80 lines
2.4 KiB
ArmAsm
80 lines
2.4 KiB
ArmAsm
;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
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;; petscii: https://www.c64-wiki.com/wiki/File:ASCII-Codes.gif
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;;https://www.c64-wiki.com/wiki/Character_set
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;; Takes in a PETSCII-code in CODE
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;; And prints it ON X_pos, Y_pos
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;; Note that this is the real positions divided by 8
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.proc char_draw; user-procedure :clobbers (A Y) :clobbers-arguments 4
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.include "char.inc"
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;;Do arithimatic to know where to read and write bytes.
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calculate_petski_position:
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;We first need to clear some memory
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LDA #$00
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STA petski_position + 1
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;;We need the relative offset for bytes to read and write.
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;; This is code *8 because 8byte is one character
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;; *8 = 2*2*2
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ASL code ;Will never owerflow, therefore 8byte
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Mult_16 petski_position, petski_position + 1
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Mult_16 petski_position, petski_position + 1
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;; Add starting position
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Add_16 petski_position, petski_position + 1, #<Character_generator_ROM , #>Character_generator_ROM, !
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calculate_screen_position:
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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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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 screen_position
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INX
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LDA big_y_offset, X
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STA screen_position + 1
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;; Y = big_x_offset
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LDA #%11111000
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AND X_pos
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STA X_pos
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;; Y += 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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CLC
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ADC X_pos
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Add_16 screen_position, screen_position + 1, A, #$00, !
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move_data:
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;; One character is 8 byte, move these bytes to screen
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LDY #$07
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$06
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$05
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$04
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$03
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$02
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$01
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LDA (petski_position), Y
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STA (screen_position), Y
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DEY ;$00
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LDA (petski_position), Y
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STA (screen_position), Y
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RTS
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.endproc
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