6.3.2. Memory Address
The 8088 processor in PC and XT has 20 address lines. To connect these 20 lines with expansion cards, the PC bus has 20 address lines.
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6.3.3. Electronic Overhead
Some bus wires just transport simple electric power; there are +5 volts, - 5 volts, +12 volts, and electric ground lines. Why are those lines there? Simple: to power a board plugged into a bus slot.
6.3.4. Control Lines
There are also a few control lines, like Reset, clock signals, and Refresh.
6.3.5. Interrupts and Direct Memory Access Channels
Add-in cards sometimes need to demand the attention of the CPU; they do that via hardware interrupts or IRQ (interrupt request) levels. There are six IRQ levels on the PC bus, labeled IRQ1 through lRQ7. Each gets a wire on the bus. There are also IRQ0 and IRQI, but they’re not available on the bus.
Some of these add-in cards also need to transfer data to the system's memory quickly; they can do that via a Direct Memory Access (DMA) channel. There are three DMA channels on thc bus, labeled DMA1 through DMA3.
6.4. The AT (ISA) Bus
When developing the AT, IBM saw, that it had to upgrade the bus. One reason was because the 80826 is a chip with a 16-bit data path. They certainly could have designed the AT with an 8-bit bus, but it'd be a terrible shame to make a 286 chip transfer data 8 bits at a time over the bus rather than utilize its full 16-bit data path. So they thought that it'd be nice to have a 16-bit bus.
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