How to manipulate specific bits of a variable

[Darkhog]

Using freepascal, you can now have bit packed records like so:

{
    Flag register:
    ---------------
    Bit[0] - Reserved
    Bit[1] - c (Unsigned Carry and Unsigned Borrow flag)
    Bit[2] - z (Zero flag)
    Bit[3] - Reserved
    Bit[4] - Reserved
    Bit[5] - Reserved
    Bit[6] - o (Signed Overflow Flag)
    Bit[7] - n (Negative Flag; aka Sign Flag)
}


type
  TBit  = 0..1;
  TFlags = bitpacked record
    _0 : TBit;
     c : TBit;
     z : TBit;
    _3 : TBit;
    _4 : TBit;
    _5 : TBit;
     o : TBit;
     n : TBit;
  end;

here, you can access all the separate bits as a separate variable in the record...rather neat!
It isn't supported under Delphi unfortunately, but if you only use freepascal, then great

Does variable names here actually bears any importance or could I just write:

type
  TBit  = 0..1;
  TByte = bitpacked record //TByte to avoid confusion with actual Byte type
    b1 : TBit;
    b2 : TBit;
    b3 : TBit;
    b4 : TBit;
    b5 : TBit;
    b6 : TBit;
    b7 : TBit;
    b8 : TBit;
  end;

//edit: I wrote (but didn't test) function that would check if specific bit is set. You'll need math unit for that one:

function IsBitSet(num:integer;bit:byte):Boolean
var bitnum:Integer;
begin
  bitnum:= Trunc(intpower(2,bit);
  Result:= (num and bitnum) = bitnum;
end;

Though it should work fine.

[User137]

//edit: I wrote (but didn't test) function that would check if specific bit is set. You'll need math unit for that one:

function IsBitSet(num:integer;bit:byte):Boolean
var bitnum:Integer;
begin
  bitnum:= Trunc(intpower(2,bit);
  Result:= (num and bitnum) = bitnum;
end;

Though it should work fine.

Dan already gave more optimized code for that

function GetBit(const Value, Bit: LongWord): Boolean;
begin
  Result := Boolean((Value shr Bit) and 1);
end;

Both trunc() and intpower() do a massive amount of extra work.

[Darkhog]

I guess it should work fine too. But why using longword for bit no.? Byte would do here much better and I don't suppose we'll need to check like 5000th bit of value soon. Therefore longword would just waste memory while byte would be much more memory-efficient here, especially when dealing with embedded systems.

[imcold]

Byte would be worse actually - more generated code. And you wouldn't save any memory, as it would be most likely passed in a register or pushed as a 4B value on the stack, depending on ABI.