Cog question

Don M

Why is a +1 added to a cognew statement such as this one from Full Duplex Serial?

okay := cog := cognew(@entry, @rx_head) + 1   

Mark_T

So that a valid cog number is logic true (ie non-zero). valid cog numbers are 0..7, add one and you can use 0 to represent no cog, 1..8 for cog numbers. So long as you subtract one again before calling cogstop in the Stop method

Don M

Thanks Mark.

Duane Degn

If just a trick to make the returned value (of cog) zero so the parent object can use like below to see if a cog was successfully launched.

returnVaule := childObj.Start
  if returnValue ' only a zero (cog not launched) will cause this not to be true
    ' do stuff if cog was successfully launched
  else
    ' do stuff if cog wasn't launched

Otherwise the poor coder would have to type:

returnVaule := childObj.Start
  if returnValue == -1
    ' do stuff if cog wasn't launched
  else
     ' do stuff if cog was successfully launched

I personally don't like the practice of adding one to the cog value but I do it anyway so my objects can be used with the ones that do add one.

I think one reason we have so many objects that add one is because Chip doesn't like to type and "if x" is easier to type than "if x == -1

Edit: I think the perferred way to use this is:

if childObj.Start
    ' do stuff if cog was launched

Mike Green

Another reason to add one is that VAR variables in Spin are initialized to zero, so the initial value of a saved cog # means "no cog running". A lot of initialization methods for objects start by calling their stop method to make sure no cogs are already running. This won't work if you don't add one to the saved cog #. You'd need some other way to indicate that the object was never initialized (and to ignore the saved cog #).