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Date: 2006-01 atari800xl.org comp.sys.atari.8bit
Crosspost: Did the cpu influence the display?  
 
1.  Michael J. Mahon  
 More options Jan 1, 5:17 am
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: "Michael J. Mahon" <mjma@aol.com> -
Date: Sat, 31 Dec 2005 12:17:54 -0800
Local: Sun, Jan 1 2006 5:17 am
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

Linards Ticmanis wrote:
> Alex Freed wrote:

>> I don't agree. No matter what the screen memory layout is, the video
>> circuit will refresh all the DRAM as long as 128 consecutive addresses
>> are accessed (or all 128 combinations of low 7 address bits are used).

> But this has to happen within a certain time frame, right?

As Alex points out, a linear mapping would do this as well.

His point is well taken--the Apple II display mapping is motivated
by the desire to display 40 characters/280 pixels per line while
leaving as few/small holes as possible and using as little logic
as possible.  DRAM refresh is not a primary consideration, since
its requirements are easily met by many mappings.

Nevertheless, the "alternate cycle free" 6502 memory usage,
coupled with a simple "DMA" video generator, negates any perceived
advantage of the Z-80 in a DRAM-based personal computer.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  

"The wastebasket is our most important design
tool--and it is seriously underused."


 
 
2.  Eric Smith  
 More options Jan 11, 11:25 am
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: Eric Smith <e@brouhaha.com> -
Date: 10 Jan 2006 18:25:12 -0800
Local: Wed, Jan 11 2006 11:25 am
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |
"Michael J. Mahon" <mjma@aol.com> writes:

> His point is well taken--the Apple II display mapping is motivated
> by the desire to display 40 characters/280 pixels per line while
> leaving as few/small holes as possible and using as little logic
> as possible.  DRAM refresh is not a primary consideration, since
> its requirements are easily met by many mappings.

And if you install the Video Associate Labs VB3 Microkeyer in your
Apple II or Apple II+, you get some added video modes including linear
mapping.

Wish I still had mine.  Sigh.


 
 
3.  Michael J. Mahon  
 More options Jan 11, 12:02 pm
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: "Michael J. Mahon" <mjma@aol.com> -
Date: Tue, 10 Jan 2006 19:02:13 -0800
Local: Wed, Jan 11 2006 12:02 pm
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

Eric Smith wrote:
> "Michael J. Mahon" <mjma@aol.com> writes:

>>His point is well taken--the Apple II display mapping is motivated
>>by the desire to display 40 characters/280 pixels per line while
>>leaving as few/small holes as possible and using as little logic
>>as possible.  DRAM refresh is not a primary consideration, since
>>its requirements are easily met by many mappings.

> And if you install the Video Associate Labs VB3 Microkeyer in your
> Apple II or Apple II+, you get some added video modes including linear
> mapping.

> Wish I still had mine.  Sigh.

That's a pretty amazing mod, since video generation is so
pervasive in the Apple II design.

How did it do that?

-michael

Music synthesis for 8-bit Apple II's!
Home page:  

"The wastebasket is our most important design
tool--and it is seriously underused."


 
 
4.  Eric Smith  
 More options Jan 11, 1:38 pm
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: Eric Smith <e@brouhaha.com> -
Date: 10 Jan 2006 20:38:30 -0800
Local: Wed, Jan 11 2006 1:38 pm
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

I wrote:
> And if you install the Video Associate Labs VB3 Microkeyer in your
> Apple II or Apple II+, you get some added video modes including linear
> mapping.
Michael J. Mahon wrote:
> That's a pretty amazing mod, since video generation is so
> pervasive in the Apple II design.

> How did it do that?

The VB3 consisted of two boards.  One was a rather long slot card, and
one was a big rectangular card that sat over the power supply.  The
two were connected by a ribbon cable.  You pulled out about a dozen
of the TTL chips in the Apple, and installed small ribbon jumpers between
them and the VB3 boards.  The slot card was mostly digital, and the
card that sat over the power supply was mostly analog.

The main purpose of the VB3 was to act as a gen lock and proc amp, make
the video fully compliant with the NTSC spec (and thus FCC broadcast
requirements) and do simple video overlay and keying.  The linear mapped
hires graphics mode was a bonus; since they replaced the entire video
timing chain anyhow, it was relatively easy to add.

Schematics weren't available, and I wasn't inclined to try to reverse-
engineer it.  I suspect that the most amazing part of the design was
getting the color phase correct.  One of the most serious deviations of
the Apple II video from NTSC spec was that it used 228 cycles of the
color carrier per scan line, rather than 227.5.  Woz did that so that
the color phase would be the same on all lines.  But that doesn't work
if you're generating true NTSC.

