Question

c) Briefly analyze the flag bits available on 8085. Develop an instruction sequence that can clear all the flag bits (So, Z0, AC-0, P-0, CY-0). Determine the number of states required

Answer 1

Sign Flag (S): After any operation if the MSB 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.

Zero Flag (Z) : After any arithmetical or logical operation if the result is 0, the zero flag becomes set i.e. 1, otherwise it becomes reset i.e. 0.

1- zero result
0- non-zero result

Auxiliary Carry Flag (AC): This flag is used in BCD number system(0-9). If after any arithmetic or logical operation generates any carry and passes on to bit 4 this flag becomes set i.e. 1, otherwise it becomes reset i.e. 0. This is the only flag register which is not accessible by the programmer

1-carry out from bit 3 on addition or borrow into bit 3 on subtraction
0-otherwise

Parity Flag (P): If after any arithmetic or logical operation the result has even parity, an even number of 1 bits, the parity register becomes set i.e. 1, otherwise it becomes reset i.e. 0.

1-accumulator has even number of 1 bits
0-accumulator has odd parity

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 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

Instruction Sequence to clear all flag bits:

MVI B,0 ; B = 0

INR B    ; B = 1 (resets Sign, Zero, Parity and Auxiliary Carry flags)

STC      ; set Carry flag

CMC      ; complement Carry flag

The INR (Increment Register) instruction sets S, Z, P and AC flags according to the result of the increment. 1 is positive and not zero, so the S and Z flags are reset. It has an odd number of '1' bits so P is reset, and there is no carry from bit 3 so AC is reset.

INR does not affect the Carry flag, so we must reset it separately. Unfortunately the 8085 does not have a 'reset carry' instruction, but it does have set carry and complement carry.