An XLR8YourMac reader may have found a solution to some PowerBook G4 models not seeing the lower RAM slot (albeit a very clunky solution):
The lower memory slot on the PowerBook G4 is not fried. It was tested by starting up with a single DIMM in the lower slot. However, it still will not recognize the lower DIMM if there is memory in both the upper & lower slots. This may be different to what others are experiencing, so your mileage may vary.
* THIS INVOLVES THE USAGE OF OPEN FIRMWARE, YOU COULD PERMANENTLY DAMAGE YOUR MACHINE FOLLOWING THIS PRODECURE, SO PERFORM AT YOUR OWN RISK *
[The Powerbook G4] is physically detecting the RAM in Open Firmware ... simply not mapping it to any address space.
Logic dictates that if the ram isn't properly mapped to an address, it will not be visible in Mac OSX. [You can] artificially reduce the size of your system RAM in open firmware by changing these mappings.
1) A note about module sizes. sizes appear to be in hex.
- 10000000 = 256mb
- 20000000 = 512mb
- 40000000 = 1gb
- and so forth...
You need to use the sizes above to map your RAM to relavent address space.
2) Next boot into open firmware. Do this by holding cmd+opt+o+f just after the boot chime of your Mac.
3) This will bring you to the white open firmware prompt "0>".
From here you need to navigate to the memory device tree by typing: 0>dev/memory
4) To display the details about the current memory mappings type:
5) This is an example of how you can map a PowerBook's memory modules to a different address space.
0> 0 encode-int 10000000 encode-int encode+
2> 10000000 encode-int 40000000 encode-int encode+
2> " reg" delete-property
2> " reg" property
The first line loads onto the Open Firmware stack - the location of the 1st memory module. In this case it starts at address space 0 and is 10000000 (256mb) in size.
The second [command] loads the 2nd (lower) memory module onto the stack starting at address 10000000 (directly after the first upper module) and is 40000000 (1gb) in size.
The third line encodes the two lines above together on the stack.
The fourth line deletes the current address space mappings (the contents of the " reg" property).
The fifth pops the new memory mappings off the stack and onto the " reg" property.
Finally, the last line, starts the boot process for your Mac.
Check system profiler - both DIMMs should be registering now.
... it is not persistant, when you reboot your machine these mappings will be lost and you will be back to square one.
XLR8YourMac is a favorite site of mine ... but it is really clunky to navigate or link to articles there ... Mike Breeden (XLR8YourMac's author) also doesn't edit ANY of his posts for grammar or proper punctuation - thus the reason for the repost with a substantial number of corrections.
I've had this happen to a number of my clients on their PowerBook G4s. I doubt I would do this as a solution as a tech, but rather inform the customer of the potential "jigger".
This really should be addressed by Apple now that someone has discovered this issue - a lot of Apple insiders read XLR8YourMac so I imagine now that they see the problem there is the possibility of it being addressed in a firmware update. I'll also monitor XLR8YourMac for a possible "better reader solution".