Q12Microprocessor and Microcontroller

Question

Explain various flags available in 8085 µP.

Answer 1

Within the 8085 microprocessor, the Flag Register is a highly specialized 8-bit status register directly hardwired to the output of the Arithmetic Logic Unit (ALU). It does not hold data; rather, it holds five independent, 1-bit boolean flags (flip-flops). These flags automatically alter their state (0 or 1) strictly based on the mathematical result of the most recently executed arithmetic or logical instruction. They act as the critical conditional decision-making brain for Jump instructions (like JZ, JC).

The Five Status Flags

The 8-bit register is organized as: $D_{7} (S), D_{6} (Z), D_{5} (X), D_{4} (A C), D_{3} (X), D_{2} (P), D_{1} (X), D_{0} (C Y)$ . The 'X' bits are undefined and unused.

1. Sign Flag (S - Bit $D_{7}$ ): This flag strictly monitors the Most Significant Bit (MSB, $D_{7}$ ) of the Accumulator after an operation. In two's complement arithmetic, a '1' in the MSB indicates a negative number. If the MSB of the result is 1, the Sign Flag is SET (1). If the MSB is 0 (positive result), the Sign Flag is RESET (0).
2. Zero Flag (Z - Bit $D_{6}$ ): This is arguably the most critical flag for looping. If the entire 8-bit result of an ALU operation is mathematically exactly zero ( $00000000$ ), the Zero Flag is SET (1). If the result is anything other than zero, the flag is RESET (0).
3. Auxiliary Carry Flag (AC - Bit $D_{4}$ ): This flag is utilized almost exclusively internally by the microprocessor for BCD (Binary Coded Decimal) arithmetic (specifically the DAA instruction). It is SET (1) if an arithmetic addition operation mathematically generates a carry-over strictly from the lower nibble (bit $D_{3}$ ) into the upper nibble (bit $D_{4}$ ). It is completely inaccessible to standard conditional Jump instructions.
4. Parity Flag (P - Bit $D_{2}$ ): This flag evaluates the bitwise parity of the result. It counts the total number of '1's present in the 8-bit output. If the result contains an EVEN number of '1's (e.g., 10101010 has four 1s), the Parity Flag is SET (1). If it contains an ODD number of '1's, it is RESET (0).
5. Carry Flag (CY - Bit $D_{0}$ ): This flag is SET (1) if an addition operation generates a mathematical carry completely out of the MSB (bit $D_{7}$ ), indicating an 8-bit overflow. In subtraction, it acts as the "Borrow" flag, SET if the subtrahend is larger than the minuend.

Q12Microprocessor and Microcontroller

Question

Answer

The Five Status Flags