in 1990 the german magazine STcomputer published in it's March issue a little addon circuit which would reenable the unused 512kB from an 1MB machine which had an added 2MB RAM expansion. That article is not yet in the online archive.
The 2MB expansion has to be bank0
The circuit does the following: it maps the second 512kB behind the original 512kB of bank1.
What is cool about this is that this works with TOS without needing any additional software. TOS checks the memory configuration and detects 2MB + 0.5MB then it fills the RAM with zeroes and continues until there is an error which then marks the end of RAM. In this case it fills 2MB+0.5MB +0.5MB and detects end of RAM at 3MB. Then TOS sets topmem to this value.
I used my ST with this 3MB configuration for years without any issues. Was so nice that I never wanted to add the last MB.
So how is it done?
You need:
1x 74LS00 (quad NAND)
1x 74LS375 (quad D-latch)
1x 74LS139 (2 bit binary decoder)
4x 68Ω
1x 1μF
Then build this (ok, the original article is 3 pages long) As soon as the article is in the STcomputer archive I will link to it (it is in german). Or if I have time, I may do a translation of the build instructions and add it here.
My construction (it's 30 years old!): https://www.exxosforum.co.uk/forum/viewt ... f=53&t=540
