Re: [oopic] Re: Serial To LCD messing up

At 02:21 PM 6/17/2008, rtstofer wrote:
>I can't speak for the new compiler but that logical expression in the
>first IF statement caused the identical symptoms in V5 and B.2.2+.
>Just breaking the statement into two if <expr> then ... end if solved
>the problem.

Ah yes, you're right. The statement:

if ((serial.received == TRUE) & (serial.value == 123))

produces this code:

55: 44 206 Get Serial.Received
57: 1 Pushb(1)
58: 152 IsEqual

59: 41 225 Get Serial.Value
61: 123 Pushb(123)
62: 152 IsEqual

63: 141 And
64: 137 0 71 BranchOnFalse(71)
130

which obviously evaluates the entire expression before deciding where
to branch to next. Gawd, I hate toy programming languages!

>I suspect there is an issue with not short circuiting the evaluation
>when the first equality is false. So what happens when you read the
>port and it hasn't received anything? That's exactly what would
>happen if the evaluation is not short circuited.

And you have a nasty race condition - if Serial.Received is false and
the trigger character arrives in time to get snagged by the "Get
Serial.Value", the entire expression fails (FALSE & TRUE = FALSE) and
the trigger character isn't detected.

>Kind of like 'what happens if I read a circular buffer ahead of the
>insert pointer'? Well, I'm going to read the entire buffer again...

Which is really dumb, since the firmware can easily do a test and
only advance the read pointer if it's not equal to the write pointer:

get_serial_dot_value:

// don't advance to next character if there isn't one

if (read_pointer != write_pointer)
increment read pointer (and wrap if now past end of buffer)

// return next available character
// (or same as last time if we don't have any new ones)

return (buffer[read_pointer])

It's really not rocket science...

Scott proclaimed one time that the (2.2+?) firmware only has one bug
in it. I doubt he meant this one.

...Andy

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