Jim Phillips (kittykat@spacestar.net)
FIND 16-BIT CHECKSUM OF A FILE
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' PBCHKSUM.bas   PowerBASIC demo program to find 16-bit checksum of a file.
' version 1.00   Written December 1999 for the public domain
'                   by Jim Phillips   kittykat@spacestar.net
'
' This program is based on C + Assembly code in The Zen of Code Optimization
' by Michael Abrash, which I translated and revised for the PowerBASIC
' compiler.  The BASIC part of the code isn't optimized to the max, but the
' Assembly code does most of the work, and that part is *fast*.  On most
' files, the computer will spend more time opening and closing the file than
' finding the checksum.  In other words, this code's speed is hardware
' dependent, so I'm fairly confident that further optimization won't gain you
' very much.  Incidentally, I recommend the Abrash book no matter what
' language you work with.  Some of his examples are real eye openers.
'
'---A Few Compiler Metastatements; Change As You Wish------------------------
$COM 0
$COMPILE MEMORY
$CPU 80386
$DEBUG MAP OFF
$DEBUG PATH OFF
$DEBUG PBDEBUG OFF
$DEBUG UNIT OFF
$DIM NONE
$ERROR ALL OFF
$EVENT OFF
$FLOAT PROCEDURE
$LIB COM OFF, LPT OFF
$LIB GRAPH OFF, CGA OFF, EGA OFF, HERC OFF, VGA ON
$LIB FULLFLOAT OFF, IPRINT OFF
$OPTIMIZE SPEED
$OPTION CNTLBREAK OFF
$SOUND 1
$STACK 1536
$STRING 32                         '(kbytes)
'---------------------------------------------------------------------------

FLEXCHR$ = CHR$(0)                 'Null, so padding won't affect checksum.
DIM TempString AS STRING * 16384   'Fixed-length buffer is easier to tie
                                   'into assembly portion.

INPUT "Enter Path and File Name: ", PathFile$

MTIMER                             'Just for testing speed....

OPEN PathFile$ FOR BINARY AS #1
CheckSum& = 0

asm push bp                 ;preserve value.
asm push si                 ;preserve value.

DO
  GET$ 1, 16384, TempString   'Get next chunk into buffer from file.  Making
                              'buffer size a power of 2 reduces disk sector
                              'reads and gains some speed.  16kbytes seems a
                              'good compromise between old and new PC's,
                              'small and large files.

  asm cld                  ;tell lodsb to increment si.
  asm lea si, TempString   ;put buffer's address offset into si.

  FOR Stepper = 1 TO 128   '128 in Stepper x 128 in cx = 16,384-byte buffer.
    asm mov cx, 128        ;initialize counter.
    asm mov ax, 0          ;initialize.
    asm mov dx, 0          ;initialize.

      'Following is the loop that actually does the check-summing.
    CheckSumLoop:
      asm lodsb            ;get next byte at ds:[si], put in al, incr si.
      asm add dx, ax       ;add byte value to intermediate sum.
    asm loop CheckSumLoop  ;continue until cx counter reaches 0.

      'The intermediate sum in dx will roll over at 65,536 so pass it on to
      'CheckSum& before rollover can happen, and re-initialize dx above.
    asm mov TempSum&, dx   ;put dx value into TempSum&.
    CheckSum& = CheckSum& + TempSum&   'Update our intermediate total.
  NEXT Stepper
LOOP UNTIL EOF(1)

asm pop si                  ;restore pushed value.
asm pop bp                  ;restore pushed value.

CLOSE #1                    'Must be sure to close the file....

ElapsedTime! = MTIMER/1000000   'For testing speed....
PRINT "  Time:"; ElapsedTime!; "second(s)"; "    Checksum:"; CheckSum&
END