HP 3000 Manuals

Accessing Files Programmer's Guide

Specifying Record Size 

You can specify the size of the records in your file by using the BUILD
(for disk files) or FILE commands, or the HPFOPEN/FOPEN intrinsic;
however, the interpretation of the requested record size can be affected
by the record structure and data format chosen as well as the device for
the file.


NOTE  Within MPE/iX and in various subsystems, the record size for an
      ASCII file is usually identified in terms of bytes (8 bits) and the
      record size for a binary file is identified in terms of half-words
      (16 bits).  This convention is a matter of convenience only, since
      most users think of ASCII files as being character oriented.


When you specify the record size for a fixed-length ASCII file, the
record size determined for the file is the same as that which you
specified for it.  The maximum record size allowed for fixed-length ASCII
files is 32767 bytes.

To maintain compatibility with pre-900 Series HP 3000 computer systems,
the following file types always begin records on half-word boundaries:

   *   ASCII files with variable-length or undefined-length record format

   *   binary files with fixed-length, variable-length, or
       undefined-length record format

For these file types, when you specify an odd-byte record size, the file
system rounds up the value to an even number to reflect the fact that
these records always begin on half-word boundaries.  The maximum record
size you can specify for these file types is 32766 bytes.

When the file is a binary file or a variable-length ASCII file, the extra
byte is available to be used for data.  Figure 2-5  illustrates the
placement of odd-bytes records and the disposition of the added byte.


          Figure 2-5.  Record Placement for ASCII Files 

Rather than specify your own record size, you can accept the default
record size for the device that you are using.  Default record sizes are
listed in Table 2-6 .  Note that subsystem defaults may be different
from MPE defaults; for example, the Editor default may be 72 or 80 bytes
(depending on text format) while the MPE standard default is the record
size configured for the device.

          Table 2-6.  Standard Default Record Sizes 

---------------------------------------------------------
|                           |                           |
|          DEVICE           |    RECORD SIZE (BYTES)    |
|                           |                           |
---------------------------------------------------------
|                           |                           |
| Disk                      |            256            |
|                           |                           |
| Magnetic Tape Unit        |            256            |
|                           |                           |
| Terminals (most cases)    |            80             |
|                           |                           |
| Line Printer              |            132            |
|                           |                           |
| Plotter                   |            510            |
|                           |                           |
| Programmable Controller   |            256            |
|                           |                           |
| Synchronous Single-Line   |            256            |
| Controller                |                           |
|                           |                           |
---------------------------------------------------------

MPE/iX 5.0 Documentation