uxn/projects/library/load-rom.tal

@load-rom ( filename* -- )
(
	Attempts to load the ROM from filename* and executes it. If the file exists
	and has non-zero length, this function does not return, because the new ROM
	is executing in its place.

	The screen and both stacks are cleared and all the device vectors are
	written to zero as a convenience. All other device bytes are left
	untouched, so they could introduce a device state to the next ROM that
	it's not expecting.
)

	.File/name DEO2
	#0000 .File/offset-hs DEO2
	#0000 .File/offset-ls DEO2

	( return if file can't be found, or zero length )
	#0001 .File/length DEO2
	;&tmp .File/load DEO2
	.File/success DEI2 ORA JMP JMP2r

	( clear wst )
	#ab
	&wst-loop
	POP
	.System/wst STH DEIr STHr ,&wst-loop JCN

	( clear rst )
	LITr ab
	&rst-loop
	POPr
	.System/rst DEI ,&rst-loop JCN

	( clear screen )
	#01 .Screen/auto DEO
	#0000 .Screen/y DEO2
	.Screen/width DEI2 #0007 ADD2 #03 SFT2 #ffff MUL2 STH2
	&screen-yloop
		#0000 .Screen/x DEO2
		STH2kr
		&screen-xloop-bg
			#00 .Screen/sprite DEO
			INC2
			ORAk ,&screen-xloop-bg JCN
		POP2
		#0000 .Screen/x DEO2
		STH2kr
		&screen-xloop-fg
			#40 .Screen/sprite DEO
			INC2
			ORAk ,&screen-xloop-fg JCN
		POP2
		.Screen/y DEI2 #0008 ADD2
		DUP2 .Screen/y DEO2
		.Screen/height DEI2 LTH2 ,&screen-yloop JCN
	POP2r
	#00 .Screen/auto DEO

	( reset device vectors )
	LIT2r 0000 #00
	&device-vector-loop
	DUP2r STHk DEO2r
	#10 ADD
	DUP ,&device-vector-loop JCN
	POP POP2r

	( copy the zero-page-loader into f0-ff )
	;&zero-page-loader LITr f0
	&copy-loop
	LDAk STHkr STZ
	INC2 INCr
	STHkr ,&copy-loop JCN
	POP2 ( leave 00 on return stack )

	( prepare the stack for the zero-page-loader )
	( the more we prepare here in advance, the less we'll have to overwrite )
	STHkr #00fe ( arguments for STZ2 at ff )
	STHkr ( argument for JMP at fe (carry on) )
	DUPk #fcfe ( arguments for STZ2 at fd and JMP (repeat) )
	OVR2 #fafe ( arguments for STZ2 at fd and JMP (repeat) )
	OVR2 #f8fe ( arguments for STZ2 at fd and JMP (repeat) )
	OVR2 #f6fe ( arguments for STZ2 at fd and JMP (repeat) )
	OVR2 #f401 ( arguments for STZ2 at fd, plus an extra 01 )
	STHkr ( first argument for ADD2 )
	.File/success ( argument for DEI2 )
	#0100 .File/load ( arguments for DEO2 )
	#ff00 .File/length DEO2
	#00f0 JMP2

	&zero-page-loader
	( f0 ) DEO2
	( f1 ) DEI2
	( f2 ) ADD2
	( f3 ) &loop DUPr
	( f4 ) STH2k
	( f5 ) STAr
	( f6 ) INC2
	( f7 ) NEQ2k
	( f8 ) ,&loop
	( f9 )
	( fa ) JCN
	( fb ) POPr
	( fc ) POP2
	( fd ) STZ2 ( deletes f4-fd through looping )
	( fe ) JMP
	( ff ) STZ2 ( deletes fe-ff )

	&tmp $1