....called in the
other ways44. The standard telemetry signal functioning as a
Digital Command System (DCS) word consisted of 35 bits. Buried in
it were an enable bit, an execute bit, and 12 information bits.
The enable and execute bits caused an interrupt, making it
possible for the data to be stored45.
The 16K program had a computation cycle
consisting of six levels: experiment input, Control Moment Gyro
gimbal rates, Workshop Computer Interface Unit tests, and the
command system processor; telemetry output; the switch-over timer
(reset each second) and 64-bit transfer register (refreshed about
once every 17 seconds); the intermediate loop (made up of Control
Moment Gyro control); the slow loop (containing timing,
navigation, maneuver, momentum management, display, redundancy,
self test, and experiment support functions); and the "wait" state
(when all functions in a particular cycle finished, about 15% of
cycle time in the flight release of the program, depending on the
number and nature of interrupts46.
The 8K Program
The 8K program was strongly related to the
16K program in that the larger version served as the model for the
smaller. Its design, released April 3, 1972, developed from the
Phase IA version of the software. IBM delivered the 8K program on
November 14, 1972 after 10 weeks of verification activity. The
functions of the short program were largely limited to attitude
control and solar experiment activity and data
handling47. It was 8,001 words in length. IBM reduced the
number of levels in the computation cycle of the 8K program to
four: Level I handled command processing and I/O to the Gyros,
Level II did telemetry, Level III consisted of the time-dependent
functions from both the original intermediate loop and slow loop,
and Level IV was the wait state48.
All mission-critical systems in Skylab
were redundant. The computer program contained 1,366 words of
redundancy management software49. At less than 10% of the total memory, it was a
bargain. Managing redundancy with stand-alone hardware and solely
mechanical switching would have added much more cost, weight, and
complexity to the workshop design, with the loss of a certain
amount of reliability.