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Forums > C64 Coding > Why does the border collapse when you reset

2019-03-29 09:23

Bones99

Registered: May 2005
Posts: 29

Why does the border collapse when you reset

Silly question maybe, but I'm curious.

Why does the horizontal border collapse when you reset or sys64738 ?

It just sounds like something that might have an interesting story behind it :P

2019-03-29 09:55

Krill

Registered: Apr 2002
Posts: 2825

Mostly guessing here, somebody might have primary sources.

Referring to http://unusedino.de/ec64/technical/aay/c64/romfce2.htm and http://unusedino.de/ec64/technical/aay/c64/vic22.htm, it looks like $d016 was supposed to be set to $ff early on, and bit 5 is described as "Reset-Bit: 1 = Stop VIC (no Video Out, no RAM refresh, no bus access)".

Now, the latter is only true for very early (pre-release) VIC-II models, if stories that bit 5 ever really did anything are to be believed.

Turning off VIC action during reset might have sped up the initialisation phase, but i guess being able to disable RAM refresh from software was considered a bad idea soon after, if only for the RAM check at start-up.

Then, however, $d016 is not set to $ff but to whatever the cartridge check at http://unusedino.de/ec64/technical/aay/c64/romfd02.htm (which might have been added later) leaves in the X register, and that would be anything in 1..5, all of which leave bit 3 at 0, disabling 40-column display and collapsing the sideborders.

So i guess it's mostly a don't-really-care thing, and that Commodore engineers early on got used to the reset "effect" so much they might not even have noticed while working hard until the extremely tight release deadline.

Edit: Or they deliberately left it at collapsing as an easy sign to see whether the reset has been issued while the RAM check delays the "READY." prompt quite a bit.

2019-03-29 11:06

Oswald

Registered: Apr 2002
Posts: 5017

first I thought the border gets opened as its display "collapses" :)

2019-03-29 11:15

Krill

Registered: Apr 2002
Posts: 2825

Nope. Now get back to finding the global border disable bit! :)

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