93 lines
2.6 KiB
ArmAsm
93 lines
2.6 KiB
ArmAsm
;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
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;;drawing line from 2 cordinates
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;;# (X_pos, Y_pos) #
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;;# * #
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;;# * #
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;;# * #
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;;# (X_end, Y_end) #
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;;NOTE THAT X_pos <= X_end, Y_pos <= Y_end. Min 45deg!
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.proc line_down_inv
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.include "line.inc"; Defines memory positions, ex X_pos
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;;We need to clear this memory
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LDA #$00
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STA <V
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STA <dx_2
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STA $FD ; for pixel_draw
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;; V = 2*(dy -dx)
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;; where: dy = Y_end - Y_start, dx = X_end - X_start
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LDA X_end
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SEC
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SBC X_pos
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STA >V
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STA >dx_2; >dx_2 = dx. Needed for dx_2 (not for V)
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LDA Y_end
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SEC
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SBC Y_pos
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STA dy
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SEC
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SBC >V
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STA >V; <V = dy - dx
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mult_16 >V, <V; V = 2*(dy -dx)
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;dx_2 = dx*2
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mult_16 >dx_2, <dx_2
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;;D = 2*dy - dx + 2*255
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;;Our D is bigger then wikipedia because D is unsigned.
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LDA >dx_2
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STA >D
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LDA <dx_2
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STA <D
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Add_16 >D, <D, #$ff, #$01
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Sub_16 >D, <D, dy, #$00
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LDY #$00
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jsr pixel_draw ;;only used first pixel. after this relative position is abused
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CLC ; this makes it that C =0 always on for_x
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for_y:
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LDX #$00
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LDA byte_to_paint
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ORA (>btp_mem_pos, X)
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STA (>btp_mem_pos, X)
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increment_y_pos:
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Add_16 >btp_mem_pos, <btp_mem_pos, #$01, #$00, ! ; I know that C ==0 on for_y
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INY
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CPY #$08 ;
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BEQ move_8px_down
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jmp increment_y_pos_end
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move_8px_down:
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;; +320-8 bytes
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LDY #$00
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Add_16 >btp_mem_pos, <btp_mem_pos,#$38 , #$01
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increment_y_pos_end:
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INC Y_pos;; legacy
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LDX Y_pos
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CPX Y_end
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BEQ end
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;;If D < %00000010 00000000: case_2
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;;else case 1.
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Lag_16 >D, <D, #$00, #$02, case_2
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case_1:
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Sub_16 >D, <D, >V, <V; D = D - V
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increment_pixel_x:
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CLC
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LDA byte_to_paint
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ROR byte_to_paint
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BCS move_8px_left
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JMP for_y
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move_8px_left:
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;; add +8 to btp_mem_pos. Find more of why in pixel_draw
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Add_16 >btp_mem_pos, <btp_mem_pos, #$08, #$00
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;; reset byte_to_paint
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LDA #%10000000
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STA byte_to_paint
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JMP for_y
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case_2: ;; C =0 because LAG_16 so we can use !
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Add_16 >D, <D, >dx_2, <dx_2, ! ;D = D + 2*dx
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JMP for_y
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end:
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RTS
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.endproc
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