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Channels (Figure 1-19)
•
Channels handle data transfers
between main storage and I/O
devices.
•
All I/O devices are connected to
channels via a standard interface.
•
There are 2 types of channels: sel-
ector and multiplexor.
•
Selector channels are designed for
high data rates.
•
Multiplexor channels are designed
for low data rates.
One of the main functions of a chan-
nel is to handle I/O requests for a main
storage cycle.
The channel receives
data from the I/O devices one byte at a
2030 Processing Unit
Main
Storage
Input/
fo--...,
Output
Channel
Control
Unit
time.
When enough data has been
received to justify the use of main
storage, the channel requests a storage
cycle.
The amount of data bandIed var-
ies depending on the particular model of
System/360.
After the data has been
placed in main storage, the channel
waits for additional information from
the input device.
For an output device
the procedure reverses.
The channel
requests a main storage cycle and brings
out data.
It passes this data to the
output device one byte at a time.
Since the channel is taking care of
main storage cycles for the I/O device,
the central processing unit is now logi-
cally free to continue processing
instructions.
We say that processing is
overlapped with the I/O operation.
Random
Access
Device
Control
Unit
Random
Access
Device
Data and Control Information .....
----~
Arithmetic,
Control, and
Register
Systems
Figure 1-19.
1-18
Control
Unit
Channel Operations
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