LORES: different between ROM 1.0 and ROM1.1?

The Oric video chip is not an easy beast to master, so any trick or method that allows to achieve nice visual results is welcome. Don't hesitate to comment (nicely) other people tricks and pictures :)
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Symoon
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LORES: different between ROM 1.0 and ROM1.1?

Post by Symoon » Thu Jul 18, 2019 3:16 am

Maybe this has been spotted and discussed before, but anyway... Did anyone notice that LORES 1 modes, which display the 2nd character set, have a different shape between Oric-1 and Oric Atmos?
Noticed that when testing an unknown little game which runs in LORES.
Atmos:
Lores1_Atmos.png
Lores1_Atmos.png (1.05 KiB) Viewed 599 times
Oric-1:
Lores1_Oric1.png
Lores1_Oric1.png (1.04 KiB) Viewed 599 times

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mikeb
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Re: LORES: different between ROM 1.0 and ROM1.1?

Post by mikeb » Thu Jul 18, 2019 5:57 pm

Yes, this is a long standing feature :)

The character cells are supposed to be an equal 2 x 3 grid for teletext/viewdata blocks

For Oric this means 3 pixels left, 3 pixels right, as there are only 6 bits in a character that become visible pixels (the other 2 bits are for attributes etc.)

xx 000 000
xx 000 111
xx 111 000
xx 111 111

But I think the routines that generate the codes for Oric-1 forgot this, and used the full 8 bits.

0000 0000
0000 1111
1111 0000
1111 1111

Meaning the shapes onscreen are wrong :-

00 0000
00 1111
11 0000
11 1111

So the left hand lo-res cell is too small, the right hand is too big. Off by one pixel.

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Re: LORES: different between ROM 1.0 and ROM1.1?

Post by Symoon » Thu Jul 18, 2019 7:34 pm

Ha ha thanks!
What's curious thought, is that the character generation routine seems to be the same for both ROMs ($F7E0 / $F816)? But I've checked quickly, maybe I'm not looking at the right place.

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Re: LORES: different between ROM 1.0 and ROM1.1?

Post by mikeb » Fri Jul 19, 2019 5:58 pm

No idea how it was fixed, $F263 is the highest address that "Rambling In The ROM" got to in OUM, so that would be where I would look for details.

There must be a subtle difference between the routines (a stray ROL or something) hidden in there that fixes it.

Edit to add: I just compared Leycester Whewell's commented ATMOS ROM disassembly (from the Advanced User Guide) with Bob Maunders un-commented Oric-1 disassembly.

You're right, the routines are the same and are both correct, the problem is GIGO -- Garbage In, Garbage Out.

ATMOS
Code: $F816 to $F860

$F861 Four Data Bytes used by the routine :-
$00 (00 000 000)
$38 (00 111 000)
$07 (00 000 111)
$3F (00 111 111)

These are correct and make allowances for the two first bits being non-visible pixels, so the cells are 3+3 pixels/bits wide.

ORIC:
Code: $F7E0 to $F82A

$F82A Data :-
$00 (00 00 0000)
$F0 (11 11 0000)
$0F (00 00 1111)
$FF (11 11 1111)

These are wrong, and assume that all bits are treated as visible, simply cleaving the byte into 2 nybbles. I've spaced them to show the 2 bits left, 4 bits right offset.

The fix for Oric, (as a post-ROM generation, user correction), is to just ROR every location down one bit, and clear the top two bits just for neatness (AND with $03F)

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