67 lines
1.7 KiB
ArmAsm
Executable file
67 lines
1.7 KiB
ArmAsm
Executable file
;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
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;;Screen print. Draws a pixel at a specified position.
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.proc pixel_draw; user-procedure :clobbers (A X Y) :clobbers-arguments 0
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.include "pixel.inc"
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;; Lets clear some memory for later
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LDA #$00
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STA btp_mem_pos + 1
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calc_byte_to_paint:
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;; X = X_pos (mod 8)
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LDA X_pos
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AND #%00000111
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TAX
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;;this is the same as: byte_to_paint = 2^X
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LDA binary_factor, X;; (see END.s)
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STA byte_to_paint
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;;FIND position to write byte_to_paint (btp_mem_pos)
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;; + + + + + > X
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;; +
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;; +
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;;\/
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;; Y
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;; The screen has a wierd memory to pixel maping!
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;; Each line of 8 pixels is one byte
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;; 8 consecutive bytes in memory makes up a chunk and are 'under each other' on the screen
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;; the chunks are orderd from the left to the right and then looping downwards.
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;; Therefore we have that: btp_mem_pos = big_x_offset + smal_y_offset + big_y_offset + bitmap_offset
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;; We use a lookup-table for big_y_offset + bitmap_offset (see END.s)
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;; (Y_pos / 8 floor)*2
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LDA Y_pos
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LSR A
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LSR A
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;; We need to remove the last digit if its still there
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;; same as LSR ASR
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AND #%11111110
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TAX
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LDA big_y_offset, X
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STA btp_mem_pos
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INX
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LDA big_y_offset,X
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STA btp_mem_pos + 1
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;; btp_mem_pos = big_x_offset
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LDA #%11111000
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AND X_pos
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Add_16 btp_mem_pos, btp_mem_pos +1, A, #$00
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;; + smal_y_offset
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LDA #%00000111 ; A = y (mod 8)
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AND Y_pos ;; offset to add
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TAY
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;Add_16 btp_mem_pos, btp_mem_pos + 1, #<Bitmap, #>Bitmap, !
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;;Let draw some stuff
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LDA byte_to_paint ;; note that both bytes are used!
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ORA (btp_mem_pos), Y
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STA (btp_mem_pos), Y
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RTS
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.endproc
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