2 (exact semantics TBD)
• INeg
  • Params: REG1
  • REG1 = REG1 * -1
  • Signed negative
• And
  • Params: REG1, REG2
  • REG1 = REG1 & REG2
• Or
  • Params: REG1, REG2
  • REG1 = REG1 | REG2
• Xor
  • Params: REG1, REG2
  • REG1 = REG1 ^ REG2
• Shl
  • Params: REG1, REG2
  • REG1 = REG1 << REG2
• Shr
  • Params: REG1, REG2
  • REG1 = REG1 >> REG2

TODO

• Add signed instructions (iadd, imul, etc)
• Overflow flag?

Control flow

• CmpEq
  • Params: REG1, REG2

  •   if REG1 == REG2 {
          FLAGS[1] = 1;
      } else {
          FLAGS[1] = 0;
      }
    

• CmpLt
  • Params: REG1, REG2

  •   if REG1 < REG2 {
          FLAGS[1] = 1;
      } else {
          FLAGS[1] = 0;
      }
    

• Jz
  • Params: REG1

  •   if FLAGS[0] == 0 {
          IP = REG1;
      }
    

• Jnz
  • Params: REG1

  •   if FLAGS[0] != 0 {
          IP = REG1;
      }
    

Data movement

• Load
  • Params: REG1, REG2

  REG1 = MEM[REG2];

• Store
  • Params: REG1, REG2

  MEM[REG2] = REG1;

• StoreImm32
  • Params: REG1, IMM_32
  • REG1 = IMM_32
• MemCopy
  • Params: REG1, REG2
  • MEM[REG1] = MEM[REG2]
• RegCopy
  • Params: REG1, REG2
  • REG1 = REG2