FPGA VAX update, now DIY TTL computers

Tony Duell ard at p850ug1.demon.co.uk
Thu Nov 3 13:53:29 CST 2005


> 
> How does parity memory work?  Must one use the extra bit for parity or 
> could one use it as a ninth data bit?

Genuine parity memory does, indeed, store 9 bits, and you can use it as 9 
bit wide RAM. Certainly some of the old 30 pin SIMMs were genuinely 9 
bits wide.

There was a hrrible thing called fake parity memory. It stored 8 bits and 
calaculted the parity bit on a read (ignored the incoming parity bit on a 
write) to keep the memory controller happy. Of course you can't store 9 
bits in that type of device.

-tony



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