Interrupts and Flags

Here is the detailed explanation of Interrupts and Flags:

Interrupts:

In the 8085 microprocessor, an interrupt is a signal that temporarily suspends the normal execution of a program and redirects the control to a specific interrupt service routine (ISR).

• Interrupts allow the microprocessor to respond to external events, such as user input, system events, or hardware signals, without the need for constant polling.

Let’s take a look at interrupts and flags in the Intel 8085:

1. )Interrupts Types:
   • RST (Restart) Interrupts: RST 7.5, RST 6.5, RST 5.5 – These are maskable hardware interrupts that cause the microprocessor to jump to specific memory locations to execute predefined routines.
   • TRAP Interrupt: Non-maskable hardware interrupt, used for high-priority tasks. The microprocessor jumps to a fixed memory location (0024H) when a TRAP interrupt occurs.
2. ) Maskable Interrupts (INTR):
   • The INTR (Interrupt Request) input is a maskable interrupt. If enabled (through the interrupt enable flip-flop), it can interrupt the microprocessor’s normal execution.
3. ) Interrupt Enable/Disable (DI, EI):
   • DI (Disable Interrupt): Disables interrupts, preventing the microprocessor from responding to subsequent interrupt requests.
   • EI (Enable Interrupt): Enables interrupts, allowing the microprocessor to respond to interrupt requests.
4. ) Interrupt Vector Table:
   • The addresses to which the microprocessor jumps when specific interrupts occur are stored in the interrupt vector table. The vector addresses are fixed and predefined for each type of interrupt.

Flags:

• The Flag register is a 5-bit register in the 8085 microprocessor that contains information about the status of the arithmetic and logic operations performed by the processor.
• The bits in the Flag register are used to indicate whether the result of an operation is zero, positive, negative, or if there was a carry or borrow during the operation.

1. Status Flags:
   • Sign (S) Flag: Set if the most significant bit of the result is 1.
   • Zero (Z) Flag: Set if the result is zero.
   • Auxiliary Carry (AC) Flag: Set if there is a carry out of the low nibble (bits 3 to 0) during arithmetic operations.
   • Parity (P) Flag: Set if the number of set bits in the result is even.
   • Carry (CY) Flag: Set if there is a carry out of the most significant bit during arithmetic operations.
   • Overflow (V) Flag: Used for signed arithmetic; set if there is overflow.
2. Control Flags:
   • Interrupt Enable (IE) Flip-Flop: Controls the enabling or disabling of interrupts. Affected by EI and DI instructions.
   • Auxiliary Carry (AC) Flip-Flop: Used internally for BCD (Binary Coded Decimal) arithmetic.
   • Sign (S), Zero (Z), and Parity (P) Flip-Flops: Used to store the respective flag conditions.
3. Flags Operations:
   • Instructions such as ADD, SUB, etc., affect the flags based on the result of the operation.
   • Some instructions (e.g., CMP) only affect the flags without changing the contents of the registers.