116 lines
3.7 KiB
ArmAsm
116 lines
3.7 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. Max 45deg!
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.proc line_down
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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 <dy_2
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STA $FD ; for pixel_draw
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;; V = 2*(dx -dy)
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SEC
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LDA dx
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SBC dy
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STA >V
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Mult_16 >V, <V
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;dy_2 = dy*2
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Mult_16 >dy_2, <dy_2 ;>dy_2 = dy (same address)
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;; This is an Bresenham's line algorithm, se wikipedia bellow.
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;;https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
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;; We need to compute the Value D = 2*dy - dx,
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;; but it may be or get negative.
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;; IN the loop we may set D = D -V
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;; Because math D needs to be at least >=V.
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;; V_max = %00000001 11111111
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;; We therefor need to add this offset to V 00000001 11111111
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;; and to its branch logic later in the loop.
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;;D = 2*dy - dx + 2*255
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Mov_16 >D, <D, >dy_2, <dy_2
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Add_16 >D, <D, #$ff, #$01, !
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Sub_16 >D, <D, dx, #$00
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selfmod:
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;; Self modifying code. Makes LDA and SBC instructions each take 1 cycle less.
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;; You can remove this if you run the loop without # at dy_2 and V.
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;;Note: The offsets like +2 etc is because there are instructions betwean the label and the
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;address that needs to be modified
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;; dy_2
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;; Modifies LDA >dy_2
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LDA >dy_2
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STA case_2 +1
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;; Modifies LDA <dy_2
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LDA <dy_2
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STA case_2 +7
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;; V
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;;Modidies SBC >V
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LDA >V
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STA case_1 +1
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;; Modifies SBC <V
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LDA <V
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STA case_1 +7
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end_selfmod:
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LDY #$01
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JSR pixel_draw ;;only used first pixel. after this relative position is abused
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LDX X_pos
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for_x:
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;; Paints A to address in |btp_mem_pos* + Y|
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;; Y is pixel position in the chunk. Therefor it may be that Y = 0, 1, 2, 3, 4, ,5 ,6 ,7.
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LDA byte_to_paint ;A byte containing a single 1. Coresponds to X position in the chunk.
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ORA (>btp_mem_pos), Y
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STA (>btp_mem_pos), Y
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increment_pixel_x:
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LSR byte_to_paint ; Rotates the pixel one bit to the left ON THE SCREEN.
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BCC increment_pixel_x_end; We need to move to the next chunk
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move_8px_left:
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;; Next chunk is 8 addresses away. Look in pixel_draw for more detail.
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;; -8.
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;; C = 1 therefore you se 07
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Add_16 >btp_mem_pos, <btp_mem_pos, #$07, #$00, !
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;; Restores byte to paint
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LDA #%10000000
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STA byte_to_paint
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increment_pixel_x_end:
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INX
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CPX X_end
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BEQ end ;We keep track on when to stop line draw with the X registry.
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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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;; D = D - V
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;; Because Lag_16:
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;; C =1 so we can use !
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;; A = >D
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Sub_16_A >D, <D, #>V, #<V, !
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increment_y_pos:
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INY ; Increment Y pos inside the buffer
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CPY #$08
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BNE for_x
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move_8px_down: ; Z=1 --> C=1
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LDY #$00
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;; Switch to chunk bellow
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; C = 1
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; So we subtract #$3F, #$01 +C
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Add_16 >btp_mem_pos, <btp_mem_pos, #$3F, #$01, !; +320
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JMP for_x
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increment_y_pos_end:
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case_2:
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Add_16 >D, <D, #>dy_2, #<dy_2, ! ;D = D + 2*dy
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JMP for_x
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end:
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RTS
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.endproc
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