Tutorial 8
Using Getkey to Detect Key presses

Overview

This will show you how to use the _getkey ROM call

What is _Getkey?

_Getkey is a ROM call similar to it's TI-Basic counterpart command, getkey. Getkey waits for a key press from the key pad. Then, depending on what you want to do, getkey can display the key code of the key pressed, execute another command, or act like a program pause.

Programming

This program will use _getkey as a program pause.

#define B_CALL(xxxx)	rst 28h \ .dw xxxx
#define B_JUMP(xxxx)	call 50h \ .dw xxxx

_clrlcdfull		=4540h		;The ususal constants and defines, look at the
                            ;New Commands section for more information
_homeup		=4558h
_getkey		=4972h

	.org	9D95h

	B_CALL(_homeup)		;Call up home screen
	B_CALL(_getkey)		;Pause until next key press
	B_CALL(_clrlcdfull)		;After key press, clear screen. Or you can use
                            ;any other clear screen command (Tutorial 3)
	ret			;Return to being a good old calculator

.end
END

New Commands

_getkey - Detect key press, store key code if nessasary

Programming

We will make our own program that displays the Hex key code of the key pressed.The code for the program is here. It looks complicated, but it really isn't. I'll comment the program along the way so you don't get lost. An important concept is the "stack". The stack holds numbers for future use or reference. In this program, it holds the hex code of each key pressed.

#define B_CALL(xxxx)	rst 28h \ .dw xxxx
#define B_JUMP(xxxx)	call 50h \ .dw xxxx

_clrlcdf		=4540h		;The ususal constants and defines, look at the
                            ;New Commands section for more information
_homeup		=4558h
_puts		=450Ah
_putc		=4504h
_getkey		=4972h
_newline		=452Eh


	.org	9D95h

	B_CALL(_clrlcdf)		; clear screen
	B_CALL(_homeup)	; home the cursor
	ld	hl,txt1		; Display the message at the txt1 label
	B_CALL(_puts)
	B_CALL(_getkey)		; Detect key press
	push	af		; Save key press in register a
	B_CALL(_clrlcdf)		; clear screen again
	B_CALL(_homeup)	; home again
	ld	hl,txt2		; Display msg2
	B_CALL(_puts)		; Display text
	pop	af		; restore reg a
    call    dishex      ; Follow commands at label dishex to find hex key     code
	B_CALL(_getkey)		; pause until next key press
	B_CALL(_newline)		; Move cursor to next line
	ret			;Return to TI-OS

dishex:	push	af	; keep copy of value
	and	0F0h		; terminate lower nibble
	rra			; shift nibble right 4
	rra
	rra
	rra
	ld	l,a		; Display the Key's hex code in Str1
	ld	h,0
	ld	de,hexStr
	push	de		; keep for other nibbler
	add	hl,de		; add offset to hexStr address
	ld	a,(hl)		; find correct ASCII character from string below to
                    ; display hex code of key pressed.
	bcall(_putc)		; display character
	pop	de		
	pop	af		
	and	0Fh		; wipeout upper nibble
	ld	l,a
	ld	h,0
	add	hl,de
	ld	a,(hl)
	bcall(_putc)
	ret			; Return to OS	

hexStr 	.db	"0123456789ABCDEF"	;These are the characters used to display
                                ;the key's hex code
txt1	.db	"Push a key.",0
txt2	.db	"Key code: ",0

.end
END

New Commands

_putc - Display character and advance cursor
push - Pushes a number on to the stack.
pop - Takes a number off the stack.
call - This looks like the TI-83 macro, and serves almost the same purpose, only for the TI-83 Plus, it jumps the program to a specified label.

Conclusion

This program wasn't exactly "lightning fast". That's because the _Getkey command was scanning the entire key pad for a key press. Later on, we will make a program like this one, only we'll narrow down the area in which _Getkey looks in for a keypress, making it faster.

Tutorial 9