Groups > comp.sys.acorn.programmer > #5981

Subtracting times

Subject	Subtracting times
Newsgroups	comp.sys.acorn.programmer
From	Alan Adams <alan@adamshome.org.uk>
Date	2020-02-18 11:55 +0000
Message-ID	<37f8364458.Alan.Adams@ArmX6.adamshome.org.uk> (permalink)
Organization	Orpheus Internet Services

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My head is hurting.

I need a BASIC routine to subtract one 5-byte time from another and return 
a correctly signed result.

Complications arise at the point where the low word goes from &7fffffff to 
&80000000, and where the high byte increases.

BASIC doesn't allow access to the carry or borrow from arithmetic 
operations.

I've developed the following which *seems* to work correctly. However I'm 
not sure about all the edge cases.

This is part of a timing system, Next February the high byte of times will 
change from &57 to &58, and I would like this code to survive working 
across this boundary. I need to reliably detect negative results, which 
can occur if a spurious finish time pulse arrives before a start time.

Subtract stamp2 from stamp1 and store the result in resultbuf.
( Although the result is 5 bytes. the elapsed time will always be 
contained in the low 4 bytes. This will only overflow after 1 year, 5 
months and 2 hours 27 minutes. The system doesn't meed to run for that 
long. )
                                                                 
PROCsubstamp(resultbuf%,stamp1%,stamp2%)
LOCAL I%,J%,K%,L%,borrow%
borrow%=0
  FOR I%=0 TO3
    J%=stamp1%?I%
    K%=stamp2%?I%
    L%=J%-K%-borrow%
    resultbuf%?I%=L%AND&FF
    IF L% < 0 THEN borrow%=1 ELSE borrow%=0
  NEXT
  I%=4
  J%=stamp1%?I%
  K%=stamp2%?I%
  L%=J%-K%-borrow%
  IF L% < 0 THEN
    resultbuf%!0 = -(resultbuf%!0)
  ENDIF
  resultbuf%?I%=L%AND&FF
ENDPROC


-- 
Alan Adams, from Northamptonshire
alan@adamshome.org.uk
http://www.nckc.org.uk/

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Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-18 11:55 +0000
  Re: Subtracting times Jean-Michel <jmc.bruck@orange.fr> - 2020-02-18 19:11 +0100
    Re: Subtracting times Steve Drain <steve@kappa.me.uk> - 2020-02-21 03:28 -0800
      Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-21 12:45 +0000
      Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-21 15:17 +0000
        Re: Subtracting times Steve Drain <steve@kappa.me.uk> - 2020-02-22 10:37 +0000
          Re: Subtracting times Steve Drain <steve@kappa.me.uk> - 2020-02-25 16:05 +0000
            Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-25 16:35 +0000
              Re: Subtracting times Steve Drain <steve@kappa.me.uk> - 2020-02-27 17:10 +0000
                Re: Subtracting times Steve Drain <steve@kappa.me.uk> - 2020-02-27 10:06 -0800
                Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-28 11:21 +0000
                Re: Subtracting times jeffrey.a.doggett@gmail.com - 2020-04-22 12:00 -0700
                Re: Subtracting times Martin <News03@avisoft.f9.co.uk> - 2020-04-23 00:07 +0100
                Re: Subtracting times jeffrey.a.doggett@gmail.com - 2020-04-23 03:13 -0700
                Re: Subtracting times Martin <News03@avisoft.f9.co.uk> - 2020-04-23 18:01 +0100
                Re: Subtracting times "John Williams (News)" <UCEbin@tiscali.co.uk> - 2020-04-23 18:06 +0100
                Re: Subtracting times jeffrey.a.doggett@gmail.com - 2020-04-23 11:18 -0700
                Re: Subtracting times Martin <News03@avisoft.f9.co.uk> - 2020-04-23 21:23 +0100
                Re: Subtracting times David Higton <dave@davehigton.me.uk> - 2020-04-24 15:24 +0100
                Re: Subtracting times David Higton <dave@davehigton.me.uk> - 2020-04-23 20:57 +0100
        Re: Subtracting times jgh@mdfs.net - 2020-02-23 17:39 -0800
          Re: Subtracting times Steve Drain <steve@kappa.me.uk> - 2020-02-24 10:23 +0000
            Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-24 11:12 +0000
              Re: Subtracting times Martin <News03@avisoft.f9.co.uk> - 2020-02-24 13:05 +0000
  Re: Subtracting times Matthew Phillips <spam2011m@yahoo.co.uk> - 2020-02-19 08:35 +0000
    Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-19 10:03 +0000
      Re: Subtracting times Sebastian Barthel <naitsabes@freenet.de> - 2020-02-19 13:04 +0000
        Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-19 16:41 +0000
          Re: Subtracting times jgh@mdfs.net - 2020-02-19 12:01 -0800
      Re: Subtracting times Matthew Phillips <spam2011m@yahoo.co.uk> - 2020-02-23 20:46 +0000
  Re: Subtracting times Martin <News03@avisoft.f9.co.uk> - 2020-02-19 23:54 +0000
    Re: Subtracting times Alan Adams <alan@adamshome.org.uk> - 2020-02-20 11:18 +0000

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