Internal 512K RAM expansion for CPC6128 from german magazine c't, issue 10/1987. The DIY project name is CPC 6512.
The c't expansion memory is said to be accessible as Video RAM (with normal expansions, only 64K are accessible as Video RAM). So, the c't expansion might be incompatible with Standard Memory Expansions, or it might be a backwards-compatible variant with additional features.
Technical
I/O Ports are unknown ...?
Scanned Article / Schematics
c't, issue 10/1987
