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https://github.com/deater/dos33fsprogs.git
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48 lines
899 B
ArmAsm
48 lines
899 B
ArmAsm
;================================
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; plot routine
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;================================
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; Xcoord in XPOS
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; Ycoord in YPOS
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; color in COLOR
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plot:
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lda YPOS ; 3
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lsr ; shift bottom bit into carry ; 2
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bcc plot_even ; 2nt/3
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plot_odd:
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ldx #$f0 ; 2
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bcs plot_c_done ; 2nt/3
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plot_even:
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ldx #$0f ; 2
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plot_c_done:
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stx MASK ; 3
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asl ; shift back (now even) ; 2
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tay ; 2
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lda gr_offsets,Y ; lookup low-res memory address ; 4
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clc ; 2
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adc XPOS ; 3
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sta GBASL ; 3
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lda gr_offsets+1,Y ; 4
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adc DRAW_PAGE ; add in draw page offset ; 3
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sta GBASH ; 3
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ldy #0 ; 2
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plot_write:
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lda MASK ; 3
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eor #$ff ; 2
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and (GBASL),Y ; 5+
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sta COLOR_MASK ; 3
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lda COLOR ; 3
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and MASK ; 3
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ora COLOR_MASK ; 3
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sta (GBASL),Y ; 6
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rts ; 6
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