JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MICROPROCESSORS AND MICROCONTROLLERS Previous Question Paper (October - 2020 - R16)

The 8086 flag register is a 16-bit register containing 9 active flags, categorized into status flags and control flags.

Status Flags:

• Carry Flag (CF): Set if there is a carry out of the MSB after addition or borrow after subtraction.
• Parity Flag (PF): Set if the result contains an even number of 1s.
• Auxiliary Carry Flag (AF): Set if there is a carry from bit 3 to bit 4 (used for BCD operations).
• Zero Flag (ZF): Set if the result of an operation is zero.
• Sign Flag (SF): Set if the MSB of the result is 1 (indicating a negative number).
• Overflow Flag (OF): Set if a signed arithmetic overflow occurs.

Control Flags:

• Trap Flag (TF): Used for single-stepping through a program for debugging.
• Interrupt Flag (IF): If set, the processor recognizes external maskable interrupts.
• Direction Flag (DF): Determines the direction of string processing; 0 for auto-increment, 1 for auto-decrement.

An interrupt is a signal sent to the processor by an external device or an internal event that causes the CPU to suspend its current execution and branch to an Interrupt Service Routine (ISR). They are used for efficient I/O handling, real-time processing, and error management.

The 8086 supports 256 interrupts, categorized as follows:

1. Hardware Interrupts: Initiated by external hardware.
   • Non-Maskable Interrupt (NMI): A high-priority interrupt that cannot be disabled by software.
   • Maskable Interrupt (INTR): A general-purpose interrupt that can be enabled or disabled via the IF flag.
2. Software Interrupts: Initiated by the INT n instruction (e.g., INT 21H for DOS services).
3. Exceptions/Internal Interrupts: Generated by the CPU itself due to errors or specific conditions, such as:
   • Divide Error (Type 0): Occurs during division by zero.
   • Single-Step (Type 1): Used for debugging when TF is set.
   • Breakpoint (Type 3): Used to pause program execution.
   • Overflow (Type 4): Occurs when the INTO instruction is executed and OF is set.