the problem is that they didn't ship a version with those populated so it's all guesswork I have a set of 256mbit 32x8bit chips on the way to try next in case the issue was that these chips don't match the existing ones also that should hopefully get me to 128MB of onboard storage with only the 4 chips that are populated now so possibly an improvement either way
This was
the very thing which got me to sign up for the MR forums in 2018.
The unused RAM slots was something I wanted to explore on my own, but I lacked the technical know-how and the soldering finesse to attempt it. In the time since, I’ve removed four RAM chips from another, long-dead Rev. C board last year and I set them aside, mindful how I’d need more than just those (such as transistors, capacitors, and resistors related to the memory bus) to have an even chance of breaking that barrier.
I speculated then — and maintain still — that Apple had planned forward back during the 1998–99 board/industrial design process with the prospect of eventually populating those spots on the board as a final revision before moving on to another board and form factor (e.g., a “Rev. D” edition, replete with its own unique colours).
But given the polarizing reactions from Apple faithful at the time that the clamshell iBook form factor looked like a toilet seat, a makeup compact, and a child’s toy, I suss that Apple forwent releasing one more revision in lieu of just rolling out a conventional form factor from May 2001 onward — which we came to know as the basic form factor they stayed with until the move to MacBooks in 2006.
That said, I also don’t know whether Open Firmware version 4.1.7f4 — the final revision for the Rev. C models (the only series I’d ever seriously explore attempting to use those RAM chip locations) — even knows to be aware of a third bank of RAM chips on that board.
In short, there are the physical logistics of finding out how those locations on the board were to be purposed (you’ll need to get your hands on the schematics for the Rev. C iBook clamshell, which are, reportedly, out there, but I’ve been wholly unsuccessful in pinpointing them personally). I also don’t know how the system might respond to simply using higher density chips sourced from, say, a PC66/100/133 SO-DIMM, sharing the same form factor.
Nevertheless, I’ll keep my eye on this thread and on your work, because you’re already much further along already than I ever got and I’m learning with you as you go along. I am also applauding your trial-and-error efforts! ?
EDIT TO ADD: Incidentally, I noticed there isn’t an IC on U34, on either of your board pics. Without schematics or even referring to the board from a ice dual-USB iBook from May 2001 (as I think the generational changes are incremental), it’s not clear what those eight connection points are related to, but a working hypothesis is it might be needed for recognizing, managing, and/or delegating memory tasks to a third bank of onboard RAM.