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Forums > C64 Coding > $39FF instead of $3FFF as alternative bit-pattern for open border
2016-10-11 14:28
YPS

Registered: Oct 2012
Posts: 2
$39FF instead of $3FFF as alternative bit-pattern for open border

I thought it's ALWAYS the last byte of the VIC-bank, that is displayed in the open border area (upper and lower).
But with "Extended Backgroundcolor Mode" on, the value of $39FF (of the VIC-Bank) is shown instead.
A usecase might be, that you can use all spritepointer together with the highest videoram - with "Extended Backgroundcolor Mode" off, the pointer of sprite 7 would be displayed as garbage in the border. Of course, there are solutions to avoid this, but maybe someone else find this usefull.
 
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2016-10-11 16:11
JackAsser

Registered: Jun 2002
Posts: 1217
Quote: I thought it's ALWAYS the last byte of the VIC-bank, that is displayed in the open border area (upper and lower).
But with "Extended Backgroundcolor Mode" on, the value of $39FF (of the VIC-Bank) is shown instead.
A usecase might be, that you can use all spritepointer together with the highest videoram - with "Extended Backgroundcolor Mode" off, the pointer of sprite 7 would be displayed as garbage in the border. Of course, there are solutions to avoid this, but maybe someone else find this usefull.


A classic use case is to avoid having the IRQ-pointer ($ffff) end up in the idle-pattern...

A bit off topic:
But if that still is a problem, like it was for me one time, I resorted to KERNEL-ROM pointers and put $00 in $ffff (RAM) since the VIC always read RAM but the CPU reads ROM in that case.
2016-10-11 16:41
Radiant

Registered: Sep 2004
Posts: 623
Quoting JackAsser
A bit off topic:
But if that still is a problem, like it was for me one time, I resorted to KERNEL-ROM pointers and put $00 in $ffff (RAM) since the VIC always read RAM but the CPU reads ROM in that case.

Or just put the IRQ handler in zeropage. ;-)
2016-10-11 19:30
Peiselulli

Registered: Oct 2006
Posts: 65
Or use the NMI instead ..
2016-10-11 19:43
Oswald

Registered: Apr 2002
Posts: 4052
or jmp abs on zeropage..
2016-10-11 23:46
algorithm

Registered: May 2002
Posts: 678
or in the displayed area, change $ffff to your hearts content and then when in the non visible area, change it back to the irq hi pointer. Can be useful in particular if you are not planning on exiting irq during the ghostbyte displaying.
2016-11-03 03:32
mrmedic
Account closed

Registered: Jul 2007
Posts: 4
you can trick the vblank d012 into thinking its no at the end and have full screen.

you have to put a sprite on the end , and it opens it every 12 lines. borderless c64 screen. i done it in a demo for s xpress in grimsby i used the sprites from hawkeye ( that one with the hawns ) it ran across the screen jumping over the scrollers at random times.
2016-11-03 10:02
JackAsser

Registered: Jun 2002
Posts: 1217
Quote: you can trick the vblank d012 into thinking its no at the end and have full screen.

you have to put a sprite on the end , and it opens it every 12 lines. borderless c64 screen. i done it in a demo for s xpress in grimsby i used the sprites from hawkeye ( that one with the hawns ) it ran across the screen jumping over the scrollers at random times.


Wait! What?! (Oo)
2016-11-03 10:10
Groepaz

Registered: Dec 2001
Posts: 8043
*plop*
2016-11-04 16:10
Radiant

Registered: Sep 2004
Posts: 623
LOL!
2016-11-05 19:49
Graham

Registered: Dec 2002
Posts: 990
ECM mode pulls two bits to 0. This is why $3FFF is pulled to $39FF.
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