Croissant Runtime
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# 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 `r0`-`r7`
- 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.
Colors use the `RRGGBB` hex format.
If input events are required, then make sure to periodically call `(sc-blit)` or `(sc-poll)`.
This may not be needed if the auto-blit function is enabled and the display is regularly written.
The default settings are 60 FPS and auto-blit enabled.
NOTE: Logging can significantly reduce crsn run speed.
Make sure the log level is at not set to "trace" when you need high-speed updates,
such as animations.
```
; Initialize the screen (opens a window)
(sc-init WIDTH HEIGHT)
; Erase the screen (fill with black)
(sc-erase)
; Fill with a custom color
(sc-erase 0xFF00FF)
; Set pixel color
(sc-px X Y COLOR)
; Set screen option
; 1 ... auto-blit (blit automatically on pixel write when needed to achieve the target FPS)
; 2 ... frame rate
(sc-opt OPTION VALUE)
; Blit (render the pixel buffer).
; This function also updates key and mouse states and handles the window close button
(sc-blit)
; Blit if needed (when the auto-blit function is enabled)
(sc-blit 0)
; Update key and mouse state, handle the window close button
(sc-poll)
; Read mouse position into two registers.
; Sets the overflow flag if the cursour is out of the window
(sc-mouse X Y)
; Check key status. Keys are 0-127. Reads 1 if the key is pressed, 0 if not.
; A list of supported keys can be found in the extension source code.
(sc-key PRESSED KEY)
; Check mouse button state
; 0-left, 1-right, 2-middle
(sc-mbtn PRESSED BTN)
```