Assembler language basics parity flag
It's probably an even parity bit.

The count of all the bits that are ones PLUS the parity bit should be an even number.

Examples:

\$0 0 0 0 0 0 1 1\$ \$Parity = 0\$,  \$\Sigma= 2 + 0 = 2\$

\$0 0 0 0 0 0 0 1\$ \$Parity = 1\$,  \$\Sigma= 1 + 1 = 2\$

\$0 0 1 0 1 0 1 1\$ \$Parity = 0\$,  \$\Sigma= 4 + 0 = 4\$

\$0 1 1 0 0 1 1 1\$ \$Parity = 1\$, \$\Sigma= 5 + 1 = 6\$