Linux Device Drivers

A Look Back: ISA

The ISA bus is quite old in design and is a notoriously poor performer, but it still holds a good part of the market for extension devices. If speed is not important, and you want to support old motherboards, an ISA implementation is preferable to PCI. An additional advantage of this old standard is that if you are an electronic hobbyist, you can easily build your own devices.

On the other hand, a great disadvantage of ISA is that it’s tightly bound to the PC architecture; the interface bus has all the limitations of the 80286 processor and is causing endless pain to system programmers. The other great problem with the ISA design (inherited from the original IBM PC) is the lack of geographical addressing, which has led to endless problems and lengthy unplug-rejumper-plug-test cycles to add new devices. It’s interesting to note that even the oldest Apple II computers were already exploiting geographical addressing, and they featured jumperless expansion boards.

Hardware Resources

An ISA device can be equipped with I/O ports, memory areas, and interrupt lines.

Even though the x86 processors support 64 kilobytes of I/O port memory (i.e., the processor asserts 16 address lines), some old PC hardware decodes only the lowest ten address lines. This limits the usable address space to 1024 ports, because any address in the range 1KB-64KB will be mistaken for a low address by any device that decodes only the low address lines. Some peripherals circumvent this limitation ...