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Forum Index : Microcontroller and PC projects : MMBasic and Bit Shifts.
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vegipete Guru  Joined: 29/01/2013 Location: CanadaPosts: 1185 |
What would you expect a bit shift by a negative number to do? Would you expect, for example, 64 << -2 to be the same as 64 >> 2? As it stands, bit shifting by a negative gives 0. Could that be changed? (CMM2, but maybe others too.) Visit Vegipete's *Mite Library for cool programs. |
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SimpleSafeName Guru Joined: 28/07/2019 Location: United StatesPosts: 351 |
Would it be up to the user not to wrap around into negative territory, or does that matter? Say 16 << -6 for example. |
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Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3368 |
Not on the Micromite. This is one for Peter but I suspect that it is a "feature" of the ARM architecture. Geoff Geoff Graham - http://geoffg.net |
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| vegipete Guru Joined: 29/01/2013 Location: CanadaPosts: 1185 |
I'd say it's up the user to keep track of shift size. And said user better have a good idea what's going on if shifting negative integers. There is the >>> operator which does treat negative integers sensibly. Although I haven't determined what it does with a negative shift amount. The effect of <<< is slightly weird - maybe a left shift of nothing, followed by a less than comparison, giving a result of 0 or 1. Visit Vegipete's *Mite Library for cool programs. |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 11868 |
The CMM2 code is identical to all other MMBasic implementations so this must be what the ARM H/W does The only thing I could do is trap negative shifts as an error. The difference between >> and >>> is described on page 56 of the 5.07.00 manual <<< makes no sense and does not exist Edited 2021-08-01 17:28 by matherp |
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| vegipete Guru Joined: 29/01/2013 Location: CanadaPosts: 1185 |
> ? 64 >> 2 ' #1 16 > ? 64 >>> 2 ' #2 16 > ? -64 >> 2 ' #3 4611686018427387888 > ? -64 >>> 2 ' #4 -16 > ? 64 >>> -2 ' #5 0 > ? -64 >>> -2 ' #6 -4294967296 > ? 16 << 2 ' #7 64 > ? 16 <<< 2 ' #8 0 > ? 1 << 6 ' #9 64 > ? 1 <<< 6 ' #10 1 #1 to #5 : as expected #6 : don't know #7, #9 : as expected #8, #10 : parsing error? My original question of whether n >> -s could be treated the same as n << s (and by extension, n << -r same as n >> r) comes about from some pixel/bit map shifting code I'm fiddling with. As it stands, I need different pieces of code for left and right shifts. Visit Vegipete's *Mite Library for cool programs. |
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twofingers Guru Joined: 02/06/2014 Location: GermanyPosts: 1769 |
@vegipete ?? Peter said: Hmm. Do I miss something? Michael ‚My name is Ozymandias, king of kings Look on my works, ye Mighty, and despair!‘ Nothing beside remains. Round the decay Of that colossal wreck, boundless and bare The lone and ... |
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| vegipete Guru Joined: 29/01/2013 Location: CanadaPosts: 1185 |
<<< does not throw an error. Instead, it returns slightly unexplainable results. As a operator, it doesn't exist. But as 3 less than signs in a row, it does exist when perhaps it should be an error. Visit Vegipete's *Mite Library for cool programs. |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6604 |
This is how the interpreter reads it: > print 16 << < 2 0 > print 1 << < 6 1 > It seems to skip over the << or treat it as 'x << 0' reducing the code to: > print 16 < 2 0 > print 1 < 6 1 > Not all errors get found by the interpreter. Edited 2021-08-02 09:18 by TassyJim VK7JH MMedit |
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