Uxn

A stack-based VM, 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.
• &macro { 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/