73 lines
1.8 KiB
C
73 lines
1.8 KiB
C
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#include "common.h"
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//#link "common.c"
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#include <cbm_petscii_charmap.h>
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#define SCROLL_TOP 0
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#define SCROLL_ROWS 10
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byte scroll_x = 0; // x scroll position
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// return the next character on the right side
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// for a given row
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char get_char_for_row(byte row) {
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return '0' + row;
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}
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void draw_right_column() {
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// get the top-right corner address of scroll area
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word addr = SCRNADR(0x400, 39, SCROLL_TOP);
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byte row;
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// draw one character per row
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for (row=0; row<SCROLL_ROWS; row++) {
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POKE(addr, get_char_for_row(row));
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addr += 40;
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}
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}
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void scroll_one_column_left() {
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// copy several rows of screen memory
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// backwards one byte
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const word start = SCRNADR(0x400, 0, SCROLL_TOP);
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const word nbytes = SCROLL_ROWS*40-1;
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memcpy((byte*)start, (byte*)start+1, nbytes);
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// draw the right column of characters
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draw_right_column();
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}
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void scroll_one_pixel_left() {
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// scroll left one pixel
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scroll_x -= 1;
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// set scroll register with lower three bits
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VIC.ctrl2 = (VIC.ctrl2 & ~0b111) | (scroll_x & 0b111);
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// move screen memory if the scroll register
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// has just gone past 0 and wrapped to 7
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if ((scroll_x & 0b111) == 0b111) {
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scroll_one_column_left();
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}
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}
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void main(void) {
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clrscr();
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printf("\n Hello Scrolling World!");
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printf("\n\n We change scroll registers");
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printf("\n\n And move screen memory");
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printf("\n\n But we don't have time");
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printf("\n\n To copy all 25 rows ");
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printf("\n\n In a single frame ");
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VIC.ctrl1 = 0x10; // 24 lines
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VIC.ctrl2 = 0x00; // 38 columns
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VIC.bordercolor = COLOR_GRAY1;
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// infinite loop
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while (1) {
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// wait for end of frame
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wait_vblank();
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// scroll one pixel to the left
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// and move screen memory every 8 pixels
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scroll_one_pixel_left();
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}
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}
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