All regular MPE disk files have one of three logical record formats: fixed length, variable-length, or undefined length. We usually speak of a file in terms of its record format, for example, "variable-length file". Spool files are variable-length files with additional overhead. Here is a quick review of the essential elements of any MPE variable-length file.
When you specify a record length and/or blocking factor for a variable length file, you tell the file system how large the physical record should be. For MPE V/E systems, the physical record determines how disk storage is allocated. We limit the present discussion to disk files. For MPE/iX systems, disk storage is allocated differently, so the physical record is now more of an abstraction that defines a fixed limit to variable-length records. In practice this means that if the next logical record to be added to the file does not fit in the current physical record, a new physical record must be started for it.
Table A-1 shows a typical physical record consisting of several variable-length logical records.
Table A-1 Physical Record (Block)
| byte count |.................................... | |
| ...................| byte count |................ | |
| [vellip] | |
| .....| byte count |.............................. | |
| ..................................| -1 |///////// | |
Because each record may have a different length, when you write a record to the file, the file system prefixes the record with a 16-bit byte count. The byte count represents the number of data bytes in the record, excluding itself and (for ASCII files) any unused odd byte at the end of the record. Within a given physical record, the file system uses these byte counts to find the start of the next logical record relative to the start of the current one.
Logical records never span a physical record boundary, so eventually there comes a time when you write a record that does not fit in the space remaining in the current physical record. When the file system discovers this situation, it writes a special byte count of -1 (octal 177777), then writes your record at the start of a new physical record. The space between the -1 and the end of the physical record is not used. (Later, when you read the file, the -1 alerts the file system that there is no more data in the current physical record and that it should retrieve your next record from the next physical record).
A spool file is a special case subset of a variable-length file:
Its physical record (block) length is always 1K (1024) bytes.
It is always an ASCII (never a binary) file.
In addition to the byte count shown above, each record contains four more 16-bit fields (eight more bytes) of overhead (described below).
The first byte of the data portion of the record may be a carriage control (CCTL) byte; if so, it too is considered overhead.
The last four bytes of each physical record contain a record count, thereby shortening by four the number of bytes available for spool file records.
Table A-2 displays a typical spool file block.
Table A-2 Spool File Block
| physical byte count | logical byte count | func | P1 | P2 |.............. | |
| .........| physical byte count | logical byte count | func | P1 | P2 |..... | |
| [vellip] | |
| ....| physical byte count | logical byte count | func | P1 | P2 |.......... | |
| ........................| -1| ////////////////// | 1st record of block | |
- Field, Description
- Physical byte count
Same as the "byte count" shown in table A-1. The new name distinguishes it from "logical byte count".
- Logical byte count
The original byte count of the data portion of the spool file record (nominally everything between the P2 field and the physical byte count field of the next record). It was included originally so that trailing blanks of a record could be left out of the spool file and reinserted at some lower printing level such as the printer driver, if necessary. In practice it has not been used this way, so its value is usually eight less than the physical byte count (remember that the physical byte count does not include itself). Further discussion is beyond the scope of this description.
- Func, P1, P2
These are low-level printer parameters. Originally they were part of the MPE V/E privileged interface to ATTACHIO. Although the native mode spooler does not use ATTACHIO, the position and function of these parameters have been retained. A complete description is beyond the scope of this appendix, but generally speaking, FUNC is a printer
function code (such as write or control), while P1 and P2 ("P" for "parameter") vary with function code and further define the operation to be performed.
- First record of block
This is an ordinal whose value is the record number (relative to the start of user data in the spool file; 0 is the first record) of the first record of the current spool file block. For example, if the first block of user data consists of 8 records (0 through 7), the field at the end of the first block is 0 and the field at the end of the second block is 8 (because record 8 is the first record in the second block). This field was added to MPE V/E spool files to allow pseudo-direct access to any record in the spool file. It has been retained in native mode spool files. Ordinarily, variable-length files must be read serially. This means that to display record 10000 you have to read and discard records 0-9999; however, using multirecord NOBUF access and binary search techniques, this field allows you to access record 10000 much more rapidly.
Figure A-1 Example: FUNC, P1, P2
PARAMETER VALUE MEANING
FUNC 1 Write
P1 1 The first byte of the data portion
of the record is a carriage
control (CCTL) byte, usually the
one specified by a user in his/her
FWRITE intrinsic call. It is not
part of the user data and should
not be displayed on the printer.
P1 <> 1 P1 is itself the CCTL byte. The
first byte of the data portion is
part of the user data and should
be displayed.
P2 Bitmask If the least significant bit (15)
is 1, the record is to be printed
in prespace mode.
If it is 0, the record should be
printed in postspace mode. See the
note below.
If bit 14 is 1, certain printer
operations are carried
out without regard for the
perforation area separating two
sheets of Z-fold paper. If bit 14
is 0, these operations skip
over the perforation area, if
necessary. Refer to the
MPE/iX Intrinsics Reference Manual (32650-90028)
for details.
All other bits are reserved.
|
In postspace mode, the data in the record is sent to the printer before the CCTL byte is interpreted and sent. In prespace mode, the CCTL byte is interpreted and sent first, followed by the data.
