Parity Checking in CPUs (was: Byte sizes...

Roger Merchberger zmerch at
Tue Mar 15 13:32:18 CST 2005

Rumor has it that William Donzelli may have mentioned these words:

>Parity checking is the job of the memory controller, not the processor. In
>fact, I am having a hard time thinking of a processor that did its own
>parity checking in software (yes, I know any processor could do it, but
>did any really do it?).

8085? There's a P bit in the condition code register (in 6809 speak) -- I 
think it's called the PSW -- Program Status Word? I'm just beginning 
learning assembly on my Tandy 10x/200 machines... Anyway, there are several 
arithmetic operations that automatically set the Parity bit and there are 
branch and return instructions that utilize the status of the P bit.

>  Even if the parity checking is a lowly 74180, like
>in a microcomputer - it is still not boggin down the processor. The
>processor really doesn't need to know about parity, unless things go bad.

Altho it's not a *huge* need, it can be handy for serial I/O... that would 
be a function that uses parity, but is not reliant on the memory controller.

Roger "Merch" Merchberger

Roger "Merch" Merchberger   | "Bugs of a feather flock together."
sysadmin, Iceberg Computers |           Russell Nelson
zmerch at          |

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