The SAM Coupé was a ZX Spectrum compatible computer with a 6MHz Z80, 256K RAM minimum, enhanced graphics and sound and an improved BASIC amongst other features. My question is about its memory. The RAM in a standard SAM is two 44256 DRAM chips of 256Kx4 bits in a 512x512 row/column arrangement, so using 9-bit row/column addresses. The Z80 only has a 7-bit refresh counter and the SAM's display (in modes 3 and 4) can be completely switched off, giving the Z80 apparently uncontended access. So what refreshes all the memory rows? (Schematic reference: http://velesoft.speccy.cz/samcoupe_schematics-cz.htm )
According to the schematic posted, the only possible component that could be performing the refresh is the chip identified as "ASIC". It has 9 pins labelled "MADD0".."MADD8", which are the only connections to the RAM address pins. I know that some work has been done reverse engineering this chip and maybe @mcleod_ideafix will let us know in more detail later, but I'm going to guess that it detects the Z80's refresh cycles and extends the refresh counter with a couple of extra bits. At 6MHz, this would generate a full cycle of refreshes in at most 2ms, so is clearly fast enough for the RAM, which requires all 512 rows refreshing in 8ms.