Conference 7.286::atarist

1292.0. "Horizontal Blank (HBL) Interrupts?" by AIDEV::MISKINIS () Mon Jun 08 1992 13:42

Hi!
 
	I'm trying to use the HBL interrupt, which I understand
	is the IPL level 2 interrupt.
 
	All I want to do is count the screen lines, so another interrupt
	(TIMER A) can change my color palette between specific scan lines.
 
	I tried doing this WITHOUT HBL, by clearing a counter in a VBL
	routine, incrementing the counter in a TIMER A routine, and
	if it hit a certain point, swapping the color palette.  This
	seemed to work, but the point at which it would change the
	palette was not always on the same scan line.  In fact it
	varied +- 3 scan lines, and thus I had a moving bar of color.
 
	After reading the internals book, I see that HBL is implemented,
	but I can't get it to fire.  I'm wondering if the IPL 2 is
	turned on by default.  I tried ORing in 0x0200 to the status
	register, but that didn't seem to help...
 
	ANY AND ALL INFORMATION IS GREATLY APPRECIATED!
 
_John_