Some tips that I learned programming for the KIM-1 and the 6502.

ALWAYS CHECK YOU DO NOT FORGET IMMEDIATE ACCESS SYNTAX!
It so easy an common to forget the ‘#’ in your assembler code. Assemblers will not complain and make it absolute or zeropage addressing and you will search for hours!

Do not use the CRLF routine at $1E2F in the monitor

This routine sends the CR and LF to the TTY output vai OUTCH. Nice, you will need that often in a console program.
But if you read the page on printing a string you see this routine also sends out 6 Null ($00) characters, this takes a lot of wasted time!

Just make your own subroutine
LDA #$0D ; CR
JSR OUTCH
LDA #$0A ; LF
JSR OUTCH
RTS
Note that this destroys A and Y!

OUTCH and GETCH

Reading and writing characters with the TTY interface, you need these a lot.

GETCH gets a character to serial TTY.
Blocking, it waits indefinitely for a character to arrive and A will have the value.
The routine also kills the Y register, returns with Y = $FF.
Also, in LED Display and Keyboard mode, it returns with A = $01 and X = 8.
Save Y before calling GETCH, and restore Y when it returns.

OUTCH sends a character to serial TTY
No handshaking, no hardware flow control, it just spits out the character.
A destroyed, Y=FF, X preserved.
Save at least Y, and saving A is recommended.

SPACE print a blank

This just calls OUTCH with A = $20, so read OUTCH for the side effects.

PRTBYT Print A as two hex characters
Uses OUTCH, so Y is lost. A is preserved.

GETBYT

X saved, Y = 0, A is hex character
Reads two hex characters (0..9 A..F) and tries to pack them in A. Any invalid hex character entered IS returned as 00, as is valid 00.
Rather useless therefore.

The audio tape routines

The DUMPT and LOADT routines are OK to call interactively to dump and load KIM-1 files on tape.
The cannot be called as subroutines. when finished they return to the KIM-1 monitor.
DUMPT return with $0000 as current address pointer in the KIM-1 monitor.
LOADT returns it status via the current address pointer in the KIM-1 monitor, $0000 is success, $FFFF is a loading error.

What you can do is:
– Have code at $0000 to return to your program (Warm start entry), so the user just has to press G
– instruct the user what to do if DUMPT returns $FFFF

The better solution is to incorporate Hypertape as callable subroutine.

Return to the KIM-1 monitor

The preferred location to call to enter the KIM-1 monitor form a user program is START at $1C4F

Reading the keyboard and lighting the display

Fill F9, FA, FB with the values to show on the LED displays
Call SCAND
Call SCAND to debounce
CaLl GETKEY
Now check the key in A:
– above $14 : no key
– 14 = PC
– 13 = GO
– 12 = +
– 11 = DA
– 10 = AD
– 0 ..9 A – F keys
Handle key and loop back to the begin