C64: For investigation - Could this prevent issues?

[pushead]

I've noticed that the function enablerasterirq() compiles to:

	; Enable raster IRQ
	lda $d01a
	ora #$01
	sta $d01a
	lda #$1B
	sta $d011

This introduces two potential issues:

• It turns the screen on at next vertical blank. So if you've forced the screen off it will be re-enabled at next vertical blank.
• It clears the HI bit (bit#8) which identifies which rasterline to generate an interrupt. This will prevent you from having interrupts above rasterline $ff.

I've got a suggestion on how to come around this potential problem. I guess it could bring havoc to older TRSE demos which use the following functions, but here it goes:

1. Remove the writing to $d011 from the enablerasterirq() funtion. So that the revised function would be only like this:

	; Enable raster IRQ
	lda $d01a
	ora #$01
	sta $d01a

2. Include the writing to $d011 in the funtction rasterirq(IRQ_HANDLER(),RASTERPOSITION,KERNAL/BASIC/IO). When the value of 'RASTERPOSTION' is equal or greater than $100 then bit #7 of $d011 is set. If the value of 'RASTERPOSITION' is less than $100 the bit #7 is cleared. NOTE: It could be that this is already the intention of TRSE. I notice that when the value is >$100 then the function includes a 'ldy #$01' but I can't see that this value is used anywhere.

Suggestion:

With a RASTERPOSITION =$FF

	lda #$ff
	sta $d012
	lda #<IRQ_HANDLER
	sta $fffe
	lda #>IRQ_HANDLER
	sta $ffff
	lda	$d011
	and	#%01111111
	sta $d011

With a RASTERPOSITION =$102

	lda #$02
	sta $d012
	lda #<IRQ_HANDLER
	sta $fffe
	lda #>IRQ_HANDLER
	sta $ffff
	lda	$d011
	ora	#%10000000
	sta $d011