# CROISSANT VIRTUAL MACHINE

Croissant (or Crsn for short) is an extensible runtime emulating a weird microcomputer (or not so micro, that depends on what extensions you install).

## FAQ

### What is this for?

F U N

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

Maybe this is not for you

# Architecture

The runtime is built as a register machine with a stack and status flags.

- All mutable state (registers and status), called "execution frame", is local to the running routine or the root of the program.
- A call pushes the active frame onto a frame stack and a clean frame is created for the callee.
- The frame stack is not accessible to the running program, it is entirely handled by the runtime.
- When a call is made, the new frame's argument registers are pre-filled with arguments passed by the caller.
- Return values are inserted into the callee's frame's result registers before its execution resumes.

## 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-, 32-bit and floating point arithmetic is not currently implemented, 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 currently unused*

### Status tests (conditions)

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.

## Program

A program has this format:

```
(
    ...<instructions and routines>...
)
```

e.g.

```
(
    (ld r0 100)          ; load value into a register
    (:again)             ; a label
    (sub r0 1            ; subtract from a register
        (nz?             ; conditional branch "not zero?"
            (j :again))) ; jump to the label :again
)
```

The same program can be written in a compact form:

```
((ld r0 100)(:again)(sub r0 1 (nz? (j :again))))
```

## Instruction

Instructions are written like this:

```
(<keyword> <args>... <conditional branches>...)
```

### Conditional instructions

All instructions can be made conditional by appending `.<cond>` to the keyword, i.e. `j.ne` means "jump if not equal".
This is used internally by the assembler when translating conditional branches to executable code.

### Instruction arguments

Args are either:
- One of the registers (`reg0`, `arg3` etc)
- Names of constants defined earlier in the program (e.g. `SCREEN_WIDTH`)
- Symbols defined as register aliases (e.g. `x`)
- The "discard register" `_` to discard an output value. That is used when you only care about side effects or status flags.
- Literal values (decimal, hex or binary)
- Label or routine name (e.g. `factorial`, `:again`)
- ...or anything else an installed crsn extension supports

### Conditional branches

Conditonal branches are written like this:

```
(<cond>? <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 most flags are cleared by instructions that affects flags.

## Routines

A routine is defined as:

```
(proc <name>/<arity> instructions...)
```

- `name` is a unique routine name
- `arity` is the number of arguments it takes, e.g. `3`.
  - you can define multiple routines with the same name and different arities, the correct one will be used depending on how it's called

Or, with named arguments:

```
(proc <name> <arguments>... instructions...)
```

Arguments are simply aliases for the argument registers that can then be used inside the routine.

Here is an example routine to calculate the factorial of `arg0`:

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

It can also be written like this:

```
(proc fac num
    ...
)
```

...or by specifying both the arity and argument names:

```
(proc fac/1 num
    ...
)
```
  
# Instruction Set

Crsn instruction set is composed of extensions.
 
Extensions can define new instructions as well as new syntax, so long as it's composed of 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 jump but without explicitly defining a label.
  All local jumps are turned into skips by the assembler.

Skipping across conditional branches may have *surprising results* - conditional branches are expanded 
to a varying number of skips and conditional instructions by the assembler. Only use skips if you really know what you're doing.

Jumping to a label is always safer than a manual skip.

## Built-in Instructions

```
; Do nothing
(nop)

; Stop execution
(halt)

; Mark a jump target.
(:LABEL)
; Numbered labels
(:#NUMBER)

; 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 that can be in another function
(fj :LABEL)

; Skip backward or forward
(s COUNT)

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

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

; Exit the current routine with return values
(ret VALUES...)

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

; Block barriers are used for routines. They are automatically skipped in execution
; and the whole pair can be jumped *across*
(barrier-open LABEL)
(barrier-close LABEL)

; 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)

; Define a register alias. The alias is only valid in the current routine or in the root of the program.
(sym ALIAS REGISTER)

; Define a constant. These are valid in the whole program.
(def NAME VALUE)
```

## 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)

; Delete an object by its handle. Objects are used by some extensions.
(drop @REG)
```

## Stacks Module

This module defines data stacks. Stacks can be shared by routines by passing a handle.

```
; Create a stack. The register then contains the stack handle.
(stack REG)

; Push to a stack (insert to the end)
(push @REG VALUE)

; Pop from a stack (remove from the end)
(pop DST @REG)

; Reverse push to a stack (insert to the beginning)
(rpush @REG VALUE)

; Reverse pop from a stack (remove from the beginning)
(rpop DST @REG)
```

To delete a stack, drop its handle - `(drop @REG)`

## Screen module

This module uses the minifb rust crate to provide a framebuffer with key and mouse input.

Documentation TBD, see the extension's source code or the example programs

.