Q59725: INTERRUPT for Clock Tick Counter Returns Negative Value

Article: Q59725
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900222-218 B_BasicCom | mspl13_basic
Last Modified: 26-MAR-1990

Interrupt 1A Hex, with Function 0, returns the current value of the
clock tick counter in registers CX and DX. When the low-order portion
of the clock tick counter (returned in DX) exceeds 32,767, it becomes
negative when stored as an integer in BASIC, because 32,767 is the
maximum value for a 2-byte signed integer. Below is an example to
convert the negative value to the equivalent unsigned long integer
value.

This information applies to Microsoft QuickBASIC Versions 4.00, 4.00b,
and 4.50, to Microsoft BASIC Compiler Versions 6.00 and 6.00b, and to
Microsoft BASIC Professional Development System (PDS) Version 7.00 for
MS-DOS.

For a more complete description of converting negative signed integers
to unsigned integers, query on the following words:

   CALL and INTERRUPT and negative and signed and integer

The following QuickBASIC program outputs only positive values by
removing the sign bit and adding the remaining number plus 1 to
32,767:

' $INCLUDE: 'qb.bi'
REM  In BASIC 7.00, use $INCLUDE: 'qbx.bi' instead of 'qb.bi'.
REM  To use CALL INTERRUPT in this program, you must do the following:
REM  If you run this program in QB.EXE, use QB /L QB.QLB.
REM  If you run this program in QBX.EXE, use QBX /L QBX.QLB.
REM  LINK with QB.LIB (or QBX.LIB for BASIC 7.00).

DIM inregs AS regtype, outregs AS regtype
LOCATE 23, 1: PRINT "Push any key to end program."
DO
inregs.ax = 0
CALL interrupt(&H1A, inregs, outregs)
high& = outregs.cx
low& = outregs.dx
IF low& < 0 THEN              'if the low-order portion is negative:
  low& = low& AND &H7FFF      'remove sign bit
  low& = low& + &H7FFF + 1    'add remaining number plus 1 to 32,767
END IF
LOCATE 24, 1: PRINT high&; ":"; low&; "   ";
LOOP UNTIL INKEY$ <> ""
END

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