kpmg3: RAM expansions with GRACKLE based Macs

I have recently bought new RAM for my B&W PowerMac. It's a 256MB module and is "Single Sided" SDRAM, so there are chips only on one side of the board. However, once installed the computer only recognizes 128MB of it. Why won't the G3 recognize this new memory for what it is?

Answer

Although not everybody realizes it, upgrading the RAM in ANY computer is a very delicate and "intimate" upgrade. The proper RAM needs to be installed. Good quality RAM modules with the correct specifications ensures flawless performance with the kind of CPU and, above all, with the Memory Controller chip soldered on the logic board.

The B&W G3 comes with a Memory Controller known as GRACKLE by Apple's engineers but which in fact is the Motorola MPC106 PCI Bridge/Memory Controller chip.

Such chip is capable to support DRAM devices up to 128 megabits in density and up to eight of them per each bank. The DRAM devices are those rectangular chips soldered on the fiber-glass SDRAM module. 8 megabits form 1 megabytes. Thus 128 megabits (aka Mb) x 8 = 128 megabytes (aka MB). Each SDRAM module bears two banks of such DRAM devices; thus 128MB x 2 = 256MB.

If the RAM module you have bought bears DRAM devices with a higher density (e.g.: 256Mb per DRAM device), the MPC106 won't be able to use the extra 128Mb provided by the DRAM device's density.

You can derive the above, by reading through pages 255 and 256 of the above Motorola MPC106 Manual (for which I have provided you the link). Also, the MPC106 addresses the DRAM devices in a "Multiplexing" fashion. If you check on page 256 of the same above Manual, you will see that the MPC106 as it is configured on B&W machines, uses the 12 or 13 row bits to address the DRAM phase; thus, as per the table 6-4 on that page, the MPC106 uses two processor addresses each time. Those processor addresses are in the form of cache memory pages.

If a B&W machine is upgraded with a ZIF G4, then (due to the G4's larger available cache size) it becomes very important that at least one of those two cache pages are released by the memory controller within the next two CPU cycles. That prevents Interrupt Requests (aka IRQs) from other components (both hardware and software) trying to attract the CPU attention, which could literally hang your DRAM query in the middle of it.

Thus, we tend to recommend to users with GRACKLE equipped machines to install SDRAM modules capable of a Cache Latency response of 2-2-2, also known as CAS2. This will provide for the best stability and performance when using a G4 CPU and in the OSX environment.

OSX, based on UNIX, is the first truly multitasking OS to appear on Macs and, due to it's pre-emptive memory protection features, requires fast CAS responses from the SDRAM modules when Multiplexing addressing is present. Otherwise you may easily experience kernel panics and general system instability.

You can fit PC-133 modules on your B&W G3, provided they are backwards compatible with the PC-100 standard, as is this Micron SDRAM Module part n. MT8LSDT3264A(I)-133_ for example, which the MPC106 will use at PC-100 frequency and timing parameters.

I hope not to have confused you and that the above makes sense to you.

Costa

P.S.: The following Helpers have contributed with me on this FAQ:
* Helper Lois Cusick
* Helper Eric
* Helper Gerry StraathofQ
* Helper Shane Scot
* Help