Day 6 - Jumps And Key Presses

Reading Keys

	We will assume that it is a normal rectangle NES pad you want to read and not
anything funky. To read to see if a key is down, you read $4016 (Pad 1) or $4017 (Pad 2)
once per key. If the key is down, Bit #0 (from right) will be set (1). And with 1 and
jump if Not Equal. Before you read anything, however, you need to reset the pad (strobe).

Strobe/Reset

	To strobe/reset the pad, we write a 1 then a 0 to $4016 or $4017 (for pad 2).
This is done like so:

	lda #$01
	sta $4016
	lda #$00
	sta $4016

That's it! Onto actually reading the pad.

Order of Reads

	On every read of $4016 or $4017, you get the status of a different key. 
The order of keys read is:

	Read # | Corresponding Key
	     1. A
	     2. B
	     3. SELECT
	     4. START
	     5. UP
	     6. DOWN
	     7. LEFT
	     8. RIGHT

	So how 'bout we learn some about conditions and jumps before we do any more?

Conditions And Jumps

	There are certain "conditions" that are set by most instructions, some, however,
can only be set by a compare instruction which'll be discussed later. These conditions are

	Conditions
		EQ - EQual - Zero
		NE - Not EQual - Not Zero
		LT - Less Than
		GT - Greater Than
		PL - Positive
		MI - Negative
		CC - Carry Clear
		CS - Carry Set
		VC - oVerflow Clear
		VS - oVerflow Set

	These conditions are set by most instructions, including loads. All of these
have a branch instruction. Put a 'B' in front of one of those abbreviations and you'll
have your branch (Conditional Jump) instruction, examples:

	; assume that there is a label called Loopto .
	beq Loopto  ; branch to Loopto if last compare was EQual or last instruction
	; resulted in a zero.

	bpl Loopto  ; branch if Bit #7 (always counted from right->left) is set.
	bmi Loopto  ; branch if Bit #7 is clear.

I hope you get the idea. I believe that only the Not Equal and Equal conditions can be
set by a non-compare instruction.

Incase you don't know, a label is just a name/word followed by a colon ':'. Examples:

	Loopto:
	aslkdfj:
	Hello_a:

I hope you get the idea. Just remember:Labels are FUN!

A Small Example of Reading Keys

	Enough talk, here's the code:

	lda #$01    ; |
	sta $4016   ;  \
	lda #$00    ;   - Setup Pad for reading.
	sta $4016   ; _/

	lda $4016  ; read for A key.
	and #1     ; check to see if down.
	bne WasDown  ; branch if it was down.
	; I'm not sure why it's a BNE for a bit AND, it just is, SO USE IT! :)

	lda $4016  ; read for B key.
	lda $4016  ; read for SELECT
	lda $4016  ; read START status
	and #1     ; see if down.
	bne StartDown  ; branch if down.

	lda $4016  ; UP
	lda $4016  ; DOWN
	lda $4016  ; LEFT
	lda $4016  ; RIGHT

	jmp NothingDown  ; the JMP instruction jumps no matter what.
	
StartDown:
	; Do stuff if START is pressed.

WasDown:
	; Do stuff if A is pressed.

NothingDown:
	; Nothing was down
	
Hope you didn't thing that was too hard! :).
Personally, I find the GBA easier (a little) to program, so for me, NES
programs take a little more studying to figure out.

This Day In Review

	First, I'd like to mention that you probably know about my GBA ASM series 
and maybe you also know about my x86 (DOS) ASM tutorial? Switching
between 3 ASM languages DOES sometimes screw me up, so if you see a something
rather wierd or any mistake feel free to let me know. Thank you.

	Also, I know Key Press Detecting isn't exactly all that exciting by itself,
but things WILL GET MORE INTERESTING! I promise. ;)

	Happy coding :),
		-Mike H a.k.a GbaGuy

Intro - Day 7