How to implement terminal scrolling

[rfudge02]

Hey, I'm completely new to operating system development. I'm currently following the bare bones and meaty skeleton tutorial and am having trouble implementing terminal scrolling. This is the contents of my tty.c:

Code: Select all

#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include <kernel/tty.h>

#include "vga.h"

static const size_t VGA_WIDTH = 80;
static const size_t VGA_HEIGHT = 25;
static uint16_t* const VGA_MEMORY = (uint16_t*) 0xB8000;

static size_t terminal_row;
static size_t terminal_column;
static uint8_t terminal_color;
static uint16_t* terminal_buffer;

void terminal_initialize(void) {
	terminal_row = 0;
	terminal_column = 0;
	terminal_color = vga_entry_color(VGA_COLOR_GREEN, VGA_COLOR_BLACK);
	terminal_buffer = VGA_MEMORY;
	for (size_t y = 0; y < VGA_HEIGHT; y++) {
		for (size_t x = 0; x < VGA_WIDTH; x++) {
			const size_t index = y * VGA_WIDTH + x;
			terminal_buffer[index] = vga_entry(' ', terminal_color);
		}
	}
}

void terminal_setcolor(uint8_t color) {
	terminal_color = color;
}

void terminal_putentryat(unsigned char c, uint8_t color, size_t x, size_t y) {
	const size_t index = y * VGA_WIDTH + x;
	terminal_buffer[index] = vga_entry(c, color);
}

void terminal_scroll(void) {
	for(size_t y = 0; y < VGA_HEIGHT; y++) {
		for (size_t x = 0; x < VGA_WIDTH; x++) {
			terminal_buffer[y * VGA_WIDTH + x] = terminal_buffer[(y + 1) * VGA_WIDTH + x];
        }
    }
}

void terminal_update(void) {
	memmove(VGA_MEMORY, terminal_buffer);
}
void terminal_putchar(char c) {
	unsigned char uc = c;
	if (c == '\n') {
		terminal_row++;
		terminal_column = 0;
	}
	else {
		terminal_putentryat(uc, terminal_color, terminal_column, terminal_row);
		if (++terminal_column == VGA_WIDTH) {
			terminal_column = 0;
			if (++terminal_row == VGA_HEIGHT) {
			terminal_scroll();
		terminal_update();
			}
		}
	}
}

void terminal_write(const char* data, size_t size) {
	for (size_t i = 0; i < size; i++)
		terminal_putchar(data[i]);
}

void terminal_writestring(const char* data) {
	terminal_write(data, strlen(data));
}

Could someone explain to me why the terminal_scroll() and terminal_update() functions are not functioning. Sorry in advance if the error is obvious

[bzt]

Hi,

This is all wrong.

First, you don't need memmove, you should use memcpy in terminal_update if the video memory and your terminal_buffer are separate buffers, and if they aren't then there's absolutely no need for terminal_update in the first place.

Second, your terminal_scroll has a buffer overflow, it reads over the terminal buffer.

Third, you should scroll by a single memmove(terminal_buffer, terminal_buffer + ONE_ROW_SIZE, TERMINAL_BUFFER_SIZE - ONE_ROW_SIZE) call, and then clear the last row by spaces and default color attribute (similar to what you do in terminal_initialize, but only for the last row).

Fourth, copying over the entire screen in terminal_putchar after each and every character printed is extremely inefficient.

Fifth, terminal_writestring scans through the string twice, that's absolutely unnecessary. You should do "while(*data) terminal_putchar(*data++);" instead.

Cheers,
bzt