Amulet

A fantasy console. This is a work in progress, spec is not finalised.

Overview

Stack-machine, 8-bit words, 16-bit addresses? 20-bit? (MMU allows access to 1MB). PPU with internal VRAM. Tile based display.

Instruction set

Opcodes

category	hex	name	desc
system	00	ZZZ	no operation
	01	SYS	SYS n system call
	0F	HLT	break/halt
stack operations	20	POP	drop/pop (a -- )
	21	DUP	duplicate (a -- a a)
	22	SWP	swap (a b -- b a)
	23	OVR	over (a b -- a b a)
	24	ROT	rotate left (a b c -- b c a) for -ROT use rot rot
	??	NIP	nip (a b --) aka SWP POP
	??	TUC	tuck (a b -- b a b) aka SWP OVR
literals & addressing	30	ST1	store (a lo hi -- ) could be named POKE
	31	ST2	store 16
	32	LD1	load (lo hi -- a) could be named PEEK
	33	LD2	load 16 bits lo hi
	34	IM1	literal, push next byte to stack ( -- a)
	35	IM2	literal, push next 2 bytes to stack ( -- lo hi) could be named PICK
alu (8-bit unless noted)	40	ADD	add (a b -- c)
	41	SUB	subtract (a b -- c)
	42	AND	bitwise and (a b -- c)
	43	IOR	bitwise or (a b -- c)
	44	XOR	bitwise xor (a b -- c)
	45	NOT	bitwise not `(a -- ~a)
	??	BSL	bit shift left
	??	BRL	bit roll left
control flow	50	CAL	perform subroutine call to address in next 2 bytes maybe JSR jump subroutine?
	51	RTN	return non-zero (cond -- )
	52	RTZ	return zero (cond -- )
	53	RET	unconditional return
	54	HOP	rel jump 8 bit (next byte)
	55	SKP	indirect jump (lo hi -- )
	56	JMP	jump 16 bit (next 2 bytes)
undecided	??	RSW	push/write to return stack (lo hi --)
undecided	??	RSR	pop/read from return stack to data stack ( -- a)
io & timing	60	VBL	blocks until VBLANK flag set (then clears)
	??	PUT	PUT n write val to io port n? (val -- )
	??	GET	GET n get byte from port number push to stack

37 SYS n emulator syscall hook (dev only) 40 IN port ( -- val8) 41 OUT port (val8 --) 42 WAIT_VBL blocks until VBLANK flag set (and clears it)