# Uxn
A [stack-based VM](https://wiki.xxiivv.com/site/uxn.html), written in ANSI C.
## Setup
If you wish to build your own emulator, you can create a new instance of Uxn like:
```
#include "uxn.h"
Uxn u;
if(!bootuxn(&u))
return error("Boot", "Failed");
if(!loaduxn(&u, argv[1]))
return error("Load", "Failed");
if(!init())
return error("Init", "Failed");
evaluxn(u, u->vreset); /* Once on start */
evaluxn(u, u->vframe); /* Each frame
```
## Assembly Syntax
- `ADD`, an opcode.
- `@label`, assign the current address to a label.
- `;variable 2`, assign an address to a label automatically.
- `:const 1a2b`, assign an address to a label manually.
- `¯o { x 2 y 2 }`, define a macro named `macro`.
- `.address`, push label address to memory.
- `,literal`, push label address to stack, prefixed with `LIT LEN`.
- `#1a`, a literal byte/short.
- `+1a`, a literal signed byte/short.
- `-1a`, a literal signed byte/short(negative).
- `=label`, helper to STR, equivalent to `,label STR`, or `label STR2`.
- `~label`, helper to LDR, equivalent to `,label LDR2`, or `,label LDR2`.
- `|0010`, move to position in the program.
- `( comment )`, toggle parsing on/off.
- `[ 0123 abcd ]`, write shorts to memory.
- `[ Hello World ]`, write text to memory.
### Operator modes
- `#1234 #0001 ADD2`, 16-bits operators have the short flag `2`.
- `#12 #11 GTH JMP?`, conditional operators have the cond flag `?`.
- `+21 -03 MULS`, signed operators have the cond flag `S`.
- `ADDS2?`, modes can be combined.
```
( hello world )
&Console { pad 8 char 1 byte 1 short 2 }
|0100 @RESET
,text1 ,print-label JSR2
,text2 ,print-label JSR2
#ab =CNSL.byte
#cdef =CNSL.short
BRK
@print-label ( text )
@print-label-loop NOP
( send ) DUP2 LDR =CNSL.char
( incr ) #0001 ADD2
DUP2 LDR #00 NEQ ^print-label-loop MUL JMPS
POP2
RTS
@text1 [ Hello 20 World 0a00 ] ( store text with a linebreak and null byte )
@text2 [ Welcome 20 to 20 UxnVM 0a00 ]
|c000 @FRAME
|d000 @ERROR
|FF00 ;CNSL Console
|FFF0 .RESET .FRAME .ERROR ( vectors )
|FFF8 [ 13fd 1ef3 1bf2 ] ( palette )
```
## TODOs
### OS Boot Disk
- Load external disk in disk2
- Build hex editor
### Assembler
- Includes
- Defines
- Jump relative
- Local loops
- Jump helpers
NOTE: OPCODES should not be relative, but there should be a relative accessor for addresses, like:
$relative_name JMP
## Notes
### Conditional Jumping
I've considered automatically popping an amount of items from the stack equal to the offset between the opcode's push/pop to make the stack length more predictable, and making the pattern JMP? POP2 unecessary, but that idea would make DUP? unusable. That change was reverted.
## Palettes
- `[ 6a03 4a0d aa0c ]`, purple/cyan
- `[ a1f3 a14d a16c ]`, grey-pink/teal
## 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/