Xerox 7240 Reference Manual page 32

1/0 FAULTS AND RECOVERY PROCEDURES
The faults and recovery procedures apply to removable disk
storage systems operating with Xerox Sigma 3,5,6,7,8, or
9 computers.
This section outlines how the status response and condition
code bits associated with specific 1/0 instructions may be
evaluated to determine whether a disk 1/0 operation has
been completed successfully or terminated due to a fault
condition. Detailed information is also provided for eval-
uating fai I ing 1/0 operations in a prescribed sequence and
taking appropriate corrective action.·
It is assumed that I/O operations are performed in an 1/0
interrupt environment and that the status response and con-
dition code bits of an AIO and TDV instruction after an
IIO interrupt provide sufficient information to determine
if the l/O operation was successfu
I.
For fai
I
ing 1/0 opera-
tions, additional information should be obtained from the
status response and condition code bits of a TIO instruction.
Note that before the status of any 1/0 instruction may be
used for testing purposes, the condition codes returned with
the 1/0 instruction must be tested to verify that the 1/0
instruction has been successfully executed and that the
status information is available in the register.
Additional assumptions are that (1) a fault can be attributed
to a specific disk order, i.e., Seek, Sense, etc., and (2)
for multisector operations, the failing sector can be deter-
mined. In the case of command lists, a sequence of opera-
tions are performed before the 1/0 interrupt occurs and
status is available. When an error occurs, the command
list sequence is terminated and the failing order (operation)
can be determined from the TIO current command address.
The programming system then has its choice of repeating
the entire command I ist or rebui I ding the command I ist (by-
passing al I nonfai I ing surface operations) to repeat the
operation that failed.
1/0 INTERRUPT ENVIRONMENT
The 1/0 interrupt environment is established by setting
flags within the Input/Output Command Doubleword
(IOCD) of the command lists and by using specified time-
out delays within software timeout routines.
SOFlWARE TIMEOUT VALUES
Recommended software timeout delay for disk operations is
a minimum of one second.
IOCD FLAGS
IOCD flag settings depend on the type of computer system.
24
1/0 Faul ts and Recovery Procedures
SIGMA 3 IOCD FLAG
The Interrupt flag (I) is set to a 1 in every IOCD. This
wi
11
cause an 1/0 interrupt to be requested whenever a
"channel end" or "unusual end" condition occurs. If data
chaining is specified, the Interrupt flag should be set to a
1 only in the last IOCD of the command list.
SIGMA 5-9 IOCD FLAGS
When the disk is operating with a Sigma 5, 6, 7, 8, or 9
computer, the fol lowing flags must be set:
ICE
IUE
HTE
SIL
Interrupt at Channel End. Th is flag is set to
a 1 only in the last IOCD of a command
I
ist.
Interrupt at Unusual End. This flag is set to
a 1 in all IOCDs.
Halt on Transmission Error. This flag is set
to a 1 in all IOCDs.
Suppress Incorrect Length. This flag is set to
a 1 whenever an incorrect length indication
is to be inhibited from causing an !OP Halt
and a subsequent "unusual end" l/O interrupt.
An incorrect length indication is generated
whenever any of the fol lowing conditions
occur:
1. During a Read, Write, or Check-Write
operation when the number of data bytes
is not an integral multiple of
1024
bytes.
2. During a Seek operation when a byte
count other than four is specified.
3. During a Header Write or Header Read
operation when a byte count other than
a multiple of eight bytes is specified.
ADDmONAL PROGRAMMING CONSIDERATIONS
The fol lowing information must also be considered when
programming a removable disk storage system.
SURFACE FLAWING
When it has been determined that a sector's data field is
unusable due to a defective surface area, all six sectors
of the associated cylinder and head should be written with
headers containing flaw marks. The use of the alternate
cylinder and head assignment fields of these headers is
optional.
SURFACE ORDERS
Automatic surface incrementing of sector and head ad-
dresses occurs afterasurfaceoperation(Read, Write, etc,);