Page 119 - Anatomy of a Robot

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104 CHAPTER THREE
Multiple buses are used inside most computer systems. To make sure we compare the
bandwidth across each bus to the requirements set by the system architecture, it’s impor-
tant to list every bus within the computer. A few of the buses are hidden from most users
and do not even have popular buzzword names. Let’s look at those buses first, and then
proceed to the popular, named buses.
Memory Bus

All computers have an interface between the memory and the processor. The processor
can only read and write to the memory at specific speeds. If the system architecture
calls for the processor to read data, manipulate it, and rewrite it, then we must be very
careful about the memory bus speeds. If the system architecture calls for the data
manipulation to be completed within a strict time budget, we must add the processor
execution time to twice the data transfer time (one transfer to read, and one transfer to
write). The transfer times may turn out to be a significant percentage of the overall tim-
ing. If this is a problem, we can look for a bigger memory bus to work with.
Sometimes the processor is just too slow on the transfers. If that is the case, we can
look for Direct Memory Access (DMA) hardware. DMA circuitry can transfer bursts of
data faster than most processors can. Sometimes DMA hardware is included within the
processor, and sometimes we can add it on with external chips. Smaller processors will
generally not have DMA capabilities. Here’s a good rule of thumb. If the analysis of the
robot’s architecture shows that the memory bus is loaded down by as much as 30 per-
cent from data moving across it, consider a faster computer, a wider memory bus, or
DMA transfers.

Video Bus

Many computer systems are used to process vast amounts of video or graphics data.
Game systems certainly are like this, and specific computer graphic buses are very fast
and flexible. The penalty for choosing the wrong graphics bus would be poor graphics,
delayed images, or system failure. If the robot design will use a great deal of graphic
display and manipulations, consult the following site as a start: www.agpforum.org/.
Many other named buses exist within a computer as well. The following URLs are
part of a superior web site. It’s a great place to start, comparing buses and looking up
their specifications:
www.interfacebus.com/Bus_Design_Top.html
www.interfacebus.com/Design_Interface_table.html