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https://github.com/deater/dos33fsprogs.git
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60 lines
936 B
ArmAsm
60 lines
936 B
ArmAsm
;================================
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; scrn routine
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;================================
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; Xcoord in XPOS
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; Ycoord in YPOS
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; color returned in A
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; assume reading from $c00
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scrn:
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lda YPOS ; 2
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; lsr ; shift bottom bit into carry ; 2
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; bcc scrn_even ; 2nt/3
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;scrn_odd:
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; ldx #$f0 ; 2
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; bcs scrn_c_done ; 2nt/3
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;scrn_even:
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; ldx #$0f ; 2
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;scrn_c_done:
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; stx MASK ; 3
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; asl ; shift back (now even) ; 2
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and #$fe ; make even
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tay
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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 #$8 ; assume reading from $c0 ; 3
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sta GBASH ; 3
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ldy #0
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lda YPOS
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lsr
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bcs scrn_adjust_even
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scrn_adjust_odd:
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lda (GBASL),Y ; top/bottom color
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jmp scrn_done
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scrn_adjust_even:
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lda (GBASL),Y ; top/bottom color
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lsr
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lsr
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lsr
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lsr
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scrn_done:
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and #$f
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rts
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