BasicMicro - Forums

EXCEPTION
This is for EXTREMELY advanced users.
EXCEPTION is used when it is necessary to break out of a subroutine for some reason before reaching the “return”(eg an exception). EXCEPTION differs from RETURN as follows:
RETURN
Retrieves the saved address from the stack, clears the address from the stack, and sets program execution to the line following the GOSUB command.
EXCEPTION
Clears the return address from the stack, and resumes program execution at the label given. The label it jumps to must be at the same gosub level as the calling function was at.

The compiler has no way of knowing at compile time what level any gosub is at. Only during runtime. The programmer/user has to determine if they are “exception”ing back to an appropriate level.

Syntax
exception label
label is the label at which program execution should continue

Example 1
looper VAR byte

SEROUT S_OUT, i9600, ["You should only see this once!", 13,13]
PAUSE 1000

MAIN
FOR looper = 1 to 150
GOSUB _GoThere

_ComeBack
NEXT ; looper
PAUSE 500 ; so you can catch the upper count
GOTO MAIN

_GoThere
IF looper > 145 THEN
SEROUT S_OUT, i9600, [DEC looper, 13]
ENDIF

IF looper < 51 THEN _skiphere
EXCEPTION _ComeBack; Return from Gosub without overflowing stack
_skiphere ; comes here if looper less than 51
SEROUT S_OUT, i9600, [DEC looper, 13]
RETURN

This example will print “You should only see this once!”, followed by 1 through 50, then 146 through 150. The “…only once!” message will be seen once, the numbers will continue to appear over and over, until boredom sets in.
Now replace the EXCEPTION line with “GOTO _ComeBack”. We now drop out of the _GoThere subroutine prematurely, without clearing the stack. Eventually there is stack overflow and the PIC goes into a death spiral which leads to a reset. And how do you know this? Because “You should only see this once!” appears every fourth or fifth loop, where it only showed once at the beginning in the example with EXCEPTION.

Example2
This program is similar to the one under GOSUB but provides an "escape" from the subroutine if the minimum weight is too low.

val var word
weightmin var word
weightmax var word

start
code to calculate minimum weight
hserout ["Minimum "]
val = weightmin
gosub outvaldec
code to calculate maximum weight
hserout ["Maximum "]
val = weightmax
gosub outvaldec
goto start

outvaldec ; start of subroutine
if weightmin > 5 then continue
exception start ; value is too low – do again
continue
hserout ["weight is ",dec val," mg",13]
return

The above program is similar to the one under GOSUB but provides an "escape" from the subroutine if the minimum weight is too low.

In the below code, subroutine "test" never RETURNs. Instead the code jumps back to main without ever reaching the RETURN or the PAUSE.

main
gosub test
pause 1000
goto main
test
exception main
return