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How does I/O performance effect the real world?

By:David K. Every
©Copyright 1999

Often performance is not just the processor but the I/O (input and output) systems. Computers do processing, but often they must move data around and bring input in to the computer, or give the user output. Sometimes this is not very critical, other times it is what people are measuring performance by.

Macs often have far better I/O performance than the PC counterparts. Contributing to this performance (or lack thereof) are the following -

  • ISA Slots - older slots that are 16 bit and slow relative to the rest of the computer - so anything on these slots can slow the whole machine down (dramatically). An example of this is PC's Sound which still goes through ISA architecture (even if it is on the motherboard) and in some extreme conditions can tie up the computers I/O for up to 40% of its time while only doing sound. (Macs do not have this problem).
  • PCI Slots - PCI slots are much faster than ISA, but many PC-motherboards implement PCI so that they are a multiples of the processors frequency. (Example - if a PC has a 150mhz processor, than the nearest even multiple for PCI would be 30mhz, instead of the 33mhz that is its max., that is a 10% drop, but with syncronization issues can be much worse). This can reduce PCI card performance by 10 or 20% easy. Macs always run PCI at max. frequency, and also split PCI sub systems and often have dedicated bridge chips (so that PCI cards can talk to other PCI cards without bothering the main computer).
  • Archaic IRQ structure - depending on the structure of IRQ's and DMA's and sharing of interrupts (and COM-ports) you not only deal with compatibility and conflict issues, but sometimes arbitration and performance issues as well.
  • Slow Serial - many PC's still have older slower serial controllers (called UART's). This keeps the serial ports running at some slow speeds (like 56kbps). (Macs serial ports can run at 2,000kbps)
  • Slow Parallel - Parallel is slow especially compared to other interfaces like SCSI or IDE. So hanging a disk drive off of parallel is a way to halve (or more) your performance. Also most older PC parallel ports are unidirectional (they can only send information and not recieve it back). (Macs use SCSI for disks, scanners, and Networks for printers so this is not an issue)
  • IDE - IDE (or EIDE) is how PC's usually access their disk drives and CD-ROMs. IDE can run at a fraction the speed of SCSI, and there are other issues. It seems that all devices on an IDE controller have to be running the same protocol. So if you have a CD-ROM and HardDrive sharing the same controller, the lowest common denominator protocol is used - which often means that your HD is slowed down even more. Some people buy another IDE controller to get around this problem. (Macs usually use SCSI and so this is not an issue).
  • Floppies are slow. So some people use a Cache (a little bit of fast RAM as a buffer to speed up reading and writing to floppies). PC's can't use a write cache on floppies because the floppy eject is not software controlled (a user could eject the disk before the cache is written causing loss of data, or swap a disk and the cache would be out of sync, etc.).

There are other gotcha's. And there are many things I did not mention like -

  • Modems. Modems run the same speed on Macs as PC's, but Macs have faster serial ports - so if you are likely to have a problem with your machine not keeping up with the modem, then it is likely to be on a PC.
  • Scanners. Scanners are likely to use SCSI on a Mac (fast) or serial or parallel on a PC. The real world results are that the Mac flavor will usually blow the PC flavor away - but that is more the type of port chosen.
  • Tablets. Those fancy drawing tablets by artists - often the Macs have high-speed serial ports, while PC's often have to add special cards to use those tablets (either serial or proprietary). The end results vary.


The I/O subsystems are just a part of computer performance - but sometimes a significant part. There are many "little" details in computer system design that effect performance. PC's tend to be commodities - and in commodities "cheap" is often much more important than "fast".

Created: 02/06/97
Updated: 11/09/02

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