SaRuMan-128 C128 static RAM for DRAM replacement board

SRAM-replacement is the only known fix for the VSP-bug. I have a similar replacement-board in my C128D that has been a VSP-bug-superspreader (so to speak) and since the SRAM-replacement is VSP-safe.

Ray Carlsen indicates he tested the product and he is very satisfied with it.

A very nicely assembled little unit! Looks pretty slick when mounted.

I am curious as to how adaptable the design is. One of the more interesting types of C128 modification is to personally add some or all of the missing onboard RAM banks.

(As we know, the system ROMs have complete support for RAM banks 0-3 even though they shipped with only banks 0-1 installed, and even the (8722? I think?) MMU includes everything needed to operate them... except for the two bank-select lines, which were not brought out on their own pins because it saved a lot of money to stick with a standard 40-pin DIP case.)

The simplest RAM-expansion mod I'm aware of piggybacks a second MMU and layer of DRAMs on the first, with a couple of the added MMU's address lines swapped; that way the original MMU does its job most of the time, but whenever you activate banks 2 or 3, the extra MMU sees it as bank 0 or 1 and activates one of its bank-select lines... which are connected to the extra layer of DRAM chips, which are used as banks 2-3.

If one of these boards is present instead of a huge array of DIP-chip DRAMs, would adding a second copy of the board work as desired? (With its much lower power draw, I would expect it to at least not come out _less_ stable, but these things continue to surprise us even after nearly 40 years.)

I, ah, did a worse-than-usual job of framing that question, didn't I? I'm sorry. Yes, you correctly deduced what I was trying to ask -- I wanted to know how the board's electrical loading compared against the older construction methods involving, I don't know, a big array of 4116s or whatever else someone had available. I _reeeeally_ didn't intend for it to sound like a snotty request for unpaid design review services. *wince*

But yes, the usual thing for the 256K mod is to just add two more flat DRAM banks of 64KiB each. Expansions beyond that involve either trying to guess how the C128's native 1-meg MMU design was _intended_ to be fully realized, or simply ignoring it entirely -- kludging in a PET-style block-swapping scheme instead, controlled through an extra 6522-type peripheral chip with I/O port(s).

I'm fairly sure that one design I saw, years ago, actually combined both approaches -- three quarters of the MMU banks were simply 64K blocks of RAM, while each of the remainder were 64K arrays of smaller, independently-mapped blocks. (I can't recall, at this point, whether those were mapped in 4K at a time or 16K at a time. I suppose it doesn't actually matter at this point.)