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lesson 17, video scroll
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‎17-video-scroll/Makefile‎

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../14-checkpoint/Makefile

‎17-video-scroll/README.md‎

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*Concepts you may want to Google beforehand: scroll*
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**Goal: Scroll the screen when the text reaches the bottom**
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For this short lesson, open `drivers/screen.c` and note that at the
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bottom of `print_char` there is a new section (line 84) which checks
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if the current offset is over the screen size and scrolls the text.
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The actual scrolling is handled by a new function, `memory_copy`. It is
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a simpler version of the standard `memcpy` but we named it differently
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to avoid namespace collisions, at least for now. Open `kernel/util.c` to
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see its implementation.
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To help visualize scrolling, we will also implement a function to
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convert integers to text, `int_to_ascii`. Again, it is a quick implementation
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of the standard `itoa`. Notice that for integers which have double digits
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or more, they are printed in reverse. This is intended. On future lessons
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we will extend our helper functions, but that is not the point for now.
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Finally, open `kernel/kernel.c`. Initially, each line displays its line
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number. You can set a breakpoint on line 14 to confirm this. Then,
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the following `kprint`s force the kernel to scroll down.

‎17-video-scroll/boot‎

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../14-checkpoint/boot/

‎17-video-scroll/drivers/ports.c‎

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../../16-video-driver/drivers/ports.c

‎17-video-scroll/drivers/ports.h‎

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../../16-video-driver/drivers/ports.h

