The ZX Spectrum +2A/+3 (compared with ZX Spectrum 128K) have the same total RAM, but with a different architecture: 4 chips of 32 kbytes each (64 kbit x 4) instead of 16 chips of 8 kbytes each (64 kbit x 1). This causes a difference in the memory contention and, in some cases, compatibility issues between Sinclair and Amstrad systems.
After adding +2a emulation to FBNeo for a few games a week or 2 ago, some things about it I'd like to share:
The Speccy 128k has 128k of ram, but it can't map all of it at once, it has to bank in 16k chunks of ram in order to use them. The +2A / +3 has the feature to unmap the bios roms and map the ram in their place, allowing the cpu/program to access 2 more 16k banks of ram concurrently than the standard 128k can.
It's this feature that the +2a/+3 only games use which won't work on a standard 128K.
best regards,
- dink