IBM System/370 Guide

The 3830 control unit is microprogram controlled. Read/write
monolithic storage contained in the control unit is used for
microprogram residence. The control unit also contains a device that
reads interchangeable magnetic cartridges (like the device in the
system console unit). This device is used for microprogram backup
storage and for storage of nonresident diagnostics for the 3330
facility. During a 3330 facility power-on sequence, the functional
microprogram is loaded from the device into control storage within
the control unit. Therefore, many engineering changes can be installed
merely by replacing the magnetic disk cartridge in use with another
cartridge that contains the new microprogram.
The 3330 facility also incorporates new error detection, correction,
and logging features, designed to improve its availability and
serviceability. The following features are implemented in the 3330
that are not provided by previously announced direct access devices:
• I/O error routine correction of recoverable data errors on read
operations with data supplied by the control unit in sense bytes
• Command retry initiated by the control unit to attempt hardware
correction of certain errors without programming assistance
• Error logging by the control unit in its control storage of
successful command retry operations
• Inline diagnostic tests, contained on magnetic disk cartridges,
which can be run on a single drive to diagnose hardware malfunctions
while other drives in the facility continue normal operations.
(Inline diagnostics are provided currently only for 2314
facilities.)
Recovery of correctable data ~ errors. When the control unit detects
a correctable data error during the reading of the data portion of
a record, it generates the information necessary to correct the
erroneous bytes.. The sense bytes presented by the control unit contain
a pattern of corrective bits and a displacement value to indicate which
of the bytes transferred to processor storage contain the errors.
The disk error recovery program need only EXCLUSIVE OR (logical
operation) the correctiv~ bit pattern with the error bytes in the input
area in processor storage to correct the errors.
Command retry. Error correction (without programming assistance) is
performed by a channel/control unit command retry procedure without
an intervening I/O interrupt in the following five situations:
1. When a correctable data error occurs during a search or read
operation on home address, record count, or record key.
During a search or read operation, the home address, count,
or key read from the disk track is placed in a buffer in control
storage within the control unit. When a correctable data error
occurs, the control unit corrects the data in the buffer and
reissues the command that caused the error. During reorientation
to the record, the control unit disconnects and frees the block
multiplexer channel. When the failing search or read command
is reexecuted, the corrected data in the buffer is used instead
of the data actually on the track.
2. When an uncorrectable data error is detected on any portion
of the record during a read or a search operation.
The failing CCW is reissued by the control unit twice. If one
of the two retries is successful, the channel program continues
normally .•
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IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English