Volex TTX2000 S Issue 1.2: Routine at 1F8D

1F8D: get_hex: Get a hexadecimal keypress.

Used by the routine at enter_page.
get_hex	1F8D	CALL get_hex_digit	Get a keypress 1-9 or A-F.
	1F90	INC HL	Write key code to next byte of page display buffer.
	1F91	LD (HL),A	Write key code to next byte of page display buffer.
	1F92	CP $41	Return if key code is less than 'A'.
	1F94	RET C	Return if key code is less than 'A'.
	1F95	ADD A,$09	Else add 9 to keycode and return.
	1F97	RET	Else add 9 to keycode and return.