Initial commit.

.gitignore

@@ -0,0 +1,48 @@
+# Project Folder (at least for now)
+.idea
+
+# Compiled Binaries
+*.bin
+
+# Compile Commands
+compile_commands.json
+
+# Prerequisites
+*.d
+
+# Compiled Object files
+*.slo
+*.lo
+*.o
+*.obj
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Compiled Dynamic libraries
+*.so
+*.dylib
+*.dll
+
+# Fortran module files
+*.mod
+*.smod
+
+# Compiled Static libraries
+*.lai
+*.la
+*.a
+*.lib
+
+# Executables
+*.exe
+*.out
+*.app
+
+# for projects that use SCons for building: http://http://www.scons.org/
+.sconsign.dblite
+
+# When configure fails, SCons outputs these
+config.log
+.sconf_temp

SConstruct

@@ -0,0 +1,39 @@
+
+import subprocess
+
+env = Environment(tools =  ['default', 'compilation_db'])
+env['AS'] = 'i686-elf-as'
+env['CC'] = 'i686-elf-gcc'
+env['CXX'] = 'i686-elf-g++'
+env['LD'] = 'i686-elf-g++'
+env.Append(CXXFLAGS = ['-ffreestanding', '-fno-exceptions', '-fno-rtti', '-std=c++20'])
+env.Append(LINKFLAGS = ['-T', 'linker.ld', '-ffreestanding', '-nostdlib'])
+env.Append(CPPPATH = ['#include'])
+
+def get_crt_object(name: str) -> str:
+    cmd = [env['CXX']]
+    cmd.extend(env['CXXFLAGS'])
+    cmd.append(f'-print-file-name={name}')
+    result = subprocess.run(cmd, stdout=subprocess.PIPE)
+    return result.stdout.decode('utf-8').strip()
+crtbegin_o = get_crt_object('crtbegin.o')
+crtend_o = get_crt_object('crtend.o')
+crti_o = env.Object('src/crt/crti.s')
+crtn_o = env.Object('src/crt/crtn.s')
+
+os_sources = Split('''
+    src/kernel/boot.s
+    src/kernel/startup.cpp
+
+    src/os/tty.cpp
+''')
+env['LINKCOM'] = env['LINKCOM'].replace('$_LIBFLAGS', f'{crti_o[0]} {crtbegin_o} $_LIBFLAGS {crtend_o} {crtn_o[0]}')
+
+prog_os = env.Program(
+    target = 'os.bin',
+    source = os_sources
+)
+env.Depends(prog_os, [crti_o, crtn_o])
+env.Default(prog_os)
+comp_db = env.CompilationDatabase(target = '#compile_commands.json')
+env.Default(comp_db)

boot/grub.cfg

@@ -0,0 +1,3 @@
+menuentry "Bad Apple OS" {
+    multiboot /boot/os.bin
+}

include/os/tty.hpp

@@ -0,0 +1,51 @@
+
+#pragma once
+
+#if !defined(OS_TTY_HPP_INCLUDED)
+#define OS_TTY_HPP_INCLUDED 1
+
+#include <stddef.h>
+#include <stdint.h>
+
+namespace tty
+{
+enum class VgaColor : uint8_t
+{
+	BLACK = 0,
+	BLUE = 1,
+	GREEN = 2,
+	CYAN = 3,
+	RED = 4,
+	MAGENTA = 5,
+	BROWN = 6,
+	LIGHT_GREY = 7,
+	DARK_GREY = 8,
+	LIGHT_BLUE = 9,
+	LIGHT_GREEN = 10,
+	LIGHT_CYAN = 11,
+	LIGHT_RED = 12,
+	LIGHT_MAGENTA = 13,
+	LIGHT_BROWN = 14,
+	WHITE = 15
+};
+
+struct VgaDoubleColor
+{
+    VgaColor foreground : 4 = VgaColor::LIGHT_GREY;
+    VgaColor background : 4 = VgaColor::BLACK;
+};
+
+[[nodiscard]] size_t strlen(const char* str); // TODO: move to standard lib
+void initialize();
+void setColor(VgaDoubleColor color);
+void putEntryAt(char chr, VgaDoubleColor color, size_t posX, size_t posY);
+void putChar(char chr);
+void write(const char* data, size_t size);
+
+inline void write(const char* data)
+{
+    write(data, strlen(data));
+}
+} // namespace tty
+
+#endif // OS_TTY_HPP_INCLUDED

