Re: Input/output on the Apple ][

[dempson@actrix.gen.nz (David Empson)]

<sam124@operamail.com> wrote:

> I would like to do single character output using data stored in the
> 6502's accumulator.  I would also like to do single character input,
> with and without echoing.

The standard routine to output a character is COUT, which is at $FDED.
This allows for redirection of output to a slot using the PR#n command,
capture by DOS 3.3 or ProDOS's BASIC.SYSTEM (e.g. output to text files),
etc.  A simple JSR $FDED will output a character in the accumulator,
typically to the screen.  The X and Y registers are preserved.

If you want to always output to the screen (bypassing redirection with
PR#n etc.) you can call COUT1 ($FDF0) directly, but this can cause
compatibility problems with 80-column display mode on some machines.

Note that the screen output routines expect characters passed to COUT to
have the high order bit set, e.g. "A" is $C1, not $41.

The screen output routines take care of the text output window,
scrolling, etc.

For keyboard input, you can do it at two levels:

(a) Direct hardware input.
(b) Call an appropriate monitor routine.

For direct hardware input, your code sits in a loop polling the keyboard
input register, which is at location $C000.  When a key is pressed, the
high order bit of this location is set, and the lower order 7 bits are
the ASCII value of the key.  You then need to reset the keyboard latch
by reading or writing location $C010, otherwise the keypress will remain
pending.  Typical code is as follows:

WAITKEY  LDA $C000
         BPL WAITKEY
         STA $C010
         RTS

This method won't display any kind of cursor and cannot be redirected
from another source (e.g. a slot or text file input).  If you want to
echo the character to the screen, call COUT after your WAITKEY routine
has returned a character, but check for control characters before
blindly outputting anything, e.g. by comparing with $A0 (space with high
order bit set) and avoiding echo of anything less than that.

The standard monitor routine for reading a single character is RDKEY,
location $FD0C.  This can be redirected to input from slots (IN#n) or
DOS/ProDOS file input.  If the standard screen input routine is used, it
will display a blinking cursor while waiting for input, but is otherwise
functionally equivalent to the WAITKEY loop above.  (This is the
equivalent of the GET statement in BASIC.)  You have to call COUT to
echo the character.

If you don't want to allow redirection from other inputs but do want a
cursor, you can call the KEYIN routine ($FD1B) directly.  It is
otherwise equivalent to RDKEY.

If you want to read a whole line of input, with echoing and editing
(same input routine used by Integer BASIC, Applesoft BASIC, the monitor,
mini-assembler and DOS 3.3 and ProDOS's BASIC.SYSTEM), call GETLN, which
is $FD6A.  You need to set up a prompt character first by storing it in
location $33.  The input line is stored in the buffer at $200-$2FF, with
a carriage return ($8D) as the last character.  The length of the line
is also returned in the X register.

[I had to look up GETLN's address - the rest of the details were from
memory, but I haven't used any of this stuff for more than ten years!]

-- 
David Empson
dempson@actrix.gen.nz