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Re: Subtracting times

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Am Wed, 19 Feb 2020 10:03:26 +0000 schrieb Alan Adams:

> In message <757ca84458.Matthew@sinenomine.freeserve.co.uk>
>           Matthew Phillips <spam2011m@yahoo.co.uk> wrote:
> 
>> In message <37f8364458.Alan.Adams@ArmX6.adamshome.org.uk>
>>  on 18 Feb 2020 Alan Adams  wrote:
> 
>>> 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.

>> But that's only if you're sure the result fits in 4 bytes.
> 
> If anyone takes a year and a half to paddle round a slalom course, we
> can time them with a calendar.
> 
>>>   resultbuf%?I%=L%AND&FF
>>> ENDPROC
> 
>> If you wnat a signed result, you can simplify by making the FOR-NEXT
>> loop with I% run from 0 to 4 and lose all the lines after the NEXT.
> 
> The following shows why this makes my head hurt.
> 
> adding two positive numbers shouldn't produce a negative result.
> 
>>A%=&7FFFFFFF PRINT A%
> 2.14748365E9
>>B%=A%+1 PRINT B%
> -2.14748365E9


But: that's no problem at all since You are NOT doing some kind of 
integer mathematics here. You are trying to change the bytes as they are 
stored in memory.

The example with B%=A%+1 results in a "higher" value in memory - but the 
BASIC interprets it as a signed value, and therefore it changes to 
negative value if the top-most bit has been set.
This only means that You can't print this values directly via PRINT - 
there is always the need to use an output-procedure if a "time" value is 
intended.


If You really know that the times measured never (!) execeed the 4 Bytes 
then I dont know why anyone should deal with the highest (the 5th) Byte. 
If these time stamp%s are taken as start and stop times then they are 
ordered since You already know wich is the higher/later one.


One thing - and thats the first I wanted to mention - more:
If this is on a 32 Bit Machine (as an Archimedes or RPC or Pi) then there 
are only 4 Bytes per integer variable% !
If You use a construct such as stamp1%?I% with I%=4 there will be a result 
- but probably not the one, wich gives the correct value for the 5th 
Byte. I think this should give You the least significant Byte of the next 
32Bit variable wich is stored in memory directly after (behind) the 
stamp1% . This means: stamp1%?4 doesn't give the 5th Byte of stamp1% as 
intendend, but will result in Byte 1 of stamp2% ( hence the same as 
stamp2%?0 ).


All the best,
SBn

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Thread

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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