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SYSTEM CONTROL OF
INPUT-OUTPUT PERIPHERAL DEVICES
In a GE-235 system up to ten input-output (I/o) devices of various types may operate simultane-
ously with the central processor and with each other. This truly active configuration i s extremely
flexible and efficient; it i s capable of a maximum throughput of 55, 000 20-bit words per second,
plus c a r d reading, card punching and printing.
The throughput may be doubled to 110, 000 words
per second, plus card reading, c a r d punching and printing, by the addition of optional dual access
1/0 controller selector channels.
This performance i s the result of two significant design concepts:
Each 1/O device controls itself and executes i t s own 1/0 commands.
All I/O devices (with exception of perforated tape reader/punch and typewriter) and the
central processor have access to memory on a time-sharing basis.
INDEPENDENT-CONTROL OF 1/0 DEVICES
The individual operation of each 1/0 device i s determined by the controller through which it is
attached to the system. The controller receives the commands appropriate to it (such commands
a s to start, stop, edit data, and rewind tape a r e typical) from the central processor and executes
these commands without further instruction.
Thus, having given a command to a controller, the
central processor is f r e e to continue with the succeeding item in the program.
TIME-SHARIIVG O F MEMORY ACCESS B Y 1/0 DEVICES
Orderly and efficient time-sharing of memory access among the central processor and 1/O
devices of the GE-235 is ensured by:
Allowing only one system element to have access to memory a t one time.
Allowing each element access to memory on a priority schedule when i t needs it and
causing
it
to relinquish access when it does not.
These conditions a r e satisfied in the GE-235 by the built-in priority control logic. The success
of this feature, in fact, accounts for the high efficiency and capacity of the system and the simul-
taneity of operation of
its
Garious elements.
Priority Control and Time-Sharing
The GE-235 priority control feature i s shown schematically in the accompanying block diagram.
F o r the numbered channels shown on the diagram, descending priority i s from left to right (from
0 to 6).
The channel assigned to a particular 1/0 device depends upon i t s information transfer
rate
-
that is, i t s memory access requirements.
F o r example, a high-speed printer has a lower priority requirement than a magnetic tape unit,
since a tape controller cannot wait a s long for access a s a printer controller can without causing
timing e r r o r s .
If the printer had the higher priority, it could possibly monopolize access to
III- 1
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