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If
the requested channel is available, the channel microcode attempts to start the
1/0
operation to the specified channel, control unit, and device.
If
the status
returned from initial device selection indicates the path is available, condition code
0 is presented, since the
1/0
operation has been started.
If
control unit busy status
with no errors is returned, the request is placed at the end of the control unit queue
and condition code 0 is presented. If any error status is returned for the control
unit, no queuing is done and condition code 1 ( CSW stored) is returned. If the
required control unit is available but the specified
1/0
device is busy, the request is
placed on the device queue. The instruction processing function resumes
processing after the appropriate condition code has been generated.
Dequeuing of requests is done when the channel becomes available (a device
disconnects), control unit end occurs, or device end occurs. When the
no-longer-busy condition is received, all requests on the associated queue (channel,
control unit, or device) are moved to a list for that channel and the operations will
be redriven as soon as the current System/370 instruction is completed.
When an
1/0
operation terminates, a delay code is stored at location 185 in
processor storage to indicate the busy condition (channel, control unit, or device)
encountered, if any, when the SIOF instruction was first processed.
20:25 Standard and Optional Features
Standard Features
The following are standard features of 43 81 Model Group 11, 12, 13, 1, and 2
Processors that are operative for both System/370 and System/370-XA modes:
Binary arithmetic
BRANCH and SAVE instructions
Byte-oriented operands
Channel indirect data addressing
Channels 0 through 5, which can be configured as one byte and five block
multiplexer channels or two byte and four block multiplexer channels
Command retry for block multiplexer channels
Conditional Swapping
Control Unit Powering (for up to 32 control units)
CPU timer and clock comparator (one microsecond resolution)
Data Streaming for all installed block multiplexer channels
Decimal arithmetic
Dual Address Space Facility
ECC on processor storage and error correction for certain double-bit errors
ECPS:MVS (tracing functions not implemented for System/370-XA mode)
EC mode of operation
Elementary Math Library Facility (Model Group 2 only)
Expanded machine check interruption class
•
Extended Addressing (Model Groups 12, 13, and 2 only)
Floating-point arithmetic
58
A Guide to the IBM 4381 Processor
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