On ZX Spectrum is very popular DIVMMC interface. DIVMMC contain system with trdos support and external fast ram 128kB or 512kB (exist two models). Then on ZX128 with DIVMMC we can access to standard 128kB ZX ram + 128 or 512kB ram switchable as 8kB ram pages on area 8182-16384 instead ZX rom. Access to this ram enable on ZX128 access to 256kB or 640kB.