ugBASIC forum

Posted: **Wed Mar 26, 2025 12:08 pm**

Hello,

I refer to https://ugbasic.iwashere.eu/manual/datatypes, especially the text:

x = y HAS BIT 4 : ' this is like y AND 32
x = y HAS NOT BIT 4 : ' this is like y AND 239
x = BIT 4 OF y : ' this is like y AND 32

it seems to me that the second line is somewhat wrong in the comment. I would think that this stands for "y and 223" (239 is 255-16, not -32)? Semantics would then be that this one bit is not set? Or should it mean that all the other bits are set? This is not so well described, I must say. It looks from this formula that all the other bits are meant to be set, no matter whether bit 4 is set or not. Is that correct?

Posted: **Wed Mar 26, 2025 8:32 pm**

Hi Hrundy and welcome!

Hrundy wrote: ↑Wed Mar 26, 2025 12:08 pm it seems to me that the second line is somewhat wrong in the comment. I would think that this stands for "y and 223" (239 is 255-16, not -32)?

I think you are right about the comment, but not on the value, that seems not exact. Infact, the HAS NOT BIT instruction will check a bit off (0) by checking the state of a specific bit inside the given data. The bit position is zero based, and starts from the less significative bit and go on. In implementation terms, the instruction is like NOT( x HAS BIT n). Since x HAS BIT n can be expressed as x AND 2^n, and NOT( a AND b) can be expressed as a AND NOT b. So, when we are talking about BYTE data type:

Code: Select all

x = y HAS NOT BIT 4 
x = NOT (y HAS BIT 4)
x = NOT (y AND 2^4)
x = NOT (y AND 16)
x = y AND NOT %00010000
x = y AND %11101111
x = y AND 239

Hrundy wrote: ↑Wed Mar 26, 2025 12:08 pmSemantics would then be that this one bit is not set?

The desired semantics would be to test that a bit is zero. Honestly, since you pointed this out to me, I'm not so sure that it's computed correctly. We should do some test cases to see that.

Hrundy wrote: ↑Wed Mar 26, 2025 12:08 pmOr should it mean that all the other bits are set? This is not so well described, I must say. It looks from this formula that all the other bits are meant to be set, no matter whether bit 4 is set or not. Is that correct?

You are absolutely right about this. At the moment the current implementation, which I repeat I do not know if it is completely correct, is the one I illustrated above. Since the semantics should be to test that a bit is zero, in reality I believe that the correct function is:

Code: Select all

x = (y AND 2^n) = 0

At this point I ask for confirmation to fix the behavior.

Thank you again!

ugBASIC forum

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Description of bit type a bit unclear

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