ZiLOG Z80-RIO Operating System User's Manual page 167

6.14
READ PREVIOUS READ PREVIOUS
Request vector:
Logical unit
Request code - 20 or 21
Data transfer area - address to which data should
be read
Length - number of bytes to read. If this is zero,
no data will be transferred. Otherwise, one
record of data will be transferred. The
number of bytes actually transferred will be
indicated here on return.
Completion return address
Error return address
Completion code
Supplemental parameter information - if the file is
open for random I/O, this field should contain
the address of a three-byte area where the
disk address of the previous record will be
returned. Otherwise, it is unused.
Action:
Unless the length is zero, the record preceding
the current one is read. In either case, the
pointer is backed up one record. The current
record will become the next record, the
previous one the current record, and the one
preceding the previous record will become the
new previous record.
Possible errors:
ALL DISK ERRORS except PROTECTION (code C3)
are possible.
POINTER ERROR (code CA) - if the forward
pointer on the previous record does not
indicate the current one, a pointer error is
reported. The data is still transferred.
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