117 lines
3.5 KiB
ArmAsm
117 lines
3.5 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 +1
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STA dy_2 +1
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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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Mult_16 A, V +1
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STA V
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;dy_2 = dy*2
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Mult_16 dy_2, dy_2 +1 ;dy_2 = dy (same address)
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;; D = dy_2 - x. (signed 16-bit)
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SEC
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LDA dy_2
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SBC dx
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STA D
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LDA dy_2 + 1
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SBC #$00
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STA D + 1
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;; because C flag is wrong value we let dy_2 be 1 to small
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Sub_16 dy_2, dy_2 +1, #$01,#$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 + 1
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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 + 3
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;; Modifies SBC <V
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LDA V + 1
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STA case_1 + 9
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end_selfmod:
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LDA X_end
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STA X_pos
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LDA Y_end
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STA Y_pos
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JSR pixel_calc ;;only used first pixel. after this relative position is abused
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STA byte_to_paint
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;; X = X_end - X_pos
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LDX dx
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Sub_16 btp_mem_pos, btp_mem_pos + 1, #$00, #$00,! ;; Y has always a offset of at least 1 = C + $0000
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INY
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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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decrement_pixel_x:
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ASL byte_to_paint ; Rotates the pixel one bit to the left ON THE SCREEN.
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BCC decrement_pixel_x_end; We need to move to the next chunk
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move_8px_right:
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;; Restores byte to paint to #%00000001
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ROL byte_to_paint
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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 = 0
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Sub_16 btp_mem_pos, btp_mem_pos + 1, #$07, #$00, !
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decrement_pixel_x_end:
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DEX
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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 < 0 goto case_2
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LDA D + 1
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BMI case_2
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case_1:
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;; D = D - V
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;; Because Sub_16 C = 1 so we can use !
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Sub_16 D, D + 1, #V, #V + 1, !
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decrement_y_pos:
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DEY ; Increment Y pos inside the buffer
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BNE for_x
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move_8px_up:
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LDY #$08
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;; Switch to chunk bellow
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; C = 1 or 0
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; So we subtract #$40, #$01
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Sub_16 btp_mem_pos, btp_mem_pos + 1, #$40, #$01;
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JMP for_x
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decrement_y_pos_end:
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case_2:
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; dy_2 is one off but C= 1 witch is off as well
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Add_16 D, D + 1, #dy_2, #dy_2 + 1, ! ;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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