3.11 Z Status Bit

The Z Status bit is a special Zero Status bit that is useful for extended precision arithmetic. The Z bit functions like a normal Z flag for all instructions, except those that use the Carry/Borrow input (ADDC, CPB, SUBB and SUBBR). For the ADDC, CPB, SUBB and SUBBR instructions, the Z bit can only be cleared and never set. If the result of one of these instructions is non-zero, the Z bit will be cleared and will remain cleared, regardless of the result of subsequent ADDC, CPB, SUBB or SUBBR operations. This allows the Z bit to be used for performing a simple zero check on the result of a series of extended precision operations.

A sequence of instructions working on multiprecision data (starting with an instruction with no Carry/Borrow input) will automatically logically AND the successive results of the zero test. All results must be zero for the Z flag to remain set at the end of the sequence of operations. If the result of the ADDC, CPB, SUBB or SUBBR instruction is non-zero, the Z bit will be cleared and remain cleared for all subsequent ADDC, CPB, SUBB or SUBBR instructions.