Radio Shack TRS-80 Model 100 Basic Manual page 166

FOR
J
= 1 TO 2121
Execute the program.
Press the Cf2) to interrupt the main program. Again you will see the message:
SUBROUTINE 2
displayed repeatedly on the LCD. While this interrupt subroutine is executing, press
~.
This time you cannot interrupt the interrupt subroutine. However, after the
subroutine is finished, the main program can
be
interrupted with any of the Function
Keys
~, ~
or
~.
The reason for this is that the KEY OFF statement in line 200 disabled the Function
Key interrupts. The KEY ON, in line 220, restored the Function Key interrupts at the
completion of the subroutine.
Rather than totally ignore an interrupt which occurs during an interrupt subroutine,
you may wish to have it execute at the completion of the current interrupt subroutine.
This can
be
accomplished with the KEY STOP statement.
Change line 200 to:
2121121 KEY STOP
Execute the program.
Press Cf2) to interrupt the main program. Then press ([j) to interrupt the interrupt
subroutine. Notice that nothing appears to happen. However, if you watch the display
carefully, you will see the message:
SUBROUTINE 1
display after the last SUBROUTINE 2 message is displayed.
This is because the KEY STOP statement in line 200 delays execution of the ([j)
interrupt until the current interrupt subroutine is finished.
Experiment #4: Checkwriter with Interrupts
In this experiment you will simulate a payroll program which allows interruption from
the keyboard. This could be used to gain access to a data file while the Computer is
engaged in a time consuming process. For example, if a payroll program was printing
a long list of paychecks, it would tie up the Computer until the printing was
completed.
Ordinarily, if you wanted to gain access to the employee data file, you would have to
wait until the printing finished or else break the program and then resume printing
later. You could use Function Key interrupts, however, to allow immediate inquiry
into the file without breaking the program. The printing would simply continue after
the inquiry was completed.
Clear memory with the NEW command and then enter the following program:
1121 CLS: KEY ON : ER$=STRING$13!21," ")
2121 ON KEY GOSUB 14121
3121 FOR 1=1 TO G:READ N$II) ,RII) ,HII):NEXTI
4121 FOR 1=1 TO G
5121 LINE 112,!21)-122G,47),1,BF
160