Question

Discuss the internal Structure of a digital computer with the
importance of each block. Also show the propose of each field
in flag register.​

Answer 1

Explanation:

Flag register of 8086 microprocessor

Prerequisite – Flag register in 8085 microprocessor

The Flag register is a Special Purpose Register. Depending upon the value of result after any arithmetic and logical operation the flag bits become set (1) or reset (0).

Figure – Format of flag register

There are total 9 flags in 8086 and the flag register is divided into two types:

(a) Status Flags – There are 6 flag registers in 8086 microprocessor which become set(1) or reset(0) depending upon condition after either 8-bit or 16-bit operation. These flags are conditional/status flags. 5 of these flags are same as in case of 8085 microprocessor and their working is also same as in 8085 microprocessor. The sixth one is the overflow flag.

The 6 status flags are:

Sign Flag (S)

Zero Flag (Z)

Auxiliary Cary Flag (AC)

Parity Flag (P)

Carry Flag (CY)

These first five flags are defined here

Overflow Flag (O) – This flag will be set (1) if the result of a signed operation is too large to fit in the number of bits available to represent it, otherwise reset (0). After any operation, if D[6] generates any carry and passes to D[7] OR if D[6] does not generates carry but D[7] generates, overflow flag becomes set, i.e., 1. If D[6] and D[7] both generate carry or both do not generate any carry, then overflow flag becomes reset, i.e., 0.

Example: On adding bytes 100 + 50 (result is not in range -128…127).

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