‎17-video-scroll/drivers/screen.c‎

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#include "screen.h"
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#include "ports.h"
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#include "../kernel/util.h"
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/* Declaration of private functions */
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int get_cursor_offset();
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void set_cursor_offset(int offset);
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int print_char(char c, int col, int row, char attr);
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int get_offset(int col, int row);
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int get_offset_row(int offset);
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int get_offset_col(int offset);
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/**********************************************************
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* Public Kernel API functions *
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**********************************************************/
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/**
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* Print a message on the specified location
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* If col, row, are negative, we will use the current offset
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*/
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void kprint_at(char *message, int col, int row) {
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/* Set cursor if col/row are negative */
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int offset;
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if (col >= 0 && row >= 0)
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offset = get_offset(col, row);
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else {
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offset = get_cursor_offset();
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row = get_offset_row(offset);
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col = get_offset_col(offset);
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}
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/* Loop through message and print it */
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int i = 0;
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while (message[i] != 0) {
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offset = print_char(message[i++], col, row, WHITE_ON_BLACK);
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/* Compute row/col for next iteration */
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row = get_offset_row(offset);
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col = get_offset_col(offset);
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}
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}
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void kprint(char *message) {
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kprint_at(message, -1, -1);
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}
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/**********************************************************
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* Private kernel functions *
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**********************************************************/
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/**
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* Innermost print function for our kernel, directly accesses the video memory
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*
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* If 'col' and 'row' are negative, we will print at current cursor location
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* If 'attr' is zero it will use 'white on black' as default
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* Returns the offset of the next character
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* Sets the video cursor to the returned offset
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*/
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int print_char(char c, int col, int row, char attr) {
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unsigned char *vidmem = (unsigned char*) VIDEO_ADDRESS;
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if (!attr) attr = WHITE_ON_BLACK;
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/* Error control: print a red 'E' if the coords aren't right */
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if (col >= MAX_COLS || row >= MAX_ROWS) {
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vidmem[2*(MAX_COLS)*(MAX_ROWS)-2] = 'E';
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vidmem[2*(MAX_COLS)*(MAX_ROWS)-1] = RED_ON_WHITE;
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return get_offset(col, row);
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}
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int offset;
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if (col >= 0 && row >= 0) offset = get_offset(col, row);
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else offset = get_cursor_offset();
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if (c == '\n') {
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row = get_offset_row(offset);
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offset = get_offset(0, row+1);
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} else {
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vidmem[offset] = c;
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vidmem[offset+1] = attr;
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offset += 2;
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}
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/* Check if the offset is over screen size and scroll */
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if (offset >= MAX_ROWS * MAX_COLS * 2) {
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int i;
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for (i = 1; i < MAX_ROWS; i++)
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memory_copy(get_offset(0, i) + VIDEO_ADDRESS,
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get_offset(0, i-1) + VIDEO_ADDRESS,
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MAX_COLS * 2);
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/* Blank last line */
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char *last_line = get_offset(0, MAX_ROWS-1) + VIDEO_ADDRESS;
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for (i = 0; i < MAX_COLS * 2; i++) last_line[i] = 0;
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offset -= 2 * MAX_COLS;
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}
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set_cursor_offset(offset);
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return offset;
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}
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int get_cursor_offset() {
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/* Use the VGA ports to get the current cursor position
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* 1. Ask for high byte of the cursor offset (data 14)
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* 2. Ask for low byte (data 15)
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*/
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port_byte_out(REG_SCREEN_CTRL, 14);
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int offset = port_byte_in(REG_SCREEN_DATA) << 8; /* High byte: << 8 */
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port_byte_out(REG_SCREEN_CTRL, 15);
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offset += port_byte_in(REG_SCREEN_DATA);
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return offset * 2; /* Position * size of character cell */
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}
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void set_cursor_offset(int offset) {
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/* Similar to get_cursor_offset, but instead of reading we write data */
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offset /= 2;
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port_byte_out(REG_SCREEN_CTRL, 14);
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port_byte_out(REG_SCREEN_DATA, (unsigned char)(offset >> 8));
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port_byte_out(REG_SCREEN_CTRL, 15);
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port_byte_out(REG_SCREEN_DATA, (unsigned char)(offset & 0xff));
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}
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void clear_screen() {
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int screen_size = MAX_COLS * MAX_ROWS;
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int i;
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char *screen = VIDEO_ADDRESS;
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for (i = 0; i < screen_size; i++) {
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screen[i*2] = ' ';
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screen[i*2+1] = WHITE_ON_BLACK;
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}
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set_cursor_offset(get_offset(0, 0));
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}
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int get_offset(int col, int row) { return 2 * (row * MAX_COLS + col); }
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int get_offset_row(int offset) { return offset / (2 * MAX_COLS); }
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int get_offset_col(int offset) { return (offset - (get_offset_row(offset)*2*MAX_COLS))/2; }

‎17-video-scroll/drivers/screen.h‎

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../../16-video-driver/drivers/screen.h

‎17-video-scroll/kernel/kernel.c‎

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#include "../drivers/screen.h"
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#include "util.h"
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void main() {
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clear_screen();
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/* Fill up the screen */
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int i = 0;
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for (i = 0; i < 24; i++) {
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char str[255];
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int_to_ascii(i, str);
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kprint_at(str, 0, i);
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}
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kprint_at("This text forces the kernel to scroll. Row 0 will disappear. ", 60, 24);
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kprint("And with this text, the kernel will scroll again, and row 1 will disappear too!");
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}

‎17-video-scroll/kernel/util.c‎

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void memory_copy(char *source, char *dest, int nbytes) {
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int i;
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for (i = 0; i < nbytes; i++) {
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*(dest + i) = *(source + i);
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}
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}
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/**
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* K&R implementation
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*/
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void int_to_ascii(int n, char str[]) {
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int i, sign;
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if ((sign = n) < 0) n = -n;
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i = 0;
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do {
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str[i++] = n % 10 + '0';
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} while ((n /= 10) > 0);
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if (sign < 0) str[i++] = '-';
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str[i] = '\0';
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/* TODO: implement "reverse" */
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}

‎17-video-scroll/kernel/util.h‎

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void memory_copy(char *source, char *dest, int nbytes);
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void int_to_ascii(int n, char str[]);

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