ZiLOG Z80-RIO Operating System User's Manual page 134

The information contained in the descriptor record is as
follows:
Bytes
0-3
Bytes
4-5
Bytes
6-7
Bytes
8-9
Bytes
10-11
Byte
12
Bytes
13-14
Bytes
15-16
Bytes
17-18
Byte
19
Bytes
20-21
Bytes
22-23
Bytes
24-31
Bytes
32-39
Bytes
40-127
Reserved for future expansion
File ID - currently unused
Pointer to directory sector holding
entry for this file
Pointer to first data record of file
Pointer to last data record of file
File type and subtype - see
description with the OPEN request
Record count
Record length
Block length - currently unused,
and set to be same as record length
File properties - see description
with the OPEN request
Starting execution address for
procedure files (entry point)
Number of bytes in last record
Date of creation
Date of last modification
Available for programmer definition
The Date of creation and Date of last modification are moved
to the descriptor from the System Global Variable DATE at the
appropriate times. Thus, if DATE is maintained to indicate
the current date, then the descriptor record can give some
historical information about the file.
The information stored in the descriptor (except for the
first
12
bytes) is available to the program accessing the
file at the time it is OPENed or while it is open by means
of a QUERY ATTRIBUTES request. It can also be supplied at
the time the file is created, or later by a SET ATTRIBUTES
request, or when the file is UPDATED or CLOSED. See the
appropriate request description for -details.
Implicit in the description of accessing a file is the
concept of the file pointer. There are actually three
pointers. The one referred to as the pointer is the
"current record pointer", the disk address of the record
considered to be the current one. This is normally the
record last handled, as, for example, in reading or
writing. The "previous record pointer" contains the disk
address of the record preceding the current one in the
sequence of the file. The "next record pointer" contains the
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