linker.ld

@@ -0,0 +1,52 @@
+/* The bootloader will look at this image and start execution at the symbol
+   designated as the entry point. */
+ENTRY(_start)
+ 
+/* Tell where the various sections of the object files will be put in the final
+   kernel image. */
+SECTIONS
+{
+	/* It used to be universally recommended to use 1M as a start offset,
+	   as it was effectively guaranteed to be available under BIOS systems.
+	   However, UEFI has made things more complicated, and experimental data
+	   strongly suggests that 2M is a safer place to load. In 2016, a new
+	   feature was introduced to the multiboot2 spec to inform bootloaders
+	   that a kernel can be loaded anywhere within a range of addresses and
+	   will be able to relocate itself to run from such a loader-selected
+	   address, in order to give the loader freedom in selecting a span of
+	   memory which is verified to be available by the firmware, in order to
+	   work around this issue. This does not use that feature, so 2M was
+	   chosen as a safer option than the traditional 1M. */
+	. = 2M;
+ 
+	/* First put the multiboot header, as it is required to be put very early
+	   in the image or the bootloader won't recognize the file format.
+	   Next we'll put the .text section. */
+	.text BLOCK(4K) : ALIGN(4K)
+	{
+		*(.multiboot)
+		*(.text)
+	}
+ 
+	/* Read-only data. */
+	.rodata BLOCK(4K) : ALIGN(4K)
+	{
+		*(.rodata)
+	}
+ 
+	/* Read-write data (initialized) */
+	.data BLOCK(4K) : ALIGN(4K)
+	{
+		*(.data)
+	}
+ 
+	/* Read-write data (uninitialized) and stack */
+	.bss BLOCK(4K) : ALIGN(4K)
+	{
+		*(COMMON)
+		*(.bss)
+	}
+ 
+	/* The compiler may produce other sections, by default it will put them in
+	   a segment with the same name. Simply add stuff here as needed. */
+}

src/crt/crti.s

@@ -0,0 +1,16 @@
+/* x86 crti.s */
+.section .init
+.global _init
+.type _init, @function
+_init:
+	push %ebp
+	movl %esp, %ebp
+	/* gcc will nicely put the contents of crtbegin.o's .init section here. */
+
+.section .fini
+.global _fini
+.type _fini, @function
+_fini:
+	push %ebp
+	movl %esp, %ebp
+	/* gcc will nicely put the contents of crtbegin.o's .fini section here. */

src/crt/crtn.s

@@ -0,0 +1,10 @@
+/* x86 crtn.s */
+.section .init
+	/* gcc will nicely put the contents of crtend.o's .init section here. */
+	popl %ebp
+	ret
+
+.section .fini
+	/* gcc will nicely put the contents of crtend.o's .fini section here. */
+	popl %ebp
+	ret

