[Color Computer] [Coco] Bug in FOR NEXT ?

[Arthur Flexser]

William's example sounds pretty much like what I remember as the minimum code
for seeing the bug appear.  Anybody care to verify that it does indeed result in
an ?NF error on a CoCo?

Art


On Thu, 1 Jun 2006, William Astle wrote:

> Arthur Flexser wrote:
> > The bug occurs when you exit an initial FOR/NEXT loop without completing it
> > (which is perfectly legal to do) and then reuse the same loop variable name in a
> > later loop.  It is critical that the NEXT statements contain the variable name
> > (i.e., NEXT Z, not just NEXT) for the bug to occur.  I think maybe the variable
> > that was previously used in an inner loop must be used later in an outer loop,
> > or possibly vice versa.  As I said, my memory is pretty vague on this, even
> > though I did at one time trace the source of the bug in the ROM code.
> > 
> > Anyone want to fiddle around with coming up with some code that produces the
> > spurious ?NF error?
> 
> I think I know how it occurs, based on my memory of reading the rom
> code. It's a combination of two things, the first being the thing I
> described in my other email (which rambled on about gosubs and
> for/next). The second is what the code does when a for loop is started.
> If I recall correctly, it does the following:
> 
> 1. look for a FOR frame on the stack using the same variable
> 2a. if a frame was found, update the parameters and reuse the stack frame
> 2b. if a frame was not found, create a new one with the correct
> parameters and push it onto the stack
> 3. go on with the execution of the program
> 
> Now, suppose you do the following:
> 
> 10 FOR X = 1 TO 10
> 20 IF X = 4 GOTO 40
> 30 NEXT X
> 40 FOR Y = 1 TO 10
> 50 FOR X = 1 TO 10
> 60 NEXT X
> 70 NEXT Y
> 
> I suspect that you would get ?NF in line 70.
> 
> The reason is due to the operation of NEXT which is, IIRC, approximately
> as follows:
> 
> 1. search the stack for a FOR frame
> 2. if no frame found, ?NF error
> 3. if frame found, is it the requested variable? (always true for no
> variable specified) If not, go back to 1
> 4. stack is now reset to point to the FOR frame that was found
> 5. apply the STEP to the variable and test the ending condition, etc
> 6. if loop is done, pop the FOR frame from the stack and resume at the
> next statement after NEXT else resume execution after the FOR statement
> 
> So, as an artifact of reusing the FOR frame from the previous FOR X, the
> order of the FOR frames on the stack is backwards so that when the NEXTX
> executes, it removes the FOR frame for FOR Y from the stack.
> 
> My guess is that this was a (possibly) misguided attempt to avoid a
> memory leak when an index variable is used repeatedly without the loop
> ever running to completion.
> 
> -- 
> William Astle
> finger lost at l-w.net for further information
> 
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