QUESTION:
What is the computer system used on the orbiter?

ANSWER from Brent Bennett on December 22, 1995:
You are certainly correct when you point out that computers have changed a
lot over the past few years.  Although Galileo was launched six years ago,
the computer was built a long time before that!  In fact, I'm not sure
when that was, but I would guess about 1978-1980. 

Because of that, it is extremely primitive compared even to the PC I am
writing this answer on!

The main computer, known as the CDS (Command and Data Subsystem) is a bus
oriented, distributed data network which forms the core of the spacecraft
information system.  The extension of the CDS bus to external subsystems
provides the primary communication path between the CDS andd other
subsystems.  The CDS distributes command, timing and parameter
information.  It collects, stores and formats data for downlink
transmission (telemetry). 

The CDS is composed of Motorola 1802 processors, which have 64K memory in
each of several modules.  There is no hard disk. 

I hope this gives you some insight into the CDS.