[G5 DDR2] SPD editing for PC2-4200 in System Profiler

[Manawyrm]

EDIT: Editing the memory ROM to report as 533 MHz (instead of 800 MHz) RAM is a purely cosmetic choice, the actual memory configuration is done before OpenFirmware even runs. PC2-6400 RAM always runs at the correct 533 MHz speed, regardless of the info in system profiler.

Spoiler

Hey folks,

I finally got the chance to buy 8 brand new DDR2 DIMMs to replace my old weird mix of DIMMs I found:

(the top module is the new one)

And while putting 8 of these gave me a beautiful 16GB of RAM, the Mac was only running them at the slow speed of PC2-3200U-288:

I found a dump of the OpenFirmware HwInit routine on here (https://forums.macrumors.com/thread...gans-to-18gb-and-beyond.2482947/post-34604974, thanks @joevt !) and noticed that it can't deal with PC2-6400 (800MHz) RAM, it'll default to the slowest speed of 400MHz (instead of 533MHz).

This is definitely _NOT_ a tutorial, please don't try this at home, buuut:
The SPD EEPROM on those Samsung RAM modules is writable from the system itself!
You could either use a normal PC or even the Mac itself using OF (like I did) to reprogram the SPD EEPROM.

You'll need to dump the SPD EEPROM first, change the value and then recalculate the checksum (there's tools for that online).

https://forums.macrumors.com/thread...gans-to-18gb-and-beyond.2482947/post-34617356 has the info on how to dump the SPD ROMs.

DON'T RUN THIS ON YOUR MEMORY MODULES WITHOUT CHANGING THE CHECKSUM! YOU'LL GET BRICKED MODULES BECAUSE THE CHECKSUM DOESN'T MATCH ANYMORE!
In my case, I ran:

" /u4@0,f8000000/i2c@f8001000" select-dev
h# 100 buffer: buf2
3d buf2 c!  buf2 1 <i2caddr> 9  write-i2c-at drop
fc buf2 c!  buf2 1 <i2caddr> 3f write-i2c-at drop

DON'T RUN THIS ON YOUR MEMORY MODULES WITHOUT CHANGING THE CHECKSUM! YOU'LL GET BRICKED MODULES BECAUSE THE CHECKSUM DOESN'T MATCH ANYMORE!
For each of the 8 memory modules (this is way easier to do over the telnet interface, highly recommended).

That set the tCKmin (byte 9) down from 0x25 (DDR2-800) to 0x3D (DDR2-533) and indeed, that seems to make the OpenFirmware happy now:

So yeah, not for the faint of heart, don't try this at home (unless you have the hardware hacking skills to externally reprogram/fix SPD EEPROMs) and have fun 🙂

Final result and benchmark:

Beautiful!

 

[Matias_]

Do you have any benchmarks to prove that this actually does anything? It has always been my understanding up until this point that this is purely a cosmetic reporting bug, and that they are still running at PC2-4200 (533 MHz).

Some references: https://forums.macrumors.com/threads/powermac-g5-quad-ram.1599796/post-17483804
https://forums.macrumors.com/thread...-reading-ram-speed-wrong.584133/post-14425608

https://forums.macrumors.com/threads/problem-with-overclocking-powerbook-g4-hi-res.2185883/post-27468959

https://forums.macrumors.com/threads/powermac-g5-quad-ram.1599796/post-17472003

EDIT, I have also found two references to "true" PC2-3200 not working:
https://forums.macrumors.com/threads/g5-ram-upgrade.1163224/post-12678927
https://forums.macrumors.com/threads/replace-watercooling-by-air-cooling.1967691/post-22837866

 

[Manawyrm]

Do you have any benchmarks to prove that this actually does anything? It has always been my understanding up until this point that this is purely a cosmetic reporting bug, and that they are still running at PC2-4200 (533 MHz).

Oops, well that's an afternoon wasted 🙂

I failed to notice that Xbench offers a memory performance benchmark and indeed, it's purely cosmetic.
I benchmarked again at the (default, reported as) PC2-3200U-288 SPD EEPROM and it's the absolute exact same score.

That's also good to know! I'll edit the main post appropriately.

 

[joevt]

Read the section 12.10 "Memory Control Registers" section of the "CPC945 Bridge and Memory Controller User Manual".

Which registers does HWInit modify?

Change the SPD. Reboot. Dump the Memory Control Registers.
Repeat for each SPD. Then compare the results. Does anything interesting change depending on the SPD?

 

[joevt]

This thread mentions a couple utilities for parsing SPD info:
https://68kmla.org/bb/threads/g4-mdd-ram-recognised-at-wrong-size.52427/post-595577