Separating bits

[renovatio]

Hello, thanks for reading my topic. Let's suppose I have a byte, and I want to copy each bit to a different variable. How can I do this in assembly?

Example:

Byte =10010011
Bit1 = 1
Bit2 = 1
Bit3 = 0
Bit4 = 0
Bit5 = 1
Bit6 = 0
Bit7 = 0
Bit8 = 1

Thanks in advance.

[AJ]

Hi,

I would use a combination of the AND operator and right rotations. There may be a more optimised way than that, though.

Cheers,
Adam

[renovatio]

Thanks for your help AJ. I found a better way with the BT instruction:

mov ax, 10010011
bt ax, 2
CF = second bit from ax

[Troy Martin]

Nifty, I wonder if that's in the 186 instruction set, that way I can use it in my kernel!

Let me guess: you're implementing printing a binary number?

EDIT: Awww, damn, 386.

[renovatio]

Not, i'm writting a fat12 driver, and i want to know the attributes of a file... . The instruction is able to use since 386 (sorry for my english )

[Combuster]

hmm 80186?

Code: Select all

  mov cx, 8
  mov dl, BYTE
  lea di, [destination]
.loop:
  ror dl, 1
  sbb al, al
  neg al
  stosb
  loop .loop

or something like that

[Owen]

The and/shift method actually executes faster than the bit instructions on anything newer than a 80486 IIRC

[DeletedAccount]

Hi,
AND is called the bit tester .

OR is called the bit setter
Regards
Shrek

[Troy Martin]

Combuster: isn't lea reg,[label] slower and bigger than mov reg,label? There are less calculations to do, IIRC.

OP: When I did my hex printing functions, I did ror ax,4 and then shr ax,4 to isolate the lower nybble. Technically, you could use that in conjunction with more shifting to isolate a single bit. That's how I'd do it with 186 instructions.

[madeofstaples]

Combuster: isn't lea reg,[label] slower and bigger than mov reg,label? There are less calculations to do, IIRC.

OP: When I did my hex printing functions, I did ror ax,4 and then shr ax,4 to isolate the lower nybble. Technically, you could use that in conjunction with more shifting to isolate a single bit. That's how I'd do it with 186 instructions.

If bits frequently need to be tested, it may be helpful to implement de Bruijn Sequence. If you are sure of your target architecture, then there's no need to calculate a de Bruijn sequence and build an appropriate hash table at run-time.

[Combuster]

LEA is fast. One clock on 486/586, i.e. the same as a regular move.

The point is, I pumped out a method that should work, used LEA to make it obvious that an address is to be used, to give one possible approach. If you're worried about clocks cycles I'd start with loop unrolling and lookup tables instead of shifts.