> that calculated the parity value on access, replacing the 9th ram chip with a very simple circuit and saving money. > It would be hard to convince the whole

> that calculated the parity value on access, replacing the 9th ram chip with a very simple circuit and saving money. > It would be hard to convince the whole industry not to make fake ECC ram, if ECC was mandatory. Assuming you don't use memory-mapped IO, that's easy to fix. On startup, generate 4 random bits a,b,c,d. Parity bit is data line 4a+2b+c, with d?even:odd parity, data bit 4a+2b+c is on data line D8. ECC on 64/72 uses more random bits, but is otherwise similar, although for modern chipsets it would probably have to be scrambled in the northbridge or southbridge (or equivalent) rather than the CPU, to allow for DMA and such. Note that there's no gate delay involved here; the multiplexing can be done with pass transistors.