source.asm

ORG 40000
INITALCLEAR	LD A,0  ; colour 2 is RED 
		OUT (0FEh),A ; set border to that colou
		; Code to clear the screen. Taken from Lab 3.
		LD	A,231
		;Colour bit. We want solid black for this.
		LD	HL,4000h
		;We're drawing to the display file, which
		;starts at address 4000h.
		LD	BC,1800h
		;Select every pixel to clear.
		LD	(HL),0
		LD	D, H
		LD	E, 1
		LDIR ;Clear the entire screen.

DRAWBLACKLINE	;Draws a black square on the screen.
		LD	HL,5800h
		LD	(HL), A
		;Dump colour bit to display file
		JP	HOLDINGPATTERN
		;Go back to the loop.

STEPUP		CALL	EDGETOP
		CALL	COLORCHECK
		;Check if at an edge, and check the current colour
		LD	A,L
		;Can only call SBC on register A, so put L into it.
		SBC	A,32
		;We must go back 32 blocks to get to the point directly above.
		CALL	C,DECH
		;Decrement H if there's a carry.
		LD	L,A
		;Load our recalculated L.
		LD	(HL),231
		;Make the new block black
		JP	WAIT

STEPDOWN	CALL	EDGEBOTTOM
		CALL	COLORCHECK
		;Check if at an edge, and check the current colour
		LD	A,L
		;Can only call SBC on register A, so put L into it.
		ADD	A,32
		;We must go forward 32 blocks to get to the point directly below.
		CALL	C,INCH
		;Increment H if there's a carry.
		LD	L,A
		;Load our recalculated L.
		LD	(HL),231
		;Make the new block black
		JP	WAIT

STEPRIGHT	CALL	EDGERIGHT
		CALL	COLORCHECK
		;Check if at an edge, and check the current colour
		INC	HL
		;Move to the next block	
		LD	(HL),231
		;Make the new block black
		JP	WAIT

STEPLEFT	CALL	EDGELEFT
		;Try to check if the block is running away
		CALL	COLORCHECK
		;Check if at an edge, and check the current colour
		DEC	HL
		;Move to the next block			
		LD	(HL),231
		;Make the new block black
		JP	WAIT

WAIT		HALT ;This
		HALT ;Makes
		HALT ;A
		HALT ;Nice
		HALT ;Delay
		JP	HOLDINGPATTERN

HOLDINGPATTERN	OR	C
		;Not entirely sure why this is here. We saw it in a lab,
		;and it breaks without it. So hey.

		;--- Start W key spotting ---
		LD	A,0FBh
		;Read Q - T.
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the W key.
		RRA
		JP	NC,STEPUP ;If W is pressed, move up.
		;--- End W key spotting ---

		;--- Start A key spotting ---
		LD	A,0FDh
		;Read A - G
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the A key
		JP	NC,STEPLEFT ; If A is pressed, move left.
		;--- End A key spotting ---

		;--- Start D key spotting ---
		LD	A,0FDh
		;Read A - G
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the D key.
		RRA
		RRA
		JP	NC,STEPRIGHT ;If D is pressed, move right.
		;--- End D key spotting ---

		;--- Start S key spotting ---
		LD	A,0FDh
		;Read A - G
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the S key.
		RRA
		JP	NC,STEPDOWN ;If S is pressed, move down.
		;--- End S key spotting ---

		;--- Start E key spotting ---
		LD	A,0FBh
		;Read Q - T.
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the E key.
		RRA
		RRA
		JP	NC,DUMPBLOCK ;If E is pressed, dump a block.
		;--- End E key spotting ---

		;--- Start Space key spotting ---
		LD	A,07Fh
		;Read break (SPACE).
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the Space key.
		CALL	NC, STARTLOOP
		;--- End Space key spotting --

		;--- Start Q key spotting ---
		LD	A,0FBh
		;Read Q - T.
		IN	A,(0FEh)
		;Read the keyboard input port.
		RRA ;Cycle to the Q key.
		CALL	NC,CLSINIT ;If Q is pressed, move up.
		;--- End Q key spotting ---

		JP	HOLDINGPATTERN ;Nothing's been pressed, so loop back.

CLSINIT		LD	HL,5800h
		CALL	CLS
		LD	HL,5800h
		LD	(HL),231
		RET

CLS		LD	(HL),63
		;Tests to ensure loops round the entire screen.
		INC	L
		LD	A,0
		CP	L
		CALL	Z,INCH

		LD	A,5Bh
		CP	H
		RET	Z
		;End loop tests.

		JP	CLS 	;Keep looping.

STARTLOOP	CALL	NC, CLEARBLOCK
		LD	HL, 5800h
		CALL	GENLOOP
		LD	HL, 5800h
		CALL	REPLACELOOP
		LD	HL, 5800h
		RET

DUMPBLOCK	LD	(HL),112	;Make the current block black
		JP	HOLDINGPATTERN	;Go back to checking for keys.

CLEARBLOCK	LD	(HL),63 	;Clears the block.
		RET

YELLOWBLOCK	LD	(HL),112
		RET

INCH		INC H ;Increment H.
		RET

DECH		DEC H ;Decrement H.
		RET

EDGELEFT	LD	A,0	;Check we're at the start
		CP	L	;of a chunk.
		RET	NZ	;And jump out if we're not.
		LD	A,88	;Check we're in the first
		CP	H	;chunk.
		RET	NZ	;And jump out if we're not.
		JP	WAIT
			;Co-ord is 1,1 - Block movement.

