# 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
* (9 unused registers)
* STATUS - Generic status code
* 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`
* Inv
    * **Params**: REG1
    * `REG1 = ~REG1`
* Not
    * **Params**: REG1
    * ```
    if REG1 == 0 {
        REG1 = 0;
    } else {
        REG1 = 1;
    }
    ```
    * Boolean NOT; equivalent of C's `!` unary operator
* Xor
    * **Params**: REG1, REG2
    * `REG1 = REG1 ^ REG2`
* Shl
    * **Params**: REG1, REG2
    * `REG1 = REG1 << REG2`
* Shr
    * **Params**: REG1, REG2
    * `REG1 = REG1 >> REG2`
    * Does not sign extend

### TODO

* Add signed instructions (iadd, imul, etc)
* Sign-extending SHR
* Overflow flag?

## Control flow

* CmpEq
    * **Params**: REG1, REG2
    * ```
        if REG1 == REG2 {
            FLAGS[1] = 1;
        } else {
            FLAGS[1] = 0;
        }
    ```
    * Sets the COMPARE flag to 1 if REG1 == REG2
* CmpLt
    * **Params**: REG1, REG2
    * ```
        if REG1 < REG2 {
            FLAGS[1] = 1;
        } else {
            FLAGS[1] = 0;
        }
    ```
    * Sets the COMPARE flag to 1 if REG1 < REG2
* Jz
    * **Params**: REG1
    * ```
        if FLAGS[1] == 0 {
            IP = REG1;
        }
    ```
    * Jumps to the address in REG1 if COMPARE flag is 0.
* Jnz
    * **Params**: REG1
    * ```
        if FLAGS[1] != 0 {
            IP = REG1;
        }
    ```
    * Jumps to the address in REG1 if COMPARE flag is 1.

## Data movement

* Load
    * **Params**: REG1, REG2
    * ```
    REG1 = MEM[REG2];
    ```
    * Sets REG1 to the value at the memory address in REG2.
* Store
    * **Params**: REG1, REG2
    * ```
    MEM[REG2] = REG1;
    ```
    * Sets the value at the memory address in REG2 to the value in REG1.
* StoreImm32
    * **Params**: REG1, IMM_32
    * `REG1 = IMM_32`
    * Sets REG1 to the specified 32-bit number.
* MemCopy
    * **Params**: REG1, REG2
    * `MEM[REG1] = MEM[REG2]`
    * Copies the value at the memory address in REG2 to the memory address in REG1.
* RegCopy
    * **Params**: REG1, REG2
    * `REG1 = REG2`
    * Copies the value in REG2 into REG1.

## Miscellaneous

* Halt
    * **Params**: (none)
    * `FLAGS[0] = 1`
    * Halts the machine
* Nop
    * **Params**: (none)
    * Does nothing

## Other instructions TODO

* Call
    * Takes address and number of bytes on the stack that are for args(?)
    * Updates SP, FP, IP, storing previous values starting at the new FP
* Ret
    * Uses FP to determine previous SP, FP, and IP and restores them
* Push
* Pop
* More immediate stores?

# General TODO

* Interrupts
* MMIO regions
* Execution pipeline
    * Helps to define when certain side effects happen (e.g. when the IP increments)
* Paging?