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ISA bus signals; replacement by PCI bus

Short answer

ISA control signals include MEMR/W, IOR/W, BALE, AEN, IRQs, DRQs, DACKs, etc. PCI replaced ISA due to higher speed (up to 528 MB/s), plug-and-play, bus mastering, and multiplexed address/data. Bandwidth of 64-bit PCI at 66 MHz is 8 bytes × 66 MHz = 528 MB/s.

List the control signals used by the ISA bus. Provide convincing arguments to justify the replacement of the ISA bus by the PCI bus. Calculate the bandwidth of a 64 bit PCI bus operating at 66 MHz.

Control signals of the ISA bus

  • MEMR', MEMW' (memory read/write, full 1 MB), SMEMR', SMEMW' (memory below 1 MB).
  • IOR', IOW' (I/O read/write).
  • BALE (address latch enable), AEN (address enable, high during DMA).
  • I/O CH RDY (wait state request), 0WS' (zero wait states).
  • IRQ2-IRQ15 (interrupt requests), DRQ0-7 (DMA requests), DACK0-7' (DMA acknowledge), T/C (terminal count).
  • RESET DRV, CLK, OSC, REFRESH', IOCS16', MEMCS16', MASTER', SBHE'.

Why PCI replaced ISA

  1. Speed: ISA is 8 MHz, 16-bit (about 8 MB/s real); PCI is 32/64 bit at 33/66 MHz (132-528 MB/s).
  2. Plug and play: PCI has automatic configuration; ISA needs jumpers/switches and manual IRQ/DMA settings.
  3. Processor independent (host bridge) and supports bus mastering with burst transfers.
  4. Multiplexed address/data lines reduce pin count and cost.
  5. Shared interrupts, 3.3 V/5 V support, parity error checking.
  6. Higher throughput is needed for graphics, disks and networks.

Bandwidth of 64-bit PCI at 66 MHz

Bandwidth = bus width in bytes x clock = (64 / 8) x 66 x 10^6 = 8 x 66 MB/s = 528 MB/s (= 4224 Mbit/s), peak value, with one data transfer in every clock in burst mode.

✓ Answer checked by Claude Sonnet 5.5

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