EDGERIGHT	LD	A,0FFh	;Check we're in the end
		CP	L	;of a chunk.
		RET	NZ	;And jump out if we're not.
		LD	A,5Ah	;Check we're in the last
		CP	H	;chunk.
		RET	NZ	;And jump out if we're not.
		JP 	WAIT
			;Co-ord is 31,24 - Block movement.

EDGETOP		LD	A,58h	;Check we're in the top
		CP	H	;chunk.
		RET	NZ	;And jump out if we're not.
		LD	A,31	;Check we're in the top
		CP	L	;row.
		RET	C	;And jump out if we're not.
		JP	WAIT
			;In very top row - Block movement.

EDGEBOTTOM	LD	A,5Ah	;Check we're in the bottom
		CP	H	;chunk.
		RET	NZ	;And jump out if we're not.
		LD	A,223	;Check we're in the bottom
		CP	L	;row
		RET	NC	;And jump out if we're not.
		JP	WAIT
			;In very bottom row - Block movement.

COLORCHECK	LD	A,(HL)
		CP	63
		CALL	Z,CLEARBLOCK
		CP	231
		CALL	Z,CLEARBLOCK
		CP	112
		CALL	Z,MARKER
		RET

MARKER		LD	D,1
		RET

GENLOOP		LD	E,0
		CALL	CHECKSUITE
		;LD	E,3		;Debug code
		CALL	GOD
		;Tests to ensure loops round the entire screen.
		INC	L
		LD	A,0
		CP	L
		CALL	Z,INCH

		LD	A,5Bh
		CP	H
		RET	Z
		;End loop tests.

		JP	GENLOOP 	;Keep looping.

REPLACELOOP	CALL	BLOCKMOVE

		;Tests to ensure loops round the entire screen.
		INC	L
		LD	A,0
		CP	L
		CALL	Z,INCH

		LD	A,5Bh
		CP	H
		RET	Z
		;End loop tests.

		JP	REPLACELOOP 	;Keep looping.
		

GOD		LD	A,E ;Carries out the relevant actions.
		CP	0
		CALL	Z,KILLBLOCK
		;CALL	Z,KEEPBLOCK
	
		LD	A,E
		CP	1
		CALL	Z,KILLBLOCK
		;CALL	Z,KEEPBLOCK

		LD	A,E
		CP	2
		;CALL	Z,KILLBLOCK
		CALL	Z,KEEPBLOCK
		
		LD	A,E
		CP	3
		;CALL	Z,KILLBLOCK
		;CALL	Z,KEEPBLOCK
		CALL	Z,CREATEBLOCK

		LD	A,E
		CP	4
		CALL	NC,KILLBLOCK
		;CALL	NC,KEEPBLOCK
		;CALL	NC,CREATEBLOCK

		LD	E,0
		
		RET

KILLBLOCK	PUSH	HL		;Store where we are in display.
		CALL	MOVEMEMORY	;Move to our temporary RAM area.
		LD	(HL),56			;Blank it.
		POP	HL				;Move back.
		RET

CREATEBLOCK	PUSH	HL	
		CALL	MOVEMEMORY
		LD	(HL),112		;Yellow it.
		POP	HL
		RET

KEEPBLOCK	PUSH	HL
		LD	D,(HL)
		CALL	MOVEMEMORY
		LD	(HL),D			;Maintain the state.
		POP	HL
		RET
		
MOVEMEMORY	INC	L
		LD	A,H
		ADD	A,40			;Move up 40*255 memory locations.
		LD	H,A
		RET

BLOCKMOVE	PUSH	HL
		CALL	MOVEMEMORY	;Find the relevant location in RAM.
		LD	D,(HL)			;Drop the colour block to temporary storage.
		POP	HL				;Return to the display.
		LD	(HL),D			;Drop the colour.
		RET

CHECKSUITE	;Check  straight up.
		PUSH	HL
		LD	A,L
		SBC	A,32
		CALL	C,DECH
		LD	L,A

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking up.

		;Check down.
		PUSH	HL
		LD	A,L
		ADD	A,32
		CALL	C,INCH
		LD	L,A

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking down.

		;Check left
		PUSH	HL

		DEC	HL

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking left.

		;Check right.
		PUSH	HL
		INC	HL

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking right.

		;Check up and left.
		PUSH	HL
		LD	A,L
		SBC	A,32
		CALL	C,DECH
		LD	L,A

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking up left.

		
		;Check up and right..
		PUSH	HL
		LD	A,L
		SBC	A,30
		CALL	C,DECH
		LD	L,A

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking up right.

		;Check down and left.
		PUSH	HL
		LD	A,L
		ADD	A,31
		CALL	C,INCH
		LD	L,A

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking down left.

		;Check down and right.
		PUSH	HL
		LD	A,L
		ADD	A,33
		CALL	C,INCH
		LD	L,A

		LD	A,(HL)
		CP	112 ;Is it yellow?
		CALL	Z,INCE

		POP	HL
		;Done checking down right.
		
		RET

INCE		INC	E
		RET