Why doesn't the Performa 630 (and friends) support 1024x768?

[mg.man]

Hey folks... spawning this query off from the discussion over on @-SE40- 's Takky Mac thread :

https://tinkerdifferent.com/threads/a-takky-mac.1363/

According to multiple sources - including https://everymac.com/systems/apple/mac_performa/specs/mac_performa_630.html - the Performa 630 has 1Mb of VRAM. According to other sources - including https://www.applerepairmanuals.com/the_manuals_are_in_here/CardCompatibilityVideo.pdf - some early Macs with 1Mb of VRAM [of note, the LC 475] can support 1024x768 x 256 colors. So... why can't the Performa 630 (and friends)? Is this because some of the VRAM is being reserved for TV/Video options?

Curious additional note... in the Apple doc above, I see the LC 630 reputedly does 1152x870?

Thoughts / comments?

 

[Trash80toG4]

There's another table on p.17 where the footnote explains the discrepancy:

c. The Macintosh 630 family computers can use larger monitors such as the Multiple
Scan 17 and 20. However the maximum resolution is 832 x 624, and at this
resolution they cannot support TV/video.

Very interesting point you bring up about VRAM being reserved for TV/Video options.

Frame buffering is quirky on those machines when it comes to what's in the TV-Tuner window. It comes up as an empty box when you do a screenshot yielding the Frame Buffer's contents. That would support buffered VRAM (DRAM) setup for TV/Video output I think?

As the IIsi Memory Controller loses the first MB of DRAM buffered for use by the Video controller, so the Video Controller of the TV/Tuner Series* would buffer V-DRAM for use by that card. There may be indication of such in the Block Diagram:

The two feeds to Valkyrie and two DMA paths from there to VDRAM are curious indeed. We'll need to take a close look at the Developer Notes for the series. No time for that now.

I'm going to say Video Controller development for the series would be the reason behind the limitations? My bet is on Valkyrie being based upon the frame buffer section of MEMCjr in the Quadra 605/LC475.

Quadra 605/LC475 Block Diagram:

LC series already had support for 640x480@60Hz VGA? Valkyrie added 800x600@60Hz SVGA to VGA output, both supporting TV/Tuner windowing.

PowerMac 6400/5400 Block Diagram shows more detail.

Video Controller development extends to Valkyrie-AR which will be interesting to contrast with Valkyrie as it doesn't appear to offer any new features? Best guess there would be that it was Valkyrie (NuBus Architecture) simply tweaked to support PCI in the Alchemy Architecture?

Thinking would be that -AR was a holdover while Apple bodged the flaky Gazelle Architecture together with its faster system bus? Apple integrated real Graphics Chipsets (ATI developed as opposed to Apple's in house ASIC designs) into the full line at about this point? It's still a custom controller with extended QuickDraw Acceleration support, but an escapee from Apple's NIH Syndrome corral.

Support for 2MB of VRAM and 1024x768@60Hz XGA finally arrived on the TV-Tuner series scene.

PowerMac 6500/5500 Block Diagram:

Anyhoo, that'd be my Video Controller development theory overlay, overly complicating your Vampire Video II theory?


*TV-Tuner Series would be what I call anything with a slot for that crazy card.

 

[-SE40-]

Hello @Trash80toG4 ,

Thats a great explanation on your theory.
Seems plausible to me, but I am no scientist
Much appreciated!

Greetings, Chris.