src/kernel/boot.s

@@ -0,0 +1,109 @@
+/* Declare constants for the multiboot header. */
+.set ALIGN,    1<<0             /* align loaded modules on page boundaries */
+.set MEMINFO,  1<<1             /* provide memory map */
+.set FLAGS,    ALIGN | MEMINFO  /* this is the Multiboot 'flag' field */
+.set MAGIC,    0x1BADB002       /* 'magic number' lets bootloader find the header */
+.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot */
+
+/* 
+Declare a multiboot header that marks the program as a kernel. These are magic
+values that are documented in the multiboot standard. The bootloader will
+search for this signature in the first 8 KiB of the kernel file, aligned at a
+32-bit boundary. The signature is in its own section so the header can be
+forced to be within the first 8 KiB of the kernel file.
+*/
+.section .multiboot
+.align 4
+.long MAGIC
+.long FLAGS
+.long CHECKSUM
+
+/*
+The multiboot standard does not define the value of the stack pointer register
+(esp) and it is up to the kernel to provide a stack. This allocates room for a
+small stack by creating a symbol at the bottom of it, then allocating 16384
+bytes for it, and finally creating a symbol at the top. The stack grows
+downwards on x86. The stack is in its own section so it can be marked nobits,
+which means the kernel file is smaller because it does not contain an
+uninitialized stack. The stack on x86 must be 16-byte aligned according to the
+System V ABI standard and de-facto extensions. The compiler will assume the
+stack is properly aligned and failure to align the stack will result in
+undefined behavior.
+*/
+.section .bss
+.align 16
+stack_bottom:
+.skip 16384 # 16 KiB
+stack_top:
+
+/*
+The linker script specifies _start as the entry point to the kernel and the
+bootloader will jump to this position once the kernel has been loaded. It
+doesn't make sense to return from this function as the bootloader is gone.
+*/
+.section .text
+.global _start
+.type _start, @function
+_start:
+	/*
+	The bootloader has loaded us into 32-bit protected mode on a x86
+	machine. Interrupts are disabled. Paging is disabled. The processor
+	state is as defined in the multiboot standard. The kernel has full
+	control of the CPU. The kernel can only make use of hardware features
+	and any code it provides as part of itself. There's no printf
+	function, unless the kernel provides its own <stdio.h> header and a
+	printf implementation. There are no security restrictions, no
+	safeguards, no debugging mechanisms, only what the kernel provides
+	itself. It has absolute and complete power over the
+	machine.
+	*/
+
+	/*
+	To set up a stack, we set the esp register to point to the top of the
+	stack (as it grows downwards on x86 systems). This is necessarily done
+	in assembly as languages such as C cannot function without a stack.
+	*/
+	mov $stack_top, %esp
+
+	/*
+	This is a good place to initialize crucial processor state before the
+	high-level kernel is entered. It's best to minimize the early
+	environment where crucial features are offline. Note that the
+	processor is not fully initialized yet: Features such as floating
+	point instructions and instruction set extensions are not initialized
+	yet. The GDT should be loaded here. Paging should be enabled here.
+	C++ features such as global constructors and exceptions will require
+	runtime support to work as well.
+	*/
+
+	/*
+	Enter the high-level kernel. The ABI requires the stack is 16-byte
+	aligned at the time of the call instruction (which afterwards pushes
+	the return pointer of size 4 bytes). The stack was originally 16-byte
+	aligned above and we've pushed a multiple of 16 bytes to the
+	stack since (pushed 0 bytes so far), so the alignment has thus been
+	preserved and the call is well defined.
+	*/
+	call kernel_main
+
+	/*
+	If the system has nothing more to do, put the computer into an
+	infinite loop. To do that:
+	1) Disable interrupts with cli (clear interrupt enable in eflags).
+	   They are already disabled by the bootloader, so this is not needed.
+	   Mind that you might later enable interrupts and return from
+	   kernel_main (which is sort of nonsensical to do).
+	2) Wait for the next interrupt to arrive with hlt (halt instruction).
+	   Since they are disabled, this will lock up the computer.
+	3) Jump to the hlt instruction if it ever wakes up due to a
+	   non-maskable interrupt occurring or due to system management mode.
+	*/
+	cli
+1:	hlt
+	jmp 1b
+
+/*
+Set the size of the _start symbol to the current location '.' minus its start.
+This is useful when debugging or when you implement call tracing.
+*/
+.size _start, . - _start

src/kernel/startup.cpp

@@ -0,0 +1,23 @@
+#include <stddef.h>
+#include <stdint.h>
+
+#include "os/tty.hpp"
+
+/* Check if the compiler thinks you are targeting the wrong operating system. */
+#if defined(__linux__)
+#error "You are not using a cross-compiler, you will most certainly run into trouble"
+#endif
+ 
+/* This tutorial will only work for the 32-bit ix86 targets. */
+#if !defined(__i386__)
+#error "This tutorial needs to be compiled with a ix86-elf compiler"
+#endif
+
+extern "C" void kernel_main(void) 
+{
+	/* Initialize terminal interface */
+	tty::initialize();
+ 
+	/* Newline support is left as an exercise. */
+	tty::write("Hello, kernel World!\n");
+}

src/os/tty.cpp

@@ -0,0 +1,79 @@
+
+#include "os/tty.hpp"
+
+namespace tty
+{
+namespace
+{
+inline const size_t VGA_WIDTH = 80;
+inline const size_t VGA_HEIGHT = 25;
+
+constexpr uint8_t vgaEntryColor(VgaDoubleColor color)
+{
+    return static_cast<uint8_t>(color.foreground) | static_cast<uint8_t>(color.background) << 4;
+}
+
+constexpr uint16_t vgaEntry(const unsigned char chr, const VgaDoubleColor color)
+{
+    return static_cast<uint16_