====== Scoring points ======
by Achim

There are three ways to code scoring.

===== 1) Decimal mode =====

This one can be found in many games. You only have to declare
a couple of variables for the player's score and switch to decimal
mode.

Example for six digits:

<code>
sed		//set decimal mode
clc
lda #$50	//50 points scored
adc score1	//ones and tens
sta score1
lda score2	//hundreds and thousands
adc #00
sta score2
lda score3	//ten-thousands and hundred-thousands
adc #00
sta score3
cld		//clear decimal mode
</code>

The score can now be printed on screen:

<code>
lda score3
and #$f0	        //hundred-thousands
lsr
lsr
lsr
lsr
ora #$30		// -->ascii
sta screenposition	//print on screen
lda score3
and #$0f		//ten-thousands
ora #$30		// -->ascii
sta screenposition+1	//print on next screen position

lda score2		//same procedure for all digits
...
</code>

Switching to decimal mode can cause problems under certain conditions:
Let's say an irq kicks in to write sprite registers while decimal mode is set.
This would mess up all registers, of course.
This might happen with a sprite mulitplexer, because these irqs are
unpredictable. Idea would be to start these irqs with
CLD. Another would be to disable irqs while calculating.


===== 2) Hexadecimal mode =====

Not much to explain. Declare your variables for the score. Use a standard hexadecimal addition
and print the result on screen (preferably with a routine like [[base:32_bit_hexadecimal_to_decimal_conversion|this]], which converts hexadecimal to decimal, then ascii).


===== 3) ASCII =====

This one's most commonly used in games. Each ascii code on screen is treated like a digit. 
An addition would look like this:

<code>
		ldx #$05
		lda #$00
!:		sta carry,x		//clear carry table
		dex
		bpl !-

		ldx #$05		//6 digits
!:		lda figure1,x           //current score, usually located in screen memory
		clc
		adc figure2,x           //add points
		adc carry,x
		and #$0f		//check bits0-3
		cmp #$0a		//if >=10
		bcc nocarry
		inc carry-1,x		//then set carry for next digit...
		sec
		sbc #$0a		//... -10 for correct ascii
nocarry:	ora #$30		//-->ascii
		sta sum,x
		dex
		bpl !-
		rts

figure1:	.byte		$31, $32, $30, $33, $37, $39 		//   120379 (example)
figure2:	.byte		$32, $32, $30, $39, $32, $31 		//  +220921
carry:		.byte		$00, $00, $00, $00, $00, $00		//tmp for carry
sum:		.byte		$00, $00, $00, $00, $00, $00		
</code>