- 2078 Bhadra · 2+6 marks
Compare status check I/O, interrupt I/O and DMA
Short answer
An instrumentation system measures, processes, displays and/or controls physical quantities. In status check I/O, the CPU polls the flag, causing high overhead and slow speed. Interrupt-driven I/O reduces CPU time by servicing only on request, while DMA uses a controller for block transfers with minimal CPU involvement and highest speed.
Define instrumentation system. Compare status check I/O, Interrupt driven I/O and DMA.
An instrumentation system is a system of devices that measures, collects, processes, displays and/or controls physical quantities. In a microprocessor-based version, the three ways to transfer data between I/O and CPU are compared below.
| Point | Status check (polling) | Interrupt driven | DMA |
|---|---|---|---|
| Who controls transfer | CPU program | CPU, on request of device | DMA controller |
| CPU activity | CPU loops, reading the status flag | CPU does other work, services ISR when interrupted | CPU free, bus only held for short time |
| CPU overhead | Very high | Medium (context saving per byte) | Very low |
| Speed | Slow | Medium | Fastest (block rate near memory speed) |
| Hardware | Simplest: port and status bit | Interrupt lines, INTA, controller (8259) | DMA controller (8237), HOLD/HLDA |
| Response time | Depends on polling loop | Fast (latency of an instruction) | Very fast |
| Suitable for | Few, simple, slow devices | Occasional events, keyboards, alarms | Block transfers: disk, ADC bursts |
| Software | Simple | ISR needed | Needs set-up of address and count |
Polling: loop: IN status; test ready? no -> loop; yes -> IN data
Interrupt: main program ... <INTR> -> ISR -> return
DMA: DREQ -> HOLD -> HLDA -> block transferred -> HOLD off
✓ Answer checked by Claude Sonnet 5.5
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