Simple Variable Question

TC

I am learning SPIN, need to have control of one bit of a variable.

If done in PBASIC:

BlaBla.bit8 = 1
Or
BlaBla.bit8 = 0

Thanks TC

JonnyMac

There are no direct bit manipulations in Spin (or PASM for that matter), but it's very easy to do with a mask. I have this in my standard template:

pub putbit(target, pos, value)

'' writes value.0 to target.pos

  if (value & 1)
    return target | (1 << pos)
  else
    return target & !(1 << pos)

To update your examples, you'd do this

blabla := putbit(blabla, 8, 1)
blabla := putbit(blabla, 8, 0)

TC

Thank You Jonny, I think I will save that one too.

Dr_Acula

Check out the bitwise decode/encode - page 160 in my manual (Help/Propeller Manual in the Proptool) though this might not be the latest manual. Search for "bitwise encode".

Pin := |<PinNum

If PinNum is 3, Pin is set equal to %00000000 00000000 00000000 00001000.

Near this section in the manual are the bitwise rotates.

Then there is AND and OR. I tend to use these rather than the bitwise encode as they are rather similar to the way that assembly works. So you have to learn less commands.

In general, to set one bit high, use an OR - eg on bit 3 %00000000_00000000_00000000_00001000

For each bit, if you OR then the truth table is 0,0=0, 0,1=1, 1,0=1 1,1=1. So for all the bits that you leave as 0, they stay the same. And the one bit you have as 1 gets forced high.

To set one bit low, use a logical AND, and a mask like %11111111_11111111_11111111_11110111

the truth table for and is 0,0=0, 0,1=0, 1,0=0 and 1,1=1. So if a bit is high in the mask and high in the data you want to change, it stays high. If it is low it stays low. The only one that changes is the one bit you set low, which forces that bit low.

X := %00101100 & %00001111
The above example ANDs %00101100 with %00001111 and writes the result, %00001100, to X.

In pasm, what you find is you can set up a series of masks in a section at the end, and then reuse them over and over.

HTH

TC

@Dr
I did look at that but I was having trouble understanding what I needed to do. The value will change and must stay intact but I must be able to change one bit without disturbing the rest of the value. Because of what Jonny gave, I understand it better. I thank you for your help.