Interrupt vs polling; interrupts in 8085

Interrupt vs polling

In polling the processor repeatedly reads the status of each I/O device in a loop to see which one needs service. In interrupt processing the device itself signals the processor when it needs service, and the processor meanwhile does other work.

Interrupt processing in the 8085

The 8085 has TRAP, RST 7.5, RST 6.5, RST 5.5 (vectored) and INTR (non-vectored), with INTA' as the acknowledge output.

1. The main program enables interrupts with EI (and SIM if RST 5.5/6.5/7.5 masks need to be set). TRAP is always enabled.
2. A device raises a request on one of the pins.
3. The 8085 finishes the current instruction and checks the pins (priority: TRAP > RST 7.5 > 6.5 > 5.5 > INTR).
4. If the interrupt is accepted, the interrupt enable flip-flop is reset, so no other maskable interrupt is accepted.
5. The PC (return address) is pushed on the stack.
6. For INTR, the 8085 sends INTA'; the device places an RST n opcode (or CALL) on the data bus. For the others the PC is loaded with a fixed address: TRAP 0024H, RST 7.5 003CH, RST 6.5 0034H, RST 5.5 002CH.
7. The interrupt service routine runs. It saves registers with PUSH, services the device, restores the registers with POP, and executes EI and then RET.
8. RET pops the PC and the main program continues.

 Main program ---- interrupt arrives
      |   finish instruction, push PC
      v
 Vector (e.g. 003CH) -> JMP ISR
      |   ISR: PUSH, service, POP, EI, RET
      v
 Pop PC -> back to main program