# Uxn
A stack-based VM, written in ANSI C.
## Build
```
cc uxn.c -std=c89 -Os -DNDEBUG -g0 -s -Wall -Wno-unknown-pragmas -o uxn
```
## Assembly Syntax
### Write
- `;variable`, set a label to an assigned address
- `:const`, set a label to a constant short
- `@label`, set a label to an address
### Read
- `,literal`, push label value to stack
- `.pointer`, read label value
### Special
- `( comment )`, toggle parsing on/off
- `|0010`, move to position in the program
- `"hello`, push literal bytes for word "hello"
- `#04`, a zero-page address, equivalent to `,0004`
### Operator modes
- `,1234 ,0001 ADD^`, 16-bits operators have the short flag `^`.
```
( hello world )
;iterator
:dev1r FFF0
:dev1w FFF1
|0100 @RESET
"hello
@loop
,dev1w STR
,incr JSU ( call incr )
,05 NEQ ,loop ROT JSC
BRK ( RESET )
@incr
,iterator LDR
,01 ADD
,iterator STR
,iterator LDR
RTS
|c000 @FRAME BRK
|d000 @ERROR BRK
|FFFA .RESET .FRAME .ERROR
```
## Mission
### Assembler
- Create a benchmark file
- Implement shorthand operators
- Signed operations
- zero-page address?
### CPU
- Signed operations
- Catch overflow/underflow
- A Three-Way Decision Routine(http://www.6502.org/tutorials/compare_instructions.html)
- Draw pixel to screen
- Detect mouse click
- SDL Layer Emulator
- Build PPU
### Devices
- Devices each have an input byte, an output byte and two request bytes.
## Refs
https://code.9front.org/hg/plan9front/file/a7f9946e238f/sys/src/games/nes/cpu.c
http://www.w3group.de/stable_glossar.html
http://www.emulator101.com/6502-addressing-modes.html
http://forth.works/8f0c04f616b6c34496eb2141785b4454
https://justinmeiners.github.io/lc3-vm/