If a channel check error is to be recorded, the recorder is entered
at DMKIOERR or DMKIOECC. The channel check handler determines the entry_ A channel check error record is formatted.
A machine check enters at DMKIOEMC. Pointers are passed from the machine check handler in registers 6 and 7 to locate a buffer where the
machine check record and length are saved. A aachine check error record
is recorded with the saved machine check logout and additional
information. The machine check error record is written onto the error
recording cylinder by using the paging routines.
Hardware environmental counter records are formed using routine DMKIOEEV. This routine is scheduled by DMKIOS after control is returned
from the ERP. Sense data information is stored in the IOERBLOK by the ERP. The record formed is called a nonstandard record. DMKIOEFM is called by DMSIFC (CPEREP command) via a DIAGNOSE instruction. DMKIOEFM 1S invoked to reset the specified error recording
cylinders (if CLEAR, CLEARF, or ZERO=Y was specified). The clear is
performed by resetting each page-header, space-available field. Pointers in storage are then updated to address the first available page
on each of the error recording cylinders. Control is then returned to
the calling routine. For details on the CPEREP com.and and EREP execution, refer to the Guide and OS/VS EREP publications.
CLEARF on a 3031, 3032, or 3033 processor clears the cylinders, then
causes the frame records to be read from the SRF device. DMKIOEFL is called by DMKCPI to find the first available page that can
be used for error recording. The paging routines, DMKRPAPT and DMKRPIGT, are used to read the error recording cylinders' pages
(4096-byte records). As each page record is read, it is examined to see
if this record is the last recorded. If so, a pointer in storage is
saved so recording can continue on that page record. Control is then
returned to the caller. If any error recording cylinder is in an
unrecognizable format, the error recording area is automatically
reformatted by CP. DASD ERROR RECOVERY, ERP (DMKDAS) Error recovery is attempted for CP-initiated I/O operations to its supported devices and for user-initiated operations to CP-supported
devices that use a DIAGNOSE interface. The primary control blocks used
for error recovery are the RDEVBLOK, the IOBLOK and the IOERBLOK. In
addition, auxiliary storage is sometimes used for recovery channel
prograas and sense buffers.
The initial error is first detected by the I/O interruption handler
which performs a SENSE operation if a unit check occurs. Unit check
errors are then passed to an appropriate ERP. If a Channel check is
encountered, the channel check interruption handler determines whether CP Introduction 1-163

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 Retry the operation 10 ti.es for 3330, 3340, 3350, and 2305 devices; twice for the 2314 and 2319. !g and Recalibrate and retry the
channel program 10 tiaes (2314/2319). No record found: Execute a READ HO!E ADDRESS and check ho.e 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. Retry the operation 10 times except that 3330/3350 seek
checks are retried by hardware.
Intervention Issue a message to console and wait for solicited
device end. This procedure is repeated once. 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. Retry the operation 10 times. !issinq Retry the operation 10 ti.es. The command is not retried. Test for command reject.
operation 10 tiaes.
If not present, retry the 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