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Flag register in 8085 microprocessor

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  • Difficulty Level : Medium
  • Last Updated : 29 Jun, 2022

Prerequisite – Registers of 8085 microprocessor The Flag register is a Special Purpose Register. Depending upon the value of the result after any arithmetic and logical operation, the flag bits become set (1) or reset (0). In 8085 microprocessor, the flag register consists of 8 bits and only 5 of them are useful. The 5 flags are: 

  1. Sign Flag (S) – After any operation if the MSB (B(7)) of the result is 1, it indicates the number is negative and the sign flag becomes set, i.e. 1. If the MSB is 0, it indicates the number is positive and the sign flag becomes reset i.e. 0. from 00H to 7F, sign flag is 0 from 80H to FF, sign flag is 1 1- MSB is 1 (negative) 0- MSB is 0 (positive) Example: MVI A 30 (load 30H in register A) MVI B 40 (load 40H in register B) SUB B (A = A – B) These set of instructions will set the sign flag to 1 as 30 – 40 is a negative number. MVI A 40 (load 40H in register A) MVI B 30 (load 30H in register B) SUB B (A = A – B) These set of instructions will reset the sign flag to 0 as 40 – 30 is a positive number.
  2. Zero Flag (Z) – After any arithmetical or logical operation if the result is 0 (00)H, the zero flag becomes set i.e. 1, otherwise it becomes reset i.e. 0. 00H zero flags is 1. from 01H to FFH zero flag is 0 1- zero-result 0- non-zero result Example: MVI A 10 (load 10H in register A) SUB A (A = A – A) These set of instructions will set the zero flag to 1 as 10H – 10H is 00H
  3. Auxiliary Carry Flag (AC) – This flag is used in the BCD number system(0-9). If after any arithmetic or logical operation D(3) generates any carry and passes it on to D(4) this flag becomes set i.e. 1, otherwise, it becomes reset i.e. 0. This is the only flag register that is not accessible by the programmer 1-carry out from bit 3 on addition or borrows into bit 3 on subtraction 0-otherwise Example: MVI A 2BH (load 2BH in register A) MVI 39H (load 39H in register B) ADD B (A = A + B) These set of instructions will set the auxiliary carry flag to 1, as on adding 2B and 39, the addition of lower-order nibbles B and 9 will generate a carry.
  4. Parity Flag (P) – If after any arithmetic or logical operation the result has even parity, an even number of 1 bit, the parity register becomes set i.e. 1, otherwise it becomes reset i.e. 0. 1-accumulator has an even number of 1 bits 0-accumulator has odd parity Example: MVI A 05 (load 05H in register A) This instruction will set the parity flag to 1 as the BCD code of 05H is 00000101, which contains an even number of ones i.e. 2.
  5. Carry Flag (CY) – Carry is generated when performing n bit operations and the result is more than n bits, then this flag becomes set i.e. 1, otherwise, it becomes reset i.e. 0. During subtraction (A-B), if A>B it becomes reset, and if (A<B) it becomes set. Carry flag is also called the borrow flag. 1-carry out from MSB bit on addition or borrow into MSB bit on subtraction 0-no carry out or borrow into MSB bit. Example: MVI A 30 (load 30H in register A) MVI B 40 (load 40H in register B) SUB B (A = A – B) These set of instructions will set the carry flag to 1 as 30 – 40 generates a carry/borrow. MVI A 40 (load 40H in register A) MVI B 30 (load 30H in register B) SUB B (A = A – B) These set of instructions will reset the sign flag to 0 as 40 – 30 does not generate any carry/borrow.

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