Demystifying the VIA shifter

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groepaz
Vic 20 Nerd
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Joined: Wed Aug 25, 2010 5:30 pm

Demystifying the VIA shifter

Postby groepaz » Thu Feb 09, 2017 9:01 pm

yay!

some days ago i have started investigating the VIA a bit, with the intention of fixing VICE and chameleon. for a start, i am looking at the VIC20 for this, simply because its more convenient to run test programs on it than the 1541 drive.

in particular i am interested in the shift register right now - because it is generally not commonly used and documented less than great :)

so, what i am looking for is actual code that uses the shift registers - may it be small code snippets or full blown programs. anything might help with testing and/or improving the emulation (which i have just added - until now VICE didnt have them _at all_).

if someone is interested in the bloody details and perhaps help with test programs - let me know and i will elaborate :)

some tests i have come up with so far are here: https://sourceforge.net/p/vice-emu/code ... 20/via_sr/ - you can easily check yourself in VICE and see what a disaster the emulation is right now =P

eslapion
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Re: Demystifying the VIA shifter

Postby eslapion » Fri Feb 10, 2017 3:03 am

@groepaz
Since it is documented that MOS VIAs and most other made in the 1980s had a bug in this shifter, are you going to test your software on the W65C22s and W65C22N which are reputed to have a fixed bit shifter ?
No one can do everything. Everyone can do something.

groepaz
Vic 20 Nerd
Posts: 597
Joined: Wed Aug 25, 2010 5:30 pm

Re: Demystifying the VIA shifter

Postby groepaz » Fri Feb 10, 2017 8:54 am

since none of my computers came with any of these - no. i'm only interested in reproducing the original behaviour exactly. (and for example find out what exactly those bugs are - which is also documented less than great)

eslapion
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Re: Demystifying the VIA shifter

Postby eslapion » Fri Feb 10, 2017 11:49 am

groepaz wrote:... find out what exactly those bugs are - which is also documented less than great

Very true.

If I may suggest; having an IC that works the way it should have in the first place could help you document what doesn't work the way it should on MOS VIAs.
No one can do everything. Everyone can do something.

groepaz
Vic 20 Nerd
Posts: 597
Joined: Wed Aug 25, 2010 5:30 pm

Re: Demystifying the VIA shifter

Postby groepaz » Fri Feb 10, 2017 1:11 pm

i dont think so - then i would have to test two chips in detail instead of one :)

Bobbi
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Occupation: Programmer

Re: Demystifying the VIA shifter

Postby Bobbi » Thu Feb 16, 2017 4:06 pm

I'm up for some of the gruesome details if you care to write it up here.


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