Updated the README.md

Makefile

@@ -1,11 +1,11 @@
 ASM=nasm
 BIN=bin
-CFLAGS=-ffreestanding -fno-pie -m32
+CC=i686-elf-gcc
-CC=gcc
+CFLAGS=-ffreestanding -fno-pie -ansi
-LD=ld
+LD=i686-elf-ld
-LDFLAGS=-melf_i386 -Ttext 0x1000 --oformat binary
+LDFLAGS=-Ttext 0x1000 --oformat binary
 
-OBJ=kernel/kernel_entry.o kernel/kernel.o
+OBJ=kernel/kernel_entry.o kernel/kernel.o kernel/ports.o kernel/util.o drivers/screen.o
 
 all: os-image
 
@@ -17,7 +17,7 @@ os-image: boot/boot_sect.bin kernel/kernel.bin
 kernel/kernel.bin: $(OBJ)
 	$(LD) $(LDFLAGS) $^ -o $@
 
-%.o: %.c ${HEADERS}
+%.o: %.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
 %.o: %.asm
@@ -29,5 +29,5 @@ kernel/kernel.bin: $(OBJ)
 .PHONY: clean
 
 clean:
-	rm -rf boot/*.bin boot/*.o
+	rm -rf boot/*.bin kernel/*.bin
 	rm -rf kernel/*.o boot/*.o drivers/*.o

README.md

@@ -4,7 +4,14 @@ This is code that I've been writing for a basic kernel from the [Writing a Simpl
 
 Compiling
 ---------
-I cross-compile from `x86_64` to `x86` (64 to 32-bit) on my machine, but all the code is plain `x86`. You will require `gcc` and `nasm` (as well as the normal GNU core utils) and you can run the final image using [QEMU](https://www.qemu.org/). To compile just run `make`.
+I cross-compile from `x86_64` to `i686` on my machine. You will require `i686-elf-gcc` and `nasm` (as well as all the rest of the GNU binutils for `i686`) and you can run the final image using [QEMU](https://www.qemu.org/). To compile just run `make`.
+
+Code Structure
+--------------
+ - `boot/`: code necessary for the boot process (i.e. loading the kernel and switching to 32-bit protected mode).
+ - `kernel/`: code related to the kernel itself.
+ - `drivers/`: interface code that simplifies interaction with hardware devices.
+ - `bin/`: binary directory where final `os-image` is stored.
 
 License
 -------

boot/boot_sect.asm

@@ -10,7 +10,7 @@ KERNEL_OFFSET: equ 0x1000      ; kernel memory offset
 	call load_kernel           ; load our kernel from memory
 	call switch_to_pm          ; switch to 32-bit protected mode
 
-	jmp $
+	;jmp $
 
 %include "print.asm"
 %include "print32.asm"
@@ -25,7 +25,7 @@ load_kernel:
 	call print_string
 
 	mov bx, KERNEL_OFFSET      ; address to load the kernel to
-	mov dh, 1                  ; number of sectors to load
+	mov dh, 3                  ; number of sectors to load
 	mov dl, [BOOT_DRIVE]       ; drive to load from
 	call disk_load
 
@@ -41,7 +41,7 @@ BEGIN_PM:
 
 	call KERNEL_OFFSET
 
-	jmp $
+	;jmp $
 
 ;;;
 ; Data

drivers/screen.c

@@ -0,0 +1,131 @@
+#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);
+	port_byte_out(REG_SCREEN_DATA, (unsigned char)offset);
+}
+
+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;
+
+	if(!attr_byte)
+		attr_byte = WHITE_ON_BLACK;
+
+	/* get the offset of the screen location specified */
+	int offset;
+	/*
+	 * if row & col are non-negative then they can be used
+	 * as an offset
+	 */
+	if(col >= 0 && row >= 0)
+		offset = get_screen_offset(col, row);
+	/* otherwise use the cursor's current location */
+	else
+		offset = get_cursor();
+
+	/* handle a newline */
+	if(c == '\n')
+	{
+		int rows = offset / (2 * MAX_COLS);
+		offset = get_screen_offset(79,rows);
+	}
+	/* else set the character */
+	else
+	{
+		vidmem[offset] = c;
+		vidmem[offset + 1] = attr_byte;
+	}
+
+	/* update the offset to the next cell */
+	offset += 2;
+	/* set scrolling adjustment */
+	offset = handle_scrolling(offset);
+	/* 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

@@ -0,0 +1,15 @@
+#pragma once
+
+#define VIDEO_ADDRESS   0xb8000
+#define MAX_ROWS        25
+#define MAX_COLS        80
+
+#define WHITE_ON_BLACK  0x0f
+
+#define REG_SCREEN_CTRL 0x3d4
+#define REG_SCREEN_DATA 0x3d5
+
+void print_char(char c, int col, int row, char attr_byte);
+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() {
-	/*
+	clear_screen();
-	 * point to first text cell of the video memory
+	print("Kernel initialized successfully!\n");
-	 */
-	char *video_memory = (char*) 0xb8000;
-	/*
-	 * store the character 'X' there
-	 */
-	*video_memory = 'X';
 }

kernel/ports.c

@@ -0,0 +1,23 @@
+unsigned char port_byte_in(unsigned short port) {
+	unsigned char res;
+	/*
+	 * `"=a" (res)' means: put result of `al' into `res' variable
+	 * `"d" (port)' means: load `port' into `dx'.
+	 */
+	__asm__("in %%dx, %%al" : "=a" (res) : "d" (port));
+	return res;
+}
+
+void port_byte_out(unsigned short port, unsigned char data) {
+	__asm__("out %%al, %%dx" : : "a" (data), "d" (port));
+}
+
+unsigned short port_word_in(unsigned short port) {
+	unsigned short res;
+	__asm__("in %%dx, %%ax" : "=a" (res) : "d" (port));
+	return res;
+}
+
+void port_word_out(unsigned short port, unsigned short data) {
+	__asm__("out %%ax, %%dx" : : "a" (data), "d" (port));
+}

kernel/ports.h

@@ -0,0 +1,18 @@
+#pragma once
+
+/*
+ * return a byte from a port.
+ */
+unsigned char port_byte_in(unsigned short port);
+/*
+ * write a byte of data to a port.
+ */
+void port_byte_out(unsigned short port, unsigned char data);
+/*
+ * return a word of data from a port.
+ */
+unsigned short port_word_in(unsigned short port);
+/*
+ * write a word of data to a port.
+ */
+void port_word_out(unsigned short port, unsigned short data);

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);