# CROISSANT VIRTUAL MACHINE

Croissant (or Crsn for short) is an extensible runtime emulating a weird microcomputer.

## FAQ

### What is this for?

F U N

#### What if I don't enjoy writing assembly that looks like Lisp?

maybe go play fortnite instead

# Architecture

## Registers

- 8 general purpose registers `reg0`-`reg7`
- 8 argument registers `arg0`-`arg7`
- 8 result registers `res0`-`res7`

All registers are 64-bit unsigned integers that can be treated as
signed, if you want to. Overflow is allowed and reported by status flags.

8-, 16-, and 32-bit arithmetic is not currently implemented (only 64-bit), but will be 
added later. Probably. Maybe.

## Status Flags

Arithmetic and other operations set status flags that can be used for conditional jumps.

- Equal … Values are equal
- Lower … A < B
- Greater … A > B
- Zero … Value is zero, buffer is empty, etc.
- Positive … Value is positive
- Negative … Value is negative
- Overflow … Arithmetic overflow or underflow, buffer underflow, etc.
- Invalid … Invalid arguments for an instruction
- Carry … Arithmetic carry *this is not currently used for anything*

### Status Tests

These keywords (among others) are used in conditional branches to specify flag tests:

- `eq` … Equal,
- `ne` … NotEqual,
- `z` … Zero,
- `nz` … NotZero,
- `lt` … Lower,
- `le` … LowerOrEqual,
- `gt` … Greater,
- `ge` … GreaterOrEqual,
- `pos` … Positive,
- `neg` … Negative,
- `npos` … NonPositive,
- `nneg` … NonNegative,
- `c` … Carry,
- `nc` … NotCarry,
- `valid` … Valid,
- `inval` … Invalid,
- `ov` … Overflow,
- `nov` … NotOverflow,

# Syntax

*The syntax is very much subject to change at the moment. The format described here
is valid at the time this file is added to version control.*

Instructions are written using S-expressions, because they are easy to parse
and everyone loves Lisp.

A program has this format:

```
(
    (RoutineName Instructions…)
    …
)
```

Instructions are written like this:

```
(Keyword Args… ConditionalBranches…)
```

Args are either:
- one of the registers (`reg0`, `arg3` etc)
- `_` to discard an output value
- a literal value (decimal, hex or binary)
- label or routine name
- condition flag keyword
- anything else an extension supports...

Conditonal branches are written like this:

```
(Condition? Instructions…)
```

If there is more than one conditional branch chained to an instruction,
then only one branch is taken - there is no fall-through. The definition order
is preserved, i.e. if the `inval` flag is to be checked, it should be done 
before checking e.g. `nz`, which is, incidentally, true by default,
because all flags start cleared.

Example routine to calculate the factorial of `arg0`:

```
(fac
    (cmp arg0 2
        (eq? (ret 2)))
    (sub r0 arg0 1)
    (call fac r0)
    (mul r0 arg0 res0)
    (ret r0)
)
```

# Instruction Set

Crsn instruction set is composed of extensions.
 
Extensions can also define special syntax for their instructions, so long as it's valid S-expressions.

## Labels, jumps and barriers

These are defined as part of the built-in instruction set (see below).

- Barrier - marks the boundary between routines to prevent overrun. Cannot be jumped across.
- Local labels - can be jumped to within the same routine, both forward and backward.
- Far labels - can be jumped to from any place in the code using a far jump (disregarding barriers).
  This is a very cursed functionality that may or may not have some valid use case.
- Skips - cannot cross a barrier, similar to a local label but without explicitly defining a label.

  Skipping across conditional branches may have *surprising results* - conditional branches are expanded 
  to series of simple and conditional skips by the assembler. Only use skips if you really know what you're doing. 
  Jumping to a label is always a safer choice.

## Built-in Instructions

```
; Do nothing
(nop)

; Stop execution
(halt)

; Mark a jump target.
(:LABEL)

; Mark a far jump target (can be jumped to from another routine).
; This label is preserved in optimized code.
(far :LABEL)

; Jump to a label
(j :LABEL)

; Jump to a label if a flag is set
(jif COND :LABEL)

; Jump to a label that can be in another function
(fj :LABEL)

; Far jump to a label if a flag is set
(fjif COND :LABEL)

; Skip backward or forward
(s COUNT)

; Skip if a flag is set
(sif COND COUNT)

; Mark a routine entry point (call target).
(routine NAME)

; Call a routine with arguments.
; The arguments are passed as argX. Return values are stored in resX registers.
(call ROUTINE ARGS…)

; Exit the current routine with return values
(ret VALS…)

; Deny jumps, skips and run across this address, producing a run-time fault with a message.
(barrier)
(barrier "message text")

; Generate a run-time fault with a debugger message
(fault)
(fault "message text")

; Copy value
(ld DST SRC)

; Store status flags to a register
(sst DST)

; Load status flags from a register
(sld SRC)
```

## Arithmetic Module

This module makes heavy use of status flags.

Many instructions have two forms:
- 3 args ... explicit source and destination
- 2 args ... destination is also used as the first argument

```
; Test properties of a value - zero, positive, negative
(tst SRC)

; Compare two values
(cmp A B)

; Add A+B
(add DST A B)
(add DST B)

; Subtract A-B
(sub DST A B)
(sub DST B)

; Multiply A*B
(mul DST A B)
(mul DST B)

; Divide A/B
(div DST A B)
(div DST B)

; Divide and get remainder
; Both DST and REM are output registers
(divr DST REM A B)
(divr DST REM B)

; Get remainder A%B
; This is equivalent to (divr _ REM A B),
; except status flags are updated by the remainder value
(mod DST A B)
(mod DST B)

; AND A&B
(and DST A B)
(and DST B)

; OR A|B
(or DST A B)
(or DST B)

; XOR A&B
(xor DST A B)
(xor DST B)

; CPL ~A (negate all bits)
(cpl DST A)
(cpl DST)

; Rotate right (wrap around)
(ror DST A B)
(ror DST B)

; Rotate left (wrap around)
(rol DST A B)
(rol DST B)

; Logical shift right (fill with zeros)
(lsr DST A B)
(lsr DST B)

; Logical shift left (fill with zeros)
(lsl DST A B)
(lsl DST B)

; Arithmetic shift right (copy sign bit)
(asr DST A B)
(asr DST B)

; Arithmetic shift left (this is identical to `lsl`, added for completeness)
(asl DST A B)
(asl DST B)
```