Makefile

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

README.md

@@ -4,14 +4,7 @@ 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 `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.
+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`.
 
 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, 3                  ; number of sectors to load
+	mov dh, 1                  ; 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

@@ -1,131 +0,0 @@
-#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

@@ -1,15 +0,0 @@
-#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

@@ -1,6 +1,10 @@
-#include "../drivers/screen.h"
-
 void main() {
-	clear_screen();
-	print("Kernel initialized successfully!\n");
+	/*
+	 * point to first text cell of the video memory
+	 */
+	char *video_memory = (char*) 0xb8000;
+	/*
+	 * store the character 'X' there
+	 */
+	*video_memory = 'X';
 }

kernel/ports.c

@@ -1,23 +0,0 @@
-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

@@ -1,18 +0,0 @@
-#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

@@ -1,7 +0,0 @@
-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

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