Here's a snippet of code for PIC processors (full offtopic here, hehe). Credit due to its author.

Work left: translate it into 8051 code, I'm too lazy now...

Txip.

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The six instructions below (provided by John Payson) will calculate the "Even" Parity for a Byte. At the end of the routine, bit 0 of "X" will have the "Even" Parity bit of the original number.

"Even" Parity means that if all the "1"s in the byte are summed along with Parity Bit, an even number will be produced.

swapf X,w
xorwf X,f

rrf X,w
xorwf X,f

btfsc X,2
incf X,f

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