The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

operating system for such a purpose. The system's device I/O handler also
makes use of this bit to determine the necessity for disk file character
I/O.
Bit 6 This bit will be set to a "1" if the file was opened with UPDATE or greater
access. It indicates to the CLOSE routine that the application has the
authority to reset the "file open bit" in the directory entry record for the
respective file. The CLOSE routine will not update the directory entry of a
file without this bit being set in the FCB.
Bit 5 This bit indicates that the opened file is a Partitioned Data Set. The
system will set this bit when the file is opened if it detects the presence
of the PDS attribute in the directory entry of the file (DIR+0, bit 5).
Bit 4 This bit is reserved for future use by the DOS.
Bit 3 This bit is reserved for future use by the DOS.
Bit 2 This bit will be set to a "1" if any WRITE operation is performed by the
system on this file while it is open. The bit is used specifically to update
the MOD flag in the file's directory entry record when the file is closed.
Bit 1 This bit is reserved for future use by the DOS.
Bit 0 This bit is reserved for future use by the DOS.
6.4.2 Input/Output Status -
This byte contains I/O buffer status flag bits used in read/write operations by the
system. The STATUS byte uses each bit as a flag per the following specifications:
Bit 7 If this bit is set to a "1", it indicates that I/O operations will be either
record operations of logical record length (LRL) less than 256 (1-255) or
character I/O. If set to a "0", only full sector operations or character I/O
will be performed. If you are going to utilize only full sector I/O, system
overhead is reduced by specifying the LRL at open time to be 0 (indicating
256). An LRL of other than 256 will set bit 7 to a "1" when the file is
first opened.
Bit 6 When a file's records have been accessed randomly rather than (or in
addition to) sequentially, the system must be prohibited from the altering
the Ending-Record-Number (ERN) unless the file is extended beyond its
current ERN. This bit is used for that status. If set to a "1", it indicates
that the ERN is to be set to the Next-Record-Number (NRN) only if the NRN
exceeds the current value of ERN. Whenever the position SVC (@POSN) is
invoked, it will automatically set bit 6. If bit 6 is set to a "0", then ERN
in the FCB will be updated on every WRITE operation.
Bit 5 It is always necessary for the system to know whether or not the file buffer
contains the current disk sector as specified by the NRN. This bit is
maintained for that use. If it is set to a "0", then the disk file buffer
contains the current sector denoted by NRN. If it is set to a "1", then the
file buffer does not contain the current sector. When a sector is read into
the disk buffer, the system will reset this bit to show that the buffer
currently holds the disk sector specified by the NRN. During character I/O,
the first character GET request will force the system to transfer a full
disk sector into the file buffer and reset the "buffer current" bit. Bit 5
is automatically set when the character in the last byte of the buffer has
been transferred to the application in the GET requests. This will then
indicate that the buffer is not current so that the next GET will force a
read of the next sector.
Bit 4 During file I/O, an application may request a repositioning of the file's
NRN-OFFSET pointer. This may be requested via an @BKSP, @POSN, @REWIND,
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