Eric


 
 
5.  Michael J. Mahon  
 More options Jan 11, 5:33 pm
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: "Michael J. Mahon" <mjma@aol.com> -
Date: Wed, 11 Jan 2006 00:33:59 -0800
Local: Wed, Jan 11 2006 5:33 pm
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

Thanks for the explanation.

Now that I know it's a genlock card, I understand both the motivation
and the niche market support for such a complex modification.

In 1979-1980, I lived in Austin, TX, and the local TV station used
a Apple equipped with a genlock card for the weather forecast graphics.
Sounds like it might have been this system.  ;-)

-michael

Music synthesis for 8-bit Apple II's!
Home page:  

"The wastebasket is our most important design
tool--and it is seriously underused."


 
 
6.  KMK  
 More options Jan 1, 8:13 am
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: KMK <d@obta.uw.edu.pl> -
Date: Sun, 01 Jan 2006 00:13:44 +0100
Local: Sun, Jan 1 2006 8:13 am
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |
A> Of course it is possible to have 40 (or say 80) chars per line and still

> have linear addressing, like in

Atari?

KMK


 
 
7.  Lyrical Nanoha  
 More options Jan 1, 10:46 am
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: Lyrical Nanoha <LyricalNan@dosius.net> -
Date: Sun, 01 Jan 2006 01:46:12 GMT
Local: Sun, Jan 1 2006 10:46 am
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

On Sun, 1 Jan 2006, KMK wrote:
> A> Of course it is possible to have 40 (or say 80) chars per line and still
>> have linear addressing, like in

> Atari?

> KMK

Commodore PET/CBM, 64 and Plus/4?  All of them had linear addressable
40x25 text mode.

-uso.


 
 
8.  Sam Gillett  
 More options Jan 1, 11:40 am
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: "Sam Gillett" <samgillettnos@diespammermsn.com> -
Date: Sun, 01 Jan 2006 02:40:16 GMT
Local: Sun, Jan 1 2006 11:40 am
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

"Lyrical Nanoha" <LyricalNan@dosius.net> wrote ...

> On Sun, 1 Jan 2006, KMK wrote:

>> A> Of course it is possible to have 40 (or say 80) chars per line and
>> still
>>> have linear addressing, like in

>> Atari?

>> KMK

> Commodore PET/CBM, 64 and Plus/4?  All of them had linear addressable 40x25
> text mode.

I started to point that out several times, but thought I would wait to see
how long it took for someone else to do it.    ;-)    Thanks for restoring my
faith in Commodore users!    :-)

I would like to add the 40 column mode of the C128 to your list.  The C128's
80 column mode may not count as the 80 column VDC used its own RAM, much like
the PC video cards of a decade ago.
--
Best regards,

Sam Gillett

Change is inevitable,
except from vending machines!


 
 
9.  Lyrical Nanoha  
 More options Jan 1, 12:28 pm
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: Lyrical Nanoha <LyricalNan@dosius.net> -
Date: Sun, 01 Jan 2006 03:28:47 GMT
Local: Sun, Jan 1 2006 12:28 pm
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

LOL, I'm not actually a Commodore user. :P

-uso.


 
 
10.  Mike Wynne  
 More options Jan 1, 7:22 pm
Newsgroups: comp.sys.sinclair, comp.sys.tandy, comp.sys.atari.8bit, comp.sys.apple2, comp.sys.cbm
From: "Mike Wynne" <mikey@hotmail.com> -
Date: Sun, 1 Jan 2006 10:22:31 -0000
Local: Sun, Jan 1 2006 7:22 pm
Subject: Re: Crosspost: Did the cpu influence the display?
| | | | | |

"Sam Gillett" wrote...
> "Lyrical Nanoha" wrote ...
>>>> Of course it is possible to have 40 (or say 80) chars per line and
>>>> still have linear addressing, like in
>>> Atari?
>> Commodore PET/CBM, 64 and Plus/4?  All of them had linear addressable
>> 40x25
>> text mode.
> I started to point that out several times, but thought I would wait to see
> how long it took for someone else to do it.    ;-)    Thanks for restoring
> my
> faith in Commodore users!    :-)

But you guys missed out the important part of the original quote:

"Of course it is possible to have 40 (or say 80) chars per line and still
have linear addressing, like in a VGA card, *but that requires a lot more
hardware.*"

In the case of the C16/+4 a custom TED Chip, the C64 uses the custom VIC-II,
the Atari it's custom Antic chip.  Try designing a video circuit in discrete
logic (Like the Apple II) then tell me how much more hardware is required to
obtain a linear 40 or 80 column mode.

MikeW


 

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