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 ^.
• ,12 ,11 GTH JMP?, conditional operators have the cond flag ?.

( hello world )

;iterator
:dev1r FFF0
:dev1w FFF1

|0100 @RESET

@word1 "hello_word ( len: 0x0b )

@loop
	,dev1w STR ( write to stdout )
	,incr JSR ( increment itr )
	,word1 ,strlen JSR ( get strlen )
	NEQ ,loop ROT JSR? ( loop != strlen )

BRK

@strlen
	,0001 ADD^ LDR
	RTS

@incr
	,iterator LDR
	,01 ADD
	,iterator STR 
	,iterator LDR
	RTS

|c000 @FRAME BRK 
|d000 @ERROR BRK 
|FFFA .RESET .FRAME .ERROR

Mission

Assembler

• Implement shorthand operators
• Signed operations

CPU

• Signed operations
• Catch overflow/underflow
• A Three-Way Decision Routine(http://www.6502.org/tutorials/compare_instructions.html)
• Draw pixel to screen
• Redo overflow/underflow mappping
• 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/