Buggy screen driver.

Makefile

@@ -5,7 +5,7 @@ CC=gcc
 LD=ld
 LDFLAGS=-melf_i386 -Ttext 0x1000 --oformat binary
 
-OBJ=kernel/kernel_entry.o kernel/kernel.o kernel/ports.o
+OBJ=kernel/kernel_entry.o kernel/kernel.o kernel/ports.o kernel/util.o drivers/screen.o
 
 all: os-image
 

drivers/screen.c

@@ -1,6 +1,67 @@
 #include "screen.h"
 
 #include "../kernel/ports.h"
+#include "../kernel/util.h"
+
+int get_screen_offset(int col, int row) {
+	return (row * MAX_COLS + col) * 2;
+}
+
+int get_cursor() {
+	/*
+	 * screens use their control register as an index to
+	 * select their internal registers of which:
+	 *  - 14: high byte of the cursor's offset
+	 *  - 15: low byte of the cursor's offset
+	 */
+	port_byte_out(REG_SCREEN_CTRL, 14);
+	/* get the high byte and shift it a byte */
+	int offset = port_byte_in(REG_SCREEN_DATA) << 8;
+	port_byte_out(REG_SCREEN_CTRL, 15);
+	/* now add the low byte with the free space from before */
+	offset += port_byte_in(REG_SCREEN_DATA);
+
+	/* remember: each cell is two bytes, not one */
+	return offset * 2;
+}
+
+void set_cursor(int offset) {
+	/* convert from cell offset to char offset */
+	offset /= 2;
+
+	port_byte_out(REG_SCREEN_CTRL, 14);
+	port_byte_out(REG_SCREEN_DATA, (unsigned char)(offset >> 8));
+	port_byte_out(REG_SCREEN_CTRL, 15);
+}
+
+int handle_scrolling(int cursor_offset) {
+	/* if within the screen then don't modify */
+	if(cursor_offset < MAX_ROWS * MAX_COLS * 2)
+		return cursor_offset;
+
+	/* translate rows back by one */
+	unsigned int i;
+	for(i = 1; i < MAX_ROWS; ++i)
+	{
+		memory_copy((char*)get_screen_offset(0, i) + VIDEO_ADDRESS,
+				(char*)get_screen_offset(0, i - 1) + VIDEO_ADDRESS,
+				MAX_COLS * 2);
+	}
+
+	/* set last line to blank */
+	char *last_line = (char*)get_screen_offset(0, MAX_ROWS - 1) + VIDEO_ADDRESS;
+	for(i = 0; i < MAX_COLS * 2; ++i)
+		last_line[i] = 0;
+
+	/*
+	 * move the cursor offset to the last row instead of
+	 * off the screen
+	 */
+	cursor_offset -= MAX_COLS * 2;
+
+	/* updated cursor position */
+	return cursor_offset;
+}
 
 void print_char(char c, int col, int row, char attr_byte) {
 	unsigned char *vidmem = (unsigned char*) VIDEO_ADDRESS;
@@ -44,3 +105,30 @@ void print_char(char c, int col, int row, char attr_byte) {
 	/* update the cursor position */
 	set_cursor(offset);
 }
+
+void print_at(const char *msg, int col, int row) {
+	if(col >= 0 && row >= 0)
+		set_cursor(get_screen_offset(col, row));
+
+	int i;
+	for(i = 0; msg[i] != 0; ++i)
+		print_char(msg[i], col, row, WHITE_ON_BLACK);
+}
+
+void print(const char *msg) {
+	print_at(msg, -1, -1);
+}
+
+void clear_screen() {
+	int row, col;
+
+	for(row = 0; row < MAX_ROWS; ++row)
+	{
+		for(col = 0; col < MAX_COLS; ++col)
+		{
+			print_char(' ', col, row, WHITE_ON_BLACK);
+		}
+	}
+
+	set_cursor(get_screen_offset(0, 0));
+}

drivers/screen.h

@@ -9,5 +9,6 @@
 #define REG_SCREEN_CTRL 0x3d4
 #define REG_SCREEN_DATA 0x3d5
 
-void print_char(char c, int col, int row, char attr_byte);
-int get_screen_offset(int col, int row);
+void print_at(const char *msg, int col, int row);
+void print(const char *msg);
+void clear_screen();

kernel/kernel.c

@@ -1,10 +1,6 @@
+#include "../drivers/screen.h"
+
 void main() {
-	/*
-	 * point to first text cell of the video memory
-	 */
-	char *video_memory = (char*) 0xb8000;
-	/*
-	 * store the character 'X' there
-	 */
-	*video_memory = 'X';
+	/*clear_screen();
+	print_at("This is a test.", 0, 0);*/
 }

kernel/ports.c

@@ -1,4 +1,4 @@
-unsigned char port_byte_in(unsigned char port) {
+unsigned char port_byte_in(unsigned short port) {
 	unsigned char res;
 	/*
 	 * `"=a" (res)' means: put result of `al' into `res' variable
@@ -8,7 +8,7 @@ unsigned char port_byte_in(unsigned char port) {
 	return res;
 }
 
-void port_byte_out(unsigned char port, unsigned char data) {
+void port_byte_out(unsigned short port, unsigned char data) {
 	__asm__("out %%al, %%dx" : : "a" (data), "d" (port));
 }
 

kernel/ports.h

@@ -3,11 +3,11 @@
 /*
  * return a byte from a port.
  */
-unsigned char port_byte_in(unsigned char port);
+unsigned char port_byte_in(unsigned short port);
 /*
  * write a byte of data to a port.
  */
-void port_byte_out(unsigned char port, unsigned char data);
+void port_byte_out(unsigned short port, unsigned char data);
 /*
  * return a word of data from a port.
  */

kernel/util.c

@@ -0,0 +1,7 @@
+void memory_copy(char *src, char *dest, unsigned int n) {
+	unsigned int i;
+	for(i = 0; i < n; ++i)
+	{
+		dest[i] = src[i];
+	}
+}

kernel/util.h

@@ -0,0 +1,2 @@
+#pragma once
+void memory_copy(char *src, char *dest, unsigned int n);