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Computer Buses

When we say a computer system has a 16-bit bus, what we mean is that it can transfer binary data around internally in chunks of 16 binary bits simultainiously. To transfer 16 binary bits simultainiously requires 16 separate wires or metal traces on a printed circuit board. Along with the 16 binary bits of data, we'll need a few more wires or traces for the control signals particular to that bus. The whole group of wires or traces is referred to as a "bus".

You can think of it as a bus that all the people related to the bus board to be transferred from the source to the destination. Actually a bus is more like a super-highway. A 16-bit bus would be a super-highway with 16 lanes, each binary bit using a separate lane (changing lanes is strictly forbidden). A 16-bit bus would actually be a super-highway with more than 16 lanes because you need those control signals.

The super-highway metaphor is more accurate because a computer bus is usually connected to several or many electronic devices. It would be like a super-highway with many on-ramps and off-ramps. The binary bits actually arrive at all on-ramps and off-ramps simultainiously, it's the control signals that determine which devices send or receive any specific chunk of binary data.

A computer has many different types of buses. The primary buses are the address bus and the data bus. The address bus identifies where the data is to come from or the data's destination. An address bus may be 64-bits wide. But there may also be specific buses for storage devices and input/output devices. And then there are different versions of each type, with a different number of bits (bus width) and a different transfer speed for each type. Let's explore some of the main types of buses.

Industry Standard Architecture (ISA) Bus

8-bit ISA bus
Image from Wikimedia Commons

The ISA bus was one of the first bus standards, and it was introduced with the first IBM PC's. It originated as an 8-bit bus, but was quickly updated to 16-bit for use with Intel's 80286 processor. The 8-bit bus ran at 4.77 MHz, the 16-bit bus ran at 6 or 8 MHz. The 8-bit bus used a 8259 Programmable Interrupt Controller (PIC), giving 8 interrupt lines. It had 4 DMA channels, 3 brought out to the expansion slots, 2 of these were normally allocated to the disk drive and hard disk controller.

The biggest problem with the ISA bus was that to add hardware to the system, you couldn't just plug it in, you were required to understand IRQ, DMA, and I/O address settings, and to manually set appropriate dip switches to configure those resources for the new hardware.

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