768MB in G3 AiO = very slow video performace

[joethezombie]

I had some 256mb sticks in my box 'o ram, so I filled up the AiO with 768MB. After an unusually long black screen at startup (memory check?), I found the video performance very slow. Moving a finder dialog, I could see the background slowly redraw itself, taking about a full second to redraw. This is under 8.1. The system recognized all the memory, and other than the slow background refresh, it seemed everything else was ok. Pulling one of the DIMMs, giving the system 512MB, made the background refresh speedy again. I tried various DIMMs, and the effect wasn't due to a bad one, but always occurred at 768MB. The foreground window seems speedy, so I wonder if this is an OS limitation rather than hardware. I'm gunna try some other versions of Mac OS.

 

[Johnnya101]

Yeah, Im gonna guess thats the problem too. Do you have any OS 9 hard drives to test it with?

 

[ArmorAlley]

From LowEndMac: http://lowendmac.com/1998/beige-power-mac-g3-all-in-one/

...The beige G3 supports 256 MB DIMMs, but they must be built using 128 Mb chips. DIMMs built with 256 Mb chips will work, but the memory controller will only see the first 128 Mb of each chip. Compatible 256 MB DIMMs will have 16 memory chips, 8 on each side...

...RAM: 32 MB (expandable to 768MB, desktop version requires low profile DIMMs), uses 3.3V unbuffered 100 MHz 168-pin SDRAM, 3 sockets accept 8, 16, 32, 64, 128, or 256MB DIMMS...

I don't know why this is the case, or even if it is true, but here is one source that may indicate why your G3 AIO is behaving in a strange way.

 

[joethezombie]

Yeah, I'm using DIMMs with 16 memory chips (8 per side), and the G3 recognizes all of the RAM.  And the system works great with 2 DIMMs installed, showing 512MB.  But when I put in the third one to bring it up to 768, desktop background refresh (and seemingly, only desktop background refresh) is SLOW.  I don't have any drives with OS9, but I just backed up the drive, so now I will try installing a later version of the OS and see what happens.  I'll start with 8.5 then 8.6, and finally 9.

 

[novusgordo]

Dumb question, but... if you're running Classic, do you really need more than half a gig of memory anyway?

 

[joethezombie]

No, but particularly strange nuances like this are interesting!

 

[trag]

This is a long shot, but if you have an ohmmeter, pull your ROM module and test pin 122 to see if it has continuity with ground.

If your ROM doesn't have pin numbers at the ends, hold it so the pins are down and the longest segment from end to notch is on the left. The leftmost pin is now 81 and moving to the right the last pin before the first notch is 110 and just to the right of the first notch is 111. You can count to 122 from there. Note that pins on the back are distinct from pins on the front. Holding it as I described you're looking at the back and pins 81 - 160. If you flip it over you'd be looking at pins 1 - 80.

You can find Ground at pins 82, 95, 123, 133 and some others.

If pin 122 is connected to ground, it disables > 512MB RAM support. Some early machines shipped with this issue and Apple did a repair/replacement program, but I'm pretty sure they just needed a replacement ROM module that doesn't tie 122 to ground.

 

[joethezombie]

Interesting... I'll have a check!

 

[trag]

Interesting... I'll have a check!

Please let us know what you find.

I had heard about the early Beige G3 > 512MB problem, but really just stumbled across this fact about pin 122. When I was building my ROM cirrcuit board to supply Rev. 3 ROMs I examined a bunch of ROM modules and documented the connections on each pin. The Beige G3/AIO module has a few differences from the x500/x600 module but the differences all made sense for various reasons, except that pin 122 is Ground on x500/x600 (and clone) modules but not on Beige modules.

So I built my module with pin 122 grounded and in testing discovered no > 512MB support. Cutting the trace on the module to pin 122 fixes the issue.

Ordinarily a ground pin on the ROM module is a ground pin because the corresponding pin in the socket connects to ground on the logic board. Pin 122 in the socket must connect to some signal line on the logic board and if 122 on the ROM module is tied to ground, the that signal line gets grounded through the ROM module.