Apple IIgs #65
Control-^ is Harder Than It Looks

Written by Dave Lyons (Sepetmber 1989)

This Technical Note describes a problem using Control-^ to change the text cursor with programs that use GETLN.


On the Apple IIGS, typing Control-^ changes the cursor to the next character typed. This feature works properly from the keyboard, but there is a problem when programs print the control sequence. Try entering the following from AppleSoft to demonstrate this problem:


    NEW
    PRINT CHR$(30);"_"

It changes the cursor into a blinking underscore, as expected. But now enter the following:


    12345 HOME
    LIST

You should see 2345 HOME, which demonstrates that the first character is ignored. This is a problem with GETLN, which AppleSoft uses to read each line of input. Even if your program does not use this routine, you should be aware of this problem since it will occur the next time another program uses GETLN.

Since changing the cursor works fine when done from the keyboard, the way to work around this problem is to have your program simulate the appropriate keypresses for GETLN.


    301: CLD                        ; required by BASIC.SYSTEM
    302: STA ($28),Y                ; remove cursor if present
    304: LDY $0300                  ; get index into simulated-keys list
    307: LDA $310,Y                 ; get a simulated keypress
    30A: INC $0300                  ; point to the next key for next time
    30B: RTS                        ; return the key to GETLN

    310: 9E DF 8D                   ; Ctrl-^, underscore, return

    100 POKE 768,0:PRINT CHR$(4);"IN#A$301":REM Start getting simulated keys
    110 INPUT "";A$
    120 PRINT CHR$(4);"IN#0":REM Get real keys again

From an assembly-language program, the equivalent of IN#A$301 is storing $01 and $03 in locations $38 and $39, while the equivalent of INPUT is JSR $FD6A (GETLN). (Store a harmless prompt character, like $80, into location $33 first.)

Further Reference


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