Spectrum ROM: Routine at 00703

Used by the routine at MASK_INT.

This subroutine is called on every occasion that a maskable interrupt occurs. In normal operation this will happen once every 20 ms. The purpose of this subroutine is to scan the keyboard and decode the key value. The code produced will, if the 'repeat' status allows it, be passed to the system variable LAST-K. When a code is put into this system variable bit 5 of FLAGS is set to show that a 'new' key has been pressed.

Used by the routine at MASK_INT. This subroutine is called on every occasion that a maskable interrupt occurs. In normal operation this will happen once every 20 ms. The purpose of this subroutine is to scan the keyboard and decode the key value. The code produced will, if the 'repeat' status allows it, be passed to the system variable LAST-K. When a code is put into this system variable bit 5 of FLAGS is set to show that a 'new' key has been pressed.
KEYBOARD	00703	CALL KEY_SCAN	Fetch a key value in the DE register pair but return immediately if the zero flag is reset.
	00706	RET NZ
A double system of 'KSTATE system variables' (KSTATE0-KSTATE3 and KSTATE4-KSTATE7) is used from now on. The two sets allow for the detection of a new key being pressed (using one set) whilst still within the 'repeat period' of the previous key to have been pressed (details in the other set). A set will only become free to handle a new key if the key is held down for about 1/10th. of a second, i.e. five calls to KEYBOARD.
	00707	LD HL,23552	Start with KSTATE0.
K_ST_LOOP	00710	BIT 7,(HL)	Jump forward if a 'set is free', i.e. KSTATE0/4 holds 255.
	00712	JR NZ,K_CH_SET	Jump forward if a 'set is free', i.e. KSTATE0/4 holds 255.
	00714	INC HL	However if the set is not free decrease its '5 call counter' and when it reaches zero signal the set as free.
	00715	DEC (HL)
	00716	DEC HL
	00717	JR NZ,K_CH_SET
	00719	LD (HL),255
After considering the first set change the pointer and consider the second set.
K_CH_SET	00721	LD A,L	Fetch the low byte of the address and jump back if the second set (KSTATE4) has still to be considered.
	00722	LD HL,23556
	00725	CP L
	00726	JR NZ,K_ST_LOOP
Return now if the key value indicates 'no-key' or a shift key only.
	00728	CALL K_TEST	Make the necessary tests and return if needed. Also change the key value to a 'main code'.
	00731	RET NC
A key stroke that is being repeated (held down) is now separated from a new key stroke.
	00732	LD HL,23552	Look first at KSTATE0.
	00735	CP (HL)	Jump forward if the codes match - indicating a repeat.
	00736	JR Z,K_REPEAT	Jump forward if the codes match - indicating a repeat.
	00738	EX DE,HL	Save the address of KSTATE0.
	00739	LD HL,23556	Now look at KSTATE4.
	00742	CP (HL)	Jump forward if the codes match - indicating a repeat.
	00743	JR Z,K_REPEAT	Jump forward if the codes match - indicating a repeat.
But a new key will not be accepted unless one of the sets of KSTATE system variables is 'free'.
	00745	BIT 7,(HL)	Consider the second set.
	00747	JR NZ,K_NEW	Jump forward if 'free'.
	00749	EX DE,HL	Now consider the first set.
	00750	BIT 7,(HL)	Continue if the set is 'free' but exit if not.
	00752	RET Z	Continue if the set is 'free' but exit if not.
The new key is to be accepted. But before the system variable LAST-K can be filled, the KSTATE system variables, of the set being used, have to be initialised to handle any repeats and the key's code has to be decoded.
K_NEW	00753	LD E,A	The code is passed to the E register and to KSTATE0/4.
	00754	LD (HL),A	The code is passed to the E register and to KSTATE0/4.
	00755	INC HL	The '5 call counter' for this set is reset to '5'.
	00756	LD (HL),5	The '5 call counter' for this set is reset to '5'.
	00758	INC HL	The third system variable of the set holds the REPDEL value (normally 0.7 secs.).
	00759	LD A,(23561)
	00762	LD (HL),A
	00763	INC HL	Point to KSTATE3/7.
The decoding of a 'main code' depends upon the present state of MODE, bit 3 of FLAGS and the 'shift byte'.
	00764	LD C,(IY+7)	Fetch MODE.
	00767	LD D,(IY+1)	Fetch FLAGS.
	00770	PUSH HL	Save the pointer whilst the 'main code' is decoded.
	00771	CALL K_DECODE
	00774	POP HL
	00775	LD (HL),A	The final code value is saved in KSTATE3/7, from where it is collected in case of a repeat.
The next three instructions are common to the handling of both 'new keys' and 'repeat keys'.
K_END	00776	LD (23560),A	Enter the final code value into LAST-K and signal 'a new key' by setting bit 5 of FLAGS.
	00779	SET 5,(IY+1)
	00783	RET	Finally return.
A key will 'repeat' on the first occasion after the delay period (REPDEL - normally 0.7s) and on subsequent occasions after the delay period (REPPER - normally 0.1s).
K_REPEAT	00784	INC HL	Point to the '5 call counter' of the set being used and reset it to 5.
	00785	LD (HL),5
	00787	INC HL	Point to the third system variable - the REPDEL/REPPER value - and decrement it.
	00788	DEC (HL)
	00789	RET NZ	Exit from the KEYBOARD subroutine if the delay period has not passed.
	00790	LD A,(23562)	However once it has passed the delay period for the next repeat is to be REPPER.
	00793	LD (HL),A
	00794	INC HL	The repeat has been accepted so the final code value is fetched from KSTATE3/7 and passed to K_END.
	00795	LD A,(HL)
	00796	JR K_END