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

@SKIP, @PEOF, or @SEEKSC SuperVisor Call. It is important for the system to
know whether or not the disk file buffer has been changed since it was read
from the file. If the buffer has been altered, it is necessary to write the
buffer back to the file prior to any movement of the file pointer. This flag
conveys such status. If it is set to a "1", it indicates that the buffer
contents have been changed since the buffer was read from the file. If it is
set to a "0", the indication is that the buffer has not been modified. The
system will set this bit whenever a WRITE operation is performed on the
buffer by either a PUT or the write of a record (of LRL < 256). The bit is
reset by the system when the buffer is physically written to the disk via
the @WRSEC SuperVisor Call request.
Bit 3 The normal method to reflect changes in a file's directory entry record data
is to update the directory entry only when the file is closed. Thus, the FCB
contains all of the information pertinent to the modifications. This keeps
the directory accesses to a minimum and results in faster file throughput.
However, it is important to note that if the system crashes after extensive
file updating (specifically where the file has been extended), the added
information will be unrecoverable without manual corrections to the file's
directory entry record. It is possible to force the system to always update
the directory whenever the system extends the file by writing another
sector. Unattended operation may utilize this extra measure of file
protection. It is specified by appending an exclamation mark "!" to the end
of a file specification when the filespec is requested at open time. This
bit will then be set by the system. It is used to specify that the directory
record is to be updated every time that the NRN exceeds the ERN.
Bits 2-0 These bits will contain the access protection level as retrieved from the
directory entry record of the file when the file is first opened. The
specific bit pattern will be adjusted to the protection level granted
according to the password (OWNER vs USER) entered at file open time.
6.4.3 PDS Member Origin Offset -
When a Partitioned Data Set (PDS) has been opened for individual member access (a sector
origin member), the PDS linkage routines will adjust the EOF contained in the FCB to be
the logical EOF of the member. The member origin offset is the number of relative sectors
between the logical ERN of the member and the first relative sector of the member. This
byte will contain that forward offset so that the linkage routines may be able to
calculate the logical beginning of the member. The calculation is required for linkage to
all SuperVisor Calls that reference file positioning forward of the NRN (@BKSP, @REWIND,
@POSN, @SEEKSC).
6.4.4 Disk File Buffer Pointer -
This is a pointer to the disk file buffer that is used for all disk I/O associated with
the file. The pointer is a 16-bit address stored in normal low-order - high-order format.
This pointer is the buffer address specified in register pair HL at open time.
6.4.5 Next Record Number Byte Offset -
When a file is accessed with either character I/O or record I/O of Logical Record Length
less than 256, requests for I/O may not necessarily require the transfer of a physical
sector from/to the disk. Therefore, the system needs a pointer to the byte position
within the buffer that is to be used for the next I/O operation. This field contains that
position - it is termed an OFFSET within the sector pointed to by the NRN. If this offset
is a zero value, then the next byte to be transferred during an I/O operation is
dependent on whether or not the buffer contains the current sector as noted by FCB+1, bit
5. The system automatically maintains this OFFSET byte during record and character I/O.
If your application is performing full sector I/O for writing data while it is
maintaining its own character buffering, then it is important for it to maintain this
byte when the file is closed if the true end-of-file offset is not at a sector boundary.
Remember, this offset is a pointer to the next available buffer position and not to the
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