BASIC stamp manual v2.2

5: BASIC Stamp Command Reference – FOR...NEXT

BASIC Stamp Syntax and Reference Manual 2.2 • www.parallax.com • Page 195

number). If you add 1 to 65535, you get 0 as the 16-bit register rolls over

(like a car’s odometer does when you exceed the maximum mileage it can

display). Similarly, if you subtract 1 from 0, you'll get 65535 as the 16-bit

If you write a FOR...NEXT loop who's StepValue would cause Counter to go

past 65535, this rollover may cause the loop to execute more times than

you expect. Try the following example:

reps VAR Word ' counter for the loop

FOR reps = 0 TO 65535 STEP 3000 ' each loop add 3000

DEBUG DEC ? reps ' show reps in Debug window

The value of reps increases by 3000 each trip through the loop. As it

approaches the EndValue, an interesting thing happens; reps is: 57000,

60000, 63000, 464, 3464... It passes the EndValue, rolls over and keeps

going. That’s because the result of the calculation 63000 + 3000 exceeds the

maximum capacity of a 16-bit number and then rolls over to 464. When

the result of 464 is tested against the range (“Is Reps > 0 and is Reps <

65535?”) it passes the test and the loop continues.

A similar symptom can be seen in a program who's EndValue is mistakenly

set higher than what the counter variable can hold. The example below

uses a byte-sized variable, but the EndValue is set to a number greater than

what will fit in a byte:

SYMBOL reps = B2 ' counter for the loop

FOR reps = 0 TO 300 ' each loop add 1

DEBUG reps ' show reps in Debug window

-- or --

reps VAR Byte ' counter for the loop

FOR reps = 0 TO 300 ' each loop add 1

DEBUG DEC ? reps ' show reps in Debug window

Here, reps is a byte variable; which can only hold the number range 0 to

255. The EndValue is set to 300, however; greater than 255. This code will

loop endlessly because when reps is 255 and the FOR…NEXT loop adds 1,

NOTE: For BS1's, change line 1 to

SYMBOL reps = W0

and line 3 to

DEBUG reps

All