Q22021: OPEN LEN=reclen Effect on Sequential File Buffering and Speed

Article: Q22021
Product(s): See article
Version(s): 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | docerr B_BasicCom | mspl13_basic
Last Modified: 19-JAN-1990

QuickBASIC Versions 2.00, 2.01, and 3.00 don't behave according to the
following statement on Page 370 of the "Microsoft QuickBASIC Compiler"
manual for Versions 2.00, 2.01, and 3.00, which discusses the
recordlength option of the OPEN statement for sequential files:

   When used to open a sequential file, recordlength specifies the
   number of characters to be loaded to the buffer before it is
   written to, or read from, the disk. A larger buffer means more room
   taken from BASIC, but faster file I/O.

Changing the LEN=reclen statement will not change the sequential
file-access speed or buffering for QuickBASIC Versions 2.00, 2.01, or
3.00, but WILL change speed and buffering for QuickBASIC 4.00 and
later versions.

For greater file-access speed, boot with a BUFFERS=20 statement in
your CONFIG.SYS file (DOS configuration file) on the root directory of
your start-up disk.

Please note that in QuickBASIC Versions 4.00, 4.00b, and 4.50 for
MS-DOS, Microsoft BASIC Compiler Versions 6.00 and 6.00b for MS-DOS
and OS/2, and Microsoft BASIC Professional Development System (PDS)
Version 7.00, the LEN=reclen statement changes both the buffer size
and sequential file-access speed. The default sequential-access buffer
size is 512 bytes in these versions.

The 512-byte size gives fast disk input/output speed because it
matches the sector size or a multiple of the sector size for most disk
drives. This is correctly documented in the "Microsoft QuickBASIC 4.0:
BASIC Language Reference" manual for Versions 4.00 and 4.00b.

By changing LEN=256 to LEN=10 or LEN=1000 in the following program, no
file-output buffering or speed changes are apparent in QuickBASIC 2.x
and 3.00 when you watch the disk-access light on the drive. However,
you will notice buffering and speed changes in later QuickBASIC
versions:

   PRINT "Start of program"
   OPEN "buf.dat" FOR OUTPUT AS #1 LEN=256
   PRINT "File was just opened"
   WHILE INKEY$="":WEND
   FOR Index% = 1 to 30
   PRINT "Record number --> ";Index%
   WRITE #1, STRING$(79,CHR$(48+Index%))
   WHILE INKEY$="":WEND
   NEXT Index%
   CLOSE #1
   END

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