Posts Tagged ‘databus’

How microprocessor perform I/O mangament:polling, direct memory acess and interrupts

May 9, 2012

This post will be abou the first method called polling. The next two posts will cover the other methods.
The first method is called polling. Polling is also called programmed I/O, and it’s also the simplest one to implement from a hardware perspective. I/O units are connected to the system data and an address buss in a  regular fashion trough necessary circuitry. Depending on the type of microprocessor they can also be connected to certain lines on the control buss.

The goal with a communication schedule is to have a well ordered procedure to decide what I/O unit is to be served by the processor next. Polling is a synchronous method since it doesn’t interrupt the program which is currently executed by the microprocessor. When polling is used the microprocessor will on regular intervals ask each unit connected to the data buss if it needs to be served.  The sensing if a unit needs to be served is in practice done by sensing a flag bit in the status registry of the unit.

The advantages of polling are:
1. It requires very little hardware and no dedicated lines.
2. it’s synchronous with the execution of the prgoram. This is a big advantage since the programmer know s exctly when an external unit is polled and who long time it takes to serve that unit.

The disadvantages of polling are:
1. It requires special hardware if interrupt management is to be done outside the microprocessor.
2. Each time an interrupt is made, there’s an overhead time to manage the interrupt

Description of polling techonology

Description of polling techonology

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Writen by: Rikard Grossman-Nielsen
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Intel 8080 and the way the 40pins limits it’s IO performance.

February 23, 2012

Another lesson on microprocessors and the intel8080 especially. The Intel 8080 has 40pins which enables it communicate with the outside world. The processors is 16bit bits internally but it only has enough pins on the databuss to fetch 8bits at a time. The processor is able to process 16bit instructions by fetching two 8bit memory positions after each other through use of a technology called multiplexing. The problem is that the multiplexing technology is slow so at best the intel 8080 is only a little bit faster than an 8bit processor of similar specifications. The only way of speeding up the Intel 8080 and making it able to load 16bits at a time from memory would be to give it more pins to communicate with the outside world.

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