8085 Microprocessor Active Status Flags Explained

The 8085 microprocessor is a central processing unit (CPU) that uses a set of internal storage locations called flags to keep track of the results of arithmetic and logical operations. These specific flags, which reflect the status or outcome of computations, are known as status flags or conditional flags.

Identifying the 8085 Status Flags

The 8085 microprocessor features several flags, but only five are considered primary status flags that are directly affected by the Arithmetic Logic Unit (ALU) operations. These flags are grouped together in the flag register.

The Five Active Status Flags

The five active status flags in the 8085 microprocessor are:

• Carry Flag (CY): This flag is set to 1 if a carry is generated out of the most significant bit (MSB) position after an addition operation, or if a borrow is required into the MSB position during a subtraction operation. It indicates an overflow condition.
• Parity Flag (P): This flag is set to 1 if the result of an operation has an even number of set bits (1s). It is reset to 0 if the result has an odd number of set bits.
• Auxiliary Carry Flag (AC): This flag is set to 1 if there was a carry from bit 3 to bit 4 (nibble boundary) during an addition, or a borrow from bit 4 into bit 3 during a subtraction. It is primarily used for Binary Coded Decimal (BCD) arithmetic.
• Zero Flag (Z): This flag is set to 1 if the result of an operation is zero. Otherwise, it is reset to 0.
• Sign Flag (S): This flag is set to 1 if the most significant bit (MSB) of the result is 1. In signed arithmetic (using two's complement), this indicates a negative number. It is reset to 0 if the MSB is 0 (indicating a positive number).

Other Flags in 8085

The 8085 also has other flags like the Interrupt Enable (I) flag, but these are generally control flags, not status flags reflecting ALU operation outcomes. Therefore, they are not included when counting the active status flags.

Conclusion

Based on the standard definition of status flags reflecting ALU results, the 8085 microprocessor has 5 active status flags.