Yeah, guess it is that link, but I don't see any info for an SRAM upgrade like Wild_Pinguin mentioned:
"as I noticed there is some SRAM upgrade instructions on the page for 256kb"
Didn't notice this reply previously (no notifications anymore).
Anyways, the resource I've been using are the "MSX Info pages". No direct linking to files on the page was allowed, and I believe the original isn't online anymore. At the moment a mirror is available here.
Also, there seems to be some confusion here, since my previous comment was poorly formed: There are no ready-made SRAM upgrade instructions available for the NMS series (I never claimed I had found one).
The only upgrade instructions for the NMS825X/80 use 4464s (several instructions of this kind, using 74LS138 or 73LS139) or SIMMs. There are generic (or cartridge, or for other MSX models) mapper instructions using SRAM available (on the page I've linked above), and these could be adapted for an internal mapper for any MSX - but one needs to know what one is knowing if going that route ;-).
