Why is the RAM limit on PMG3s 768MB/1GB?

[Doq]

This is a bit of an odd inquiry. I was researching on memory upgrades for the Beige Restoration Project, and many, many sources cite (unofficially) 768MB of RAM being the maximum on Power Macintosh G3s beiges (1GB for Blue and Whites with the one extra slot), filling all slots with 256MB SDRAM DIMMs.

But I know that 512MB SDRAM DIMMs exist and can be easily had on the internet or in person. So why is the unofficial cap 768/1024MB? Is there a technical reason for this? Is it compatibility? I'm considering picking up 512MB DIMMs because it should work-- it would be compatible on paper, though running at a slower speed depending on the DIMMs-- but if it doesn't maybe I should steer clear and stick with a 256MB set.

But if 512s don't work, why does it not work?

 

[B S Magnet]

This is a bit of an odd inquiry. I was researching on memory upgrades for the Beige Restoration Project, and many, many sources cite (unofficially) 768MB of RAM being the maximum on Power Macintosh G3s beiges (1GB for Blue and Whites with the one extra slot), filling all slots with 256MB SDRAM DIMMs.

But I know that 512MB SDRAM DIMMs exist and can be easily had on the internet or in person. So why is the unofficial cap 768/1024MB? Is there a technical reason for this? Is it compatibility? I'm considering picking up 512MB DIMMs because it should work-- it would be compatible on paper, though running at a slower speed depending on the DIMMs-- but if it doesn't maybe I should steer clear and stick with a 256MB set.

But if 512s don't work, why does it not work?

I don’t have a definitive answer, but I have a pair of hypotheses. One is more plausible than the other.

The first, less plausible, may be open firmware-related:

The beige G3s were the last of the Old World models, whereas the Blue & White was among the very first of the New World Macs (with components of the Old World systems still in use). There might not be accommodations for memory mapping beyond 1GB on Old World systems.

But this hypothesis falls short, as the B&W is, indeed, the first Power Mac without a gestalt ID and the first to use the New World model syntax still around today.

This brings up the second hypothesis:

The PCI-based Macs were not designed with a means to accommodate RAM modules more dense than 256MB per stick. The Blue & White models, as with the Yikes! G4, all capped at 1GB limits, despite all of these having four RAM slots. The AGP-based Yosemite Power Mac G4s, released concurrently with Yikes!, but with a completely redesigned logic board, can accommodate up to 2GB (though in Mac OS/OS X, only 1.5GB is recognized).

When looking to the laptop offerings from 1999, the Lombard PowerBook G3 shipped with PCI graphics and the means to support up to 512MB across two slots (2x256), but not 2x512MB (as with the later, AGP-based Pismo); the iBook G3 Rev. A, released two months and change later, was the first Mac to ship with an AGP bus and, unlike the Lombard, can accept a 512MB SO-DIMM in its single slot.

So if I were to hazard an educated guess, the PCI/AGP-based architecture of the northbridge probably has something to do with the limits. Beyond that, someone like dosdude1 or someone with similar, applied engineer mindedness may be able to verify and/or expand on this.

 

[VivienM]

This is a bit of an odd inquiry. I was researching on memory upgrades for the Beige Restoration Project, and many, many sources cite (unofficially) 768MB of RAM being the maximum on Power Macintosh G3s beiges (1GB for Blue and Whites with the one extra slot), filling all slots with 256MB SDRAM DIMMs.

But I know that 512MB SDRAM DIMMs exist and can be easily had on the internet or in person. So why is the unofficial cap 768/1024MB? Is there a technical reason for this? Is it compatibility? I'm considering picking up 512MB DIMMs because it should work-- it would be compatible on paper, though running at a slower speed depending on the DIMMs-- but if it doesn't maybe I should steer clear and stick with a 256MB set.

But if 512s don't work, why does it not work?

You might want to broaden the scope of your inquiry, because I think the same thing applies in x86 Intel land. A 440BX chipset will do 4x256 megs of SDRAM, an i815 will do 2x256 megs of SDRAM. Neither one is believed to support 512MB SDRAM DIMMs.

I am not an electrical engineer, nor do I play one on YouTube, but my guess is that you need specific hardware features to support specific amounts of RAM. Take, say, the 128K Mac where you could unsolder the chips and solder chips for 512K of RAM - that was only possible because Burrell Smith had helpfully put the required traces for 512K on the board (see https://www.folklore.org/StoryView.py?project=Macintosh&story=Diagnostic_Port.txt ). So that means that you would design your board/memory controller/etc based on a reasonable degree of forward thinking.

