IBM Multiprise 3000 Reference Manual page 58

The Internal Disk Subsystem will threshold unit checks for Incomplete Domain
exceptions for normally ending Locate Record operations with write intent, to
reduce the amount of sense data going to LOGREC caused by faulty software that
did not previously have this exception condition checked by the control unit. The
first occurrence of this exception following a system or power on reset results in
unit check. Thereafter, only every 256th occurrence is reported. When a unit
check is presented, along with the sense data a message is written to the system
console.
Orient – '00' Operation Code: This operation code prepares the control unit to
initiate track access in accordance with the seek address and sector number
parameters.
When the Locate Record command is followed by a search or read command, the
search operations specified by the search argument and orientation modifiers is
performed after the track access is complete. When the Locate Record command
is not followed by a search or read command, a search operation is not performed.
The Locate Record domain ends when the track accessing or search operation
completes. Although the search operation is performed after the following CCW is
received, the following CCW operates outside the Locate Record domain.
Write Data – '01' Operation Code: This operation code prepares the control unit
to update write one or more consecutive user data records. The search argument
parameter specifies the first record. The count parameter specifies the number of
records to update. This operation can also update the key and/or data areas of
record zero when the search argument is set equal to the record ID in the record
zero count area.
If the file mask inhibits all write operations (bits 0 and 1 are '01'), the command is
rejected with unit check status. The sense data contains command reject with
format 0, message 2.
If the Define Extent command specified CKD conversion mode (byte 1, bit 2 is '1'),
the Locate Record must specify count orientation. If Read Count suffix (byte 1, bit
7) in the Locate Record command is set to '1' when CKD conversion mode is
specified, the count must be 2, otherwise, the count must be 1. If these conditions
are not met, the command is rejected with unit check status. The sense data
contains command reject with format 0, message 4.
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Internal Disk Subsystem Reference Guide
data transfer to the disk subsystem. If the exception condition is related to the
execution of a write command that had already presented clean ending status,
sense data may also include imprecise ending (byte 1, bit 7).
If an Incomplete Domain exception condition is detected after final status is
presented for the last command in the chain and a subsequent exception
condition prevents data from being successfully written to the logical volume,
the next channel program to that logical volume will receive status that includes
unit check. The sense data contains incomplete domain (byte 0, bit 7),
permanent error (byte 1, bit 0), and imprecise ending (byte 1, bit 7). Sense
bytes 22 and 23 will contain the exception category causing the write failure,
not the incomplete domain exception category. This unit check will not be
subject to thresholding.