OEL TTX2000 Issue 2.3: Routine at 1155

1155: get_dec_digit: Wait for a keypress in the range 0-9.

Used by the routines at enter_page, enter_subcode_and_clear and select_channel.
get_dec_digit	1155	PUSH HL	Preserve HL.
get_dec_digit_0	1156	CALL get_keypress	Get a keypress.
	1159	CP $30	Loop until key code is greater than or equal to $30 and less than $3A ('0' to '9').
	115B	JR C,get_dec_digit_0
	115D	CP $3A
	115F	JR NC,get_dec_digit_0
	1161	POP HL	Restore HL and return.
	1162	RET	Restore HL and return.