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158 lines
3.0 KiB
ArmAsm
158 lines
3.0 KiB
ArmAsm
; Autumn, based on the code in Hellmood's Autumn
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; by deater (Vince Weaver) <vince@deater.net>
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; Zero Page Addresses
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XCOORDL = $F6
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XCOORDH = $F7
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YCOORDL = $F8
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YCOORDH = $F9 ; shapetable ROT
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EBP1 = $FA
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EBP2 = $FB
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EBP3 = $FC
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EBP4 = $FD
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COLORL = $FE
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COLORH = $FF
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; Soft Switches
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KEYPRESS= $C000
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KEYRESET= $C010
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; ROM routines
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; Some of these are in the Applesoft ROMs so need at least an Apple II+
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; or later to run
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TEXT = $FB36 ; Set text mode
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HGR2 = $F3D8 ; Set full-screen hi-res mode using page 2 ($4000)
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; 280x192 6-colors
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HPLOT0 = $F457 ; Plot point, (Y,X) = Horizontal, (A=Vertical)
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HCOLOR = $F6EC ; Set color in X, must be 0..7
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autumn:
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;===================
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; init screen ; Instruction Length
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jsr HGR2 ; 3
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autumn_forever:
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; save old Xcoord value to X/Y for later
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; push/pop is 1 byte each but have to get
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; value into accumulator first
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ldx XCOORDL ; 2
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ldy XCOORDH ; 2
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; 16-bit subtraction x=x-y
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; need to set carry before subtraction on 6502
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txa ; 1
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sec ; 1
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sbc YCOORDL ; 2
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sta XCOORDL ; 2
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tya ; 1
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sbc YCOORDH ; 2
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; 16-bit arithmatic shift right of X
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; 6502 has no asr instruction
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; cmp #$80 sets carry if high bit set
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cmp #$80 ; 2 ; XCOORDH still in A from before
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ror ; 1
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sta XCOORDH ; 2
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ror XCOORDL ; 2
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; 16-bit add, ycoord=ycoord+oldx
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clc ; 1
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txa ; 1
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adc YCOORDL ; 2
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sta YCOORDL ; 2
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tya ; 1
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adc YCOORDH ; 2
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; 16-bit arithmatic shift right of y-coord
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cmp #$80 ; 2 ; YCOORDH still in A from before
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ror ; 1
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sta YCOORDH ; 2
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ror YCOORDL ; 2
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; 32 bit rotate of low bit shifted out of Y-coordinate
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ror EBP1 ; 2
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ror EBP2 ; 2
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ror EBP3 ; 2
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ror EBP4 ; 2
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; branch if carry set
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bcs label_11f ; 2
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; 16-bit increment of color
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inc COLORL ; 2
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bne no_oflo ; 2
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inc COLORH ; 2
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no_oflo:
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; 16-bit add of X-coord by 0x80
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; this keeps the drawing roughly to the 280x192 screen
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; carry should still be clear (inc doesn't set it)
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lda XCOORDL ; 2
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adc #$80 ; 2
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sta XCOORDL ; 2
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bcc no_oflo2 ; 2
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inc XCOORDH ; 2
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no_oflo2:
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; 16-bit negate of Y-coord
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sec ; 1
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lda #0 ; 2
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sbc YCOORDL ; 2
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sta YCOORDL ; 2
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lda #0 ; 2
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sbc YCOORDH ; 2
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sta YCOORDH ; 2
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label_11f:
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; skipping the color manipulation done here by original
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; mix colors a bit?
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; 16-bit shift of color
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; 2nd is a rotate as asl doesn't shift in cary
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lda COLORL ; 2
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asl ; 1 ; shl %ax
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rol COLORH ; 2
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eor COLORH ; 2
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sta COLORL ; 2
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; get color mapping
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; using a color lookup table looks best but too many bytes
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; you can approximate something close to the lookup with
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; two extra instructions
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and #$7 ; 2
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ora #$2 ; 2
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tax ; 1
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; if ycoord negative, loop
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lda YCOORDH ; 2
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bmi autumn_forever ; 2
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; if top bits of xcoord, loop
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lda XCOORDH ; 2
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and #$f0 ; 2
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bne autumn_forever ; 2
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put_pixel:
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; actually set the color
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jsr HCOLOR ; 3
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; set up paramaters for HPLOT call
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ldx XCOORDL ; 2
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ldy XCOORDH ; 2
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lda YCOORDL ; 2
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jsr HPLOT0 ; 3
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jmp autumn_forever ; 3 ; if we get under 128 bytes
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