Bit and Byte questions. Help needed.

Don M

I have a byte that is comprised of these 8 bits: aabbcccc and want to be able to write to specific bits or pairs of bits.

I know (after learning from Jonny Macs help) that I can read / write to the upper and lower bits by "& $0F" or ">> 4" but I don't understand how I could read / write to the "aa" or "bb" bits.

Thanks.
Don

JonnyMac

Here's how to write to the aa bits -- you should be able to work the rest out from here:

workbyte := (workbyte & %0011_1111) | ((newbits & %11) << 6)

Don M

Thanks Jon. I understand it now.

Bits

Would this work for your needs, requires a global variable 'Globalvar"?

[FONT=arial]Pub SetBit(_in)[/FONT]
[FONT=arial]
[/FONT]
[FONT=arial]  Globalvar := (|< _in) | Globalvar

[/FONT]

Don M

Bits wrote: »

Would this work for your needs, requires a global variable 'Globalvar"?

[FONT=arial]Pub SetBit(_in)[/FONT]
[FONT=arial]
[/FONT]
[FONT=arial]  Globalvar := (|< _in) | Globalvar

[/FONT]

Bits- I'll look at your suggestion too. Thanks for your help.

Don M

I am having trouble grasping how to get an individual bit of a byte. I was looking through the manual at bitwise decode and others but am not sure how best to do this (read- don't understand it).

Lets say I have byte abcd (a, b, c, d just represent the bit positions in the byte). Below is what I was thinking for a way to test it but was wondering if this is the best / easiest way to get a, b, c or d.

byte abcd

abcd >> 3    ' to get bit a

(abcd & $8) >> 3   ' another way to get bit a

(abcd & $4) >> 2  ' to get bit b

(abcd & $2) >> 1  ' to get bit c

abcd & $1          ' to get bit d

Thanks.
Don

Mike G

Try this

CON
  _clkmode = xtal1 + pll16x     
  _xinfreq = 5_000_000

  CR            = $0D
  LF            = $0A
  NULL          = $00

DAT
  data  byte  %10101011
VAR


OBJ
  pst           : "Parallax Serial Terminal"
  
PUB Main | i
  pst.Start(115_200)
  pause(500)

  repeat i from 0 to 7
    pst.str(string("Bit "))
    pst.dec(i)
    pst.str(string(" = "))
    pst.dec(GetBit(data, i))
    pst.char(CR)

  return

PUB GetBit(value, position)
  if (value & |< position)
    return 1
  else
    return 0

PRI pause(Duration)  
  waitcnt(((clkfreq / 1_000 * Duration - 3932) #> 381) + cnt)
  return

JonnyMac

I am having trouble grasping how to get an individual bit of a byte.

I suspect you're making it harder than it is. This will return 1 or 0 from value.pos.

pub get_bit(value, pos)

  return (value >> pos) & 1

While we're at it, here are complimentary methods, set (to 1) and clear (to 0):

pub set_bit(value, pos)

  return (value | (1 << pos))


pos clear_bit(value, pos)

  return (value & !(1 << pos))

Don M

Thanks Mike & Jon.