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There are three ways to code scoring.
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:
sed //set decimal mode clc lda #$50 //50 points scored adc score1 //ones and tens sta score1 lda score2 //hundreds and thousands adc #00 sta score2 lda score3 //ten-thousands and hundred-thousands adc #00 sta score cld //clear decimal mode
The score can now be printed on screen:
lda score3 and #$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 ...
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.
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 this, which converts hexadecimal to decimal, then 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:
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