Avalon: Routine at 812D

812D: Menu 2: Check for ENTER or LOAD keys being pressed


Second menu (811B) - checks for one of two key press options (both checked using the same keyboard port).
812D	LD BC,$BFFE	Keyboard port address for keys H, J, K, L, Enter
8130	IN A,(C)	Read port input
8132	LD C,A	Temp store value in C register for next check
Check if ENTER is being pressed
8133	OR %11111110	Need to check bit 0, so set all other bits
8135	CP %11111110	Check for the ENTER key
8137	LD A,$00	Set "New Game" flag (0) in advance of a positive check
8139	JR Z,$8150	Finish check/jump out here if ENTER is pressed
Check if LOAD ("J") is being pressed
813B	LD A,C	Retrieve keyboard port address input from C register
813C	OR %11110111	Need to check bit 3, so set all other bits
813E	CP %11111111	Check for "J" key
8140	JR Z,$812D	If it's not being pressed, jump back and keep scanning for ENTER or LOAD keys
LOAD ("J") has been pressed
8142	CALL $815B	LOAD has been selected. Clear the screen (pixels)
8145	LD DE,$8CCD	Get address of text string ("LOAD SAVED GAME")
8148	LD HL,$48C8	Set screen position for printing text
814B	CALL $8179	Print "LOAD SAVED GAME" on screen
814E	LD A,$01	Set "Load game" flag (1)
ENTER or LOAD has now been selected, and the A register contains flag for either new game (0) or load game (1)
8150	LD ($8BAB),A	Store game flag (0 = New Game, 1 = Load Saved Game)
8153	CP $01	Store game flag (0 = New Game, 1 = Load Saved Game)
8155	JR Z,$815A	Load game - don't clear the screen (skip the next instruction to leave "LOAD SAVED GAME" message on-screen)
8157	CALL $815B	New game - clear the screen (pixels)
815A	RET	New game - clear the screen (pixels)