The first bus was called Industry Standard Architecture (ISA). This was a simple bus, and the Model B had five expansion slots. Each one offered an 8-bit data path, and each card had to 'wait its turn,' if another expansion card was busy with the bus.
It quickly was realized by IBM that it was business, not homeowners who would buy the 5150. That led to the Model Z, which featured 8 expansion buses.
When IBM released the AT, the corporation extended the ISA slot with an extra connector to make the ISA an 8 or 16-bit slot. By just adding the extra connector, the slot was backwards compatible with 8-bit cards.
To this day, the 16-bit ISA slot runs at 8Mhz. What 8Mhz means in real delivery of data on the bus is seen in Chapter 0100. In plain English, and in todays world, the nice word is legacy. To be more blunt, today it is considered unacceptably slow.
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