QUESTION: 
What kind of onboard software does the HST have -- what programming
language was it written in, and about how many lines of code are in
use?

ANSWER from Leslye Boyce on May 1, 1996:
There are three computers on Hubble right now and they are all 1970
technology;  
the Hubble was built in the late 1970's and early 1980's.  In the next
servicing 
mission (2/97) we will be adding two new Science Instruments with powerful 
micropocessors.  Historically, we have used assembly language,  but now we are 
moving to C.  The issues we must consider are:

1) Memory limitations.  Our memory is typically plated wire and that technology 
use to be very expensive.  Assembly language applications allow us to utilize 
every memory location.  This also helps us make patches to the current
programs. 
Prior to the First Servicing Mission (12/93) our main pointing computer
(DF-224) 
memory units were failing.  On this computer, we had a total of 6 - 8K units.  
We require 4 at any time.  We had lost 2 and could not afford to lose any
more.  

A team of engineers (hardware & software) designed and built a coprocessor
which 
provided us with 64K x24 bits of additional RAM storage. That computer also 
contains an Intel 80386 Processor.    


2) Performance.  In flight computers, we require the system to operate at real-
time.  This means that we want to read our sensors and send commands to our 
actuators immediately.  As a result, we often place heavy requirements on the 
CPU and that requires we track our the utilization very carefully.  In the
older 
technology computers which did not run very fast, we typically go above 90% CPU 
utilization.  That limits what we can do with the telescope.  We are in the 
process of rewriting some of Assembly code in C and migrating those
functions to 
our 386 processor.  It is a very exciting, yet busy time for the flight
software 
group. 

We are also going to replace the DF-224/Coprocessor with a new computer in
1999.  

It is a 80486 Intel Processor, with 33 MHz clocking.  The computer language of 
choice is C; we do not plan to use assembly.  We chose C because we believe, 1) 
it will be able to achieve our required performance with the increased CPU
speed  

and 2) it will help reduce our maintenance costs over the life of the mission 
(assy. programmers are sometimes hard to find).  

The estimated lines of flight software code is 22K for all 3 of the current 
onboard computers.  The two new Science Instrument computers will contain about 
50K lines of code combined; this is based on early estimates.  We also maintain 
over 200K lines of test facility unique code (programs and test procedures).  
These facilities are used to test changes to the flight software.