or not retry is possible and passes control to an ERP through the I/O interruption handler. DASD errors are processed as described below.
Channel ~§ • I/O interface inoperative on a 3031, reflected to the virtual .achine if
Otherwise, a wait state X'0002' occurs. 3032, or 3033 processor is
the channel is dedicated. • Channel control check is treated as seek check. It is retried 10 times. • Interface control check is treated as seek check. It is retried 10 times. • Channel data check is treated as data check. It is retried 10 ti.es.
Equipment ch~k: Retry the operation 10 ti.es for 3330, 3340, 3350, and 2305 devices; twice for the 2314 and 2319. !g ~ord !gy~g and ~issing ~~dre§§ ~arks: Recalibrate and retry the
channel program 10 tiaes (2314/2319). No record found: Execute a READ HO!E ADDRESS and check ho.e address agains~seek--address. If they are the same, consider the error
peraanent. If they are not equal recalibrate and retry the channel
program 10 ti.es (2314/2319). For other devices, return to caller. ~eek £h~ck: Retry the operation 10 times except that 3330/3350 seek
checks are retried by hardware.
Intervention ~equire~: Issue a message to console and wait for solicited
device end. This procedure is repeated once. ~Y2 2Y1 check: One retry of the operation.
Data checks: For 2314/2319 retry the operation 256 tiaes, with a
recalibrate being executed every 16th tiae. For the 2305/3340, retry
the operation 10 times. For the 3330/3350, the operation is retried by hardware. Ov!££~~: Retry the operation 10 times. !issinq add~§§ .ark~~: Retry the operation 10 ti.es. ~~g rej~£!: The command is not retried. ~hainiDg chec~: Test for command reject.
operation 10 tiaes.
If not present, retry the ~nviroA!enta! dat~ pres~1: Issue a BUFFER UNLOAD com.and and retry the
operation.
Track condition check: On condItion check-is-received alternate track software
permanent error. CP does
devices; this support is
Recovery, ERP (D!KTRK)." CP I/O and Diagnose I/O, when a track
fro. a device for which CP does not provide
recovery, the condition is treated as a
provide alternate track support for other
described in the section "Alternate Track
1-164 IB! V!/370 System Logic and Problem Deter.ination--Volume 1

The error recovery routine keeps track of the number of retries in
the IOBRCNT field of the IOBLOK. This count determines if a retry limit has been exceeded for a particular error. On initial entry from DMKICS for an error condition, the count is zero. Each time a retry is
attempted, the count is increased by one.
The ERP preserves the original error CSW and sense information by placing a pointer to the original IOERBLOK in the RDEVBLOK. Additional IOERBLOKs, which are received from DMKIOS on failing restart attempts,
are discarded. The original IOERBLOK is thus preserved for recording
purposes.
If after a specified number of retries, DMKDAS fails to correct the
error, the operator mayor may not be notified of the error. Control is
returned to DMKIOS.DMKIOS is notified of the permanent error by posting
the IOBLOK (IOBST1T=IOBFATAL). The error is recorded via DMKIOS by D~KIOERR~ if DMKDAS and DMKIOE determine that the error warrants
recording.
If the error is corrected by a restart, the temporary or transient
error is not recorded. Control is returned to DMKIOS with the error flag
off.
Before returning control to DMKIOS on either a permanent error or a
successful recovery, the ERP frees all auxiliary storage gotten for
recovery CCWs, buffers, and IOERBLOKs, and updates the statistical
counters for 2314 and 2319 devices.
The DMKIOS interface with the ERP uses the IOBSTAT and IOBFLAG fields
of the IOBLOK to determine the action required when the ERP returns to DMKIOS. When retry is to be attempted, the ERP turns on the restart bit of
the IOBFLAG field. The ERP bit of the IOBFL1G field is also turned on to
indicate to DMKIOS that the ERP wants control back when the task has
finished. This enables the ERP to receive control even if the retry was
successful and allows the freeing of all storage gotten for CCWs and temporary buffers. The IOBRCAW is set to the recovery CCW string
address.
In handling an intervention-required situation~ the ERP sends a
message to the operator and then waits for the device end to arrive.
This is accomplished by a return to DMKIOS with the ERP bit in the
IOBFLAG field set on and the IOBSTRT bit in the IOBFLAG field set off. When the device end interruption arrives, the original channel program
which was interrupted is then started.
The ERP flags of the IOERBLOK are also used to indicate when special
recovery is being attempted. For example, a READ HOME ADDRESS command when a no record found error occurs.
The other two indicators are self-explanatory and are explained in
Figure 22. CP Introduction 1-165