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https://github.com/deater/dos33fsprogs.git
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86 lines
1.2 KiB
ArmAsm
86 lines
1.2 KiB
ArmAsm
; try to get sine table from ROM
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; zero page
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GBASL = $26
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GBASH = $27
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YY = $69
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ROW_SUM = $70
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HGR_X = $E0
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HGR_XH = $E1
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HGR_Y = $E2
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HGR_COLOR = $E4
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HGR_PAGE = $E6
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FRAME = $FC
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SUM = $FD
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SAVEX = $FE
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SAVEY = $FF
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;================================
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; Clear screen and setup graphics
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;================================
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rom_sine:
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;==========================================
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; create sinetable using ROM cosine table
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ldy #0
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sinetable_loop:
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tya ; 2
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and #$3f ; wrap sine at 63 entries ; 2
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cmp #$20
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php ; save pos/negative for later
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and #$1f
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beq sin_noadjust
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cmp #$10
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bcc sin_left ; blt
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bne sin_right
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lda #$20 ; force sin(16) to $20 instead of $1F
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bne sin_noadjust
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sin_right:
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; sec carry should be set here
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sbc #$10 ; X-16 (x=16..31)
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bne sin_both ; bra
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sin_left:
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; clc ; carry should be clear
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eor #$FF ; 16-X (but plus one twos complement)
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adc #$11
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sin_both:
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tax
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lda sinetable_base,X ; 4+
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lsr ; rom value is *256
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lsr ; we want *32
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lsr
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sin_noadjust:
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plp
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bcc sin_done
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sin_negate:
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; carry set here
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eor #$ff
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; adc #0 ; off by one, does it matter?
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sin_done:
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sta sinetable,Y
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iny
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bne sinetable_loop
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end:
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jmp end
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sinetable_base = $F5BA
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sinetable=$6000
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