135 lines
2.9 KiB
ArmAsm
135 lines
2.9 KiB
ArmAsm
;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
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.proc circle
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.include "circle.inc"
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;; We use the algorithm jerkos method
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;; https://schwarzers.com/algorithms/
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;; X_rel = radius (share the same address)
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;;Y_rel =0
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LDA #$00
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STA Y_rel
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;; t1 = radius >> 4
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LDA radius
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LSR
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LSR
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LSR
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LSR
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STA t1
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draw_center_px_in_circle:
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JSR pixel_draw
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LDA btp_mem_pos
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STA btp_mem_pos_center
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LDA btp_mem_pos + 1
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STA btp_mem_pos_center + 1
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;; btp_mem_pos_center_two = 2*btp_mem_pos_center | used later for calculating btp_mem_pos_inv
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btp_mem_pos_center_two = btp_mem_pos_center
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Mult_16 btp_mem_pos_center, btp_mem_pos_center + 1
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draw_right_px_in_circle:
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CLC
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LDA X_pos
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ADC radius
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STA X_pos
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;; We only draw the first pixel using absolute position.
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;; After that we use relative position.
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JSR pixel_draw
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;; This sets byte_to_paint, btp_mem_pos and Y
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while_x_bigger_then_y:
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;STY temp
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;LDY #$00
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draw_pixel:
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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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draw_pixel_inv:
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SEC
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LDA btp_mem_pos_center_two
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SBC btp_mem_pos
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STA btp_mem_pos_inv
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LDA btp_mem_pos_center_two + 1
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SBC btp_mem_pos + 1
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STA btp_mem_pos_inv + 1
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;; calculate the inverted y value Y = 7-Y
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STY temp
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LDA #$07
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SEC
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SBC temp
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TAY
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;; calculate byte_to_paint_inv (could be faster with lookup table)
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LDA byte_to_paint
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STA temp_
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LDA #%10000000
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jmp start_calc
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calc:
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LSR A
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start_calc:
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LSR temp_
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BCC calc
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; A = byte_to_paint_inv
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ORA (btp_mem_pos_inv), Y
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STA (btp_mem_pos_inv), Y
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;;Recover the Y value (we changed it because evrything is inverted)
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ldy temp
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increment_y_pos:
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INC Y_rel ; y++
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INY
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CPY #$08
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BNE increment_y_pos_end
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move_8px_down:
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LDY #$00
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;; Switch to chunk bellow
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; So we subtract #$4000
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Add_16 btp_mem_pos, btp_mem_pos + 1, #$40, #$01 ;+320
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increment_y_pos_end:
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;;t1 += y
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CLC
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LDA t1
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ADC Y_rel
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STA t1
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;; t2 = t1 - x
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SEC
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LDA t1
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SBC X_rel
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STA t2
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;; if t2 < 0 then skip to endif
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CMP #$00
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BMI endif
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if:
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decrement_x_pos:
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DEC X_rel
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ASL byte_to_paint
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BCC decrement_x_pos_end
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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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Sub_16 btp_mem_pos, btp_mem_pos + 1, #$08, #$00
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;; Restores byte to paint
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LDA #%00000001
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STA byte_to_paint
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decrement_x_pos_end:
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LDA t2
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STA t1 ; t1 = t2
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endif:
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;; repeat if X > Y
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LDA X_rel
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CMP Y_rel
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BCS while_x_bigger_then_y
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RTS
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.endproc
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