Best way to address bits in a byte?

RichardF

I need to be able to read and write individual bits within a byte variable in order to keep track of eight switch positions (on/off). This is easy to do in pBasic using the .BITn form, but I don't see an easy way (or frankly any way) to do this in Spin. Would appreciate some suggestions.
Thanks,
Richard

Mike Green

byteVar &= !Mask ' To set a bit off
byteVar |= Mask ' To set a bit on
byteVar ^= Mask ' To toggle a bit
if byteVar & Mask ' To test if a bit is on
where
CON Mask = %10000000 ' or something similar

Rayman

Usually, I'd do something like

Mask:=1<<bit

to make a mask

tekochip

Here's some examples with functions to do the same:

CON
  #0, Serve, Dispense, Flash 'Declare some bits
 
PUB Demo |coils
 
  OnBit(@coils, Serve)
  ToggleBit(@coils, Flash)
 
  if TestBit(@coils, Flash)
    OffBit(@coils, Serve)

PUB OnBit(x, bit)
{{Turn a bit on}}
  long[noparse][[/noparse]x]|=1<<bit
 
PUB OffBit(x, bit)
{{Turn a bit off}}  
  long[noparse][[/noparse]x]&= !(1<<bit)
 
PUB ToggleBit(x, bit)
{{Toggle a bit}}
  long[noparse][[/noparse]x]^=1<<bit
 
PUB TestBit(x, bit)
{{Return true if a bit is on}}
  return long[noparse][[/noparse]x] & (1<<bit)        

RichardF

Thanks everyone. Makes me appreciate pBasic (smile)
Richard

hippy

For reading bits you could create a method which handles that, "var.bit4" becomes "bit4(var)", and option which is consistent and works for read and write is "GetBit4(@var)" and "SetBit4(@var,bitVal)", something like ...

PUB GetBit4( varPtr )
  result := GetBitX(4, varPtr )
PUB GetBitX( bitNum, varPtr )
  result := ( byte[noparse][[/noparse] varPtr ] & |< bitNum ) <> 0

etc

Ariba

Another idea is to use the outb and dirb registers as bitaddressable variables (Prop 1 has this registers but no PortB hardware):

1)
  outb[noparse][[/noparse] 4] := 1
  if outb[noparse][[/noparse] 5] == 1
    outb[noparse][[/noparse] 4] := 0
2)
  outb[noparse][[/noparse] 4]~~
  if outb[noparse][[/noparse] 5]
    outb[noparse][[/noparse] 4]~

1) is better readable and 2) is more optimized.
You get up to 64 bits with this methode. And if you don't use the timers, also frqa, frqb, phsa, phsb are good for bitaddressing.

Andy

Edit: the space after [noparse][[/noparse] is only for the forum software

Post Edited (Ariba) : 3/28/2008 4:02:15 PM GMT

RichardF

Two more great ideas, thank you.
Ariba, your use of the b registers is really cool. I have always assumed they were not addressable, since there are no associated pins.

Beau Schwabe

Ariba,
·
outb is ok to use on the Current Propeller Chip, but it will not be backwards compatible on the next Propeller Chip
·
Instead, if you are not using any or one of the counters, you could use the frqa, frqb, phsa, and phsb registers to do basically the same thing that you are suggesting.
Stay away from the ctra, and ctrb registers as you could startup one of the counters without intending to if you write to them.
·


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Rayman

Ariba, Beau,

I like this idea! I rarely use frqa, frqb, phsa, or phsb in the main cog. So, this is a very nice way to access bits!

Javalin

Chaps,

Surely:

if byteVar & Mask ' To test if a bit is on
where
  'CON Mask = %10000000 ' or something similar

should be

if byteVar & Mask == Mask ' to test if a bit is on

so -·if byteVar is %00001111 & %00001000 would leave %00001000 so surely wouldn't equate to TRUE/FALSE or 1 or 0?

??

James

Post Edited (Javalin) : 3/28/2008 7:37:55 PM GMT

Rayman

I think zero == false and non-zero == true...

Paul Baker

correct, just like C, false is 0 and anything else is true, so·it is equivalent to·"if (byteVar & Mask)<>0"

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 3/29/2008 2:52:31 AM GMT

Javalin

ah - ok!

J