If I had to guess why they would have picked the design they did, RAM was fairly expensive back in the day. These were machines that shipped with 32-128 megs of RAM. At the time they were in the marketplace, I think only modules up to 128 megs were fairly readily available - I think 256 meg DIMMs became affordable around, oh, late 2001ish. And the life expectancy of these machines was expected to be... ~5 years. And RAM had been expensive for most of the 90s - look at, say, Mac IIs with 8 SIMM slots - upgrading those machines to 128 megs would still have been prohibitively expensive by the time most people were phasing them out (1993-1995). I remember paying $250CAD for 4 megs of 72-pin RAM for a 486 in 1995; that would have been the time at which these memory controllers would have been being designed. If you had wanted to put a 32-meg 72-pin SIMM in your LC475 in 1994, my guess is that the memory might have cost you more than the LC475 itself.

Look at the beige G3 itself - they officially supported 3x64MB modules. My guess is that 64MB modules was the biggest available at the time the machine shipped, and manufacturers typically don't like asserting their product supports a memory capacity that they haven't been able to test. In reality, they designed the memory controller for 3x256, so four times more. If you've already added that much extra margin to your design, why would you bother adding even more to support 3x512?

Also, the other thing I would suggest is to try and figure out when JEDEC standardized 512MB SDRAM DIMMs. It may be that they did so... after all of these memory controllers had been designed. There are plenty of things that use RAM that aren't desktop/notebook computers. And also, typically, laptops, because they were designed later in that era, tend to support higher density modules. You can find 512 meg SDRAM SODIMMs for laptops and printers or 4 gig DDR2 SODIMMs for laptops that work with Intel memory controllers; the same capacities in full-sized DIMMs are far more rare and far less supported.

It's also worth noting that this has continued to be a thing with newer standards. Go and look on eBay for, say, 4 gig DDR2 DIMMs - most will be labelled "AMD-only" because they use a chip configuration that Intel's DDR2 memory controllers did not support - Intel had already moved on to DDR3 by that point. (I believe Intel does support some form of 4GB DDR2 DIMMs in their last DDR2 chipsets, but DIMMs that are compatible with that are far more rare than the 'AMD-only' ones on eBay) But 4 gig DDR2 SODIMMs, while rareish, do work just fine with Intel's later DDR2 laptop chipsets.

 

[joevt]

Early Beige G3 documentation (Gossamer SW ERS and Gossamer HW ERS) from Apple mentions 384MB as the max RAM using 128MB DIMMs.

The memory controller for the Beige G3 is the MPC106 (aka Grackle)
https://www.nxp.com/docs/en/data-sheet/MPC106EC.pdf
https://www.nxp.com/docs/en/reference-manual/MPC106UM.pdf

I'm not sure if there were different versions of Grackle? The SW ERS document mentions "Grackle 4".

The MPC106 documents say it supports 8 banks up to 128 MB each which is 1024 MB.
So can you have two 512 MB DIMMs which are each 4 banks of 128 MB?

 

[joevt]

The MPC106 documents say it supports 8 banks up to 128 MB each which is 1024 MB.

This limit is clear: the MPC106 Memory Boundary Registers described in section 3.2.8.1 shows that the maximum ram address is 0x3fffffff = 1024 MB.

So can you have two 512 MB DIMMs which are each 4 banks of 128 MB?

This is not clear or it's complicated. A solution might require some motherboard modification or a non-standard DIMM?

Gossamer HW ERS says it supports memory modules that conform to the JEDEC specification for a 3.3V unbuffered 64bit 168pin module. It also says the 168pin memory module supports up to 2 banks of memory and not more than 16 memory devices are allowed on each module to limit loading on the memory address bus.

Gossamer HW ERS says GRACKLE has 14 address bits. I'm not sure what that means. I suppose those bits are for row and column. 2^(14+14) = 256MB, but each address is 64 bits because that's how many bits are transferred from a DIMM at once, so that would be 2048MB?
MPC106 doc says 13 address bits and 2 bank select bits (4 banks?) but the second bank select bit also corresponds to the most significant address bit?

You can download various JEDEC standards for 168 pin DIMMs. I'm not sure I can interpret them correctly. It seems the JEDEC Standard No. 21–C doesn't list anything with more than 2 banks. It does show 512MB DIMMs so I guess those have 2 banks of 256MB. It does have a single mention of a 4 bank option for SDRAM using A12 as BS1 but I don't know if that's useful.

 

[Doq]

Thanks for the various answers. It looks like I'll just stick to the 3x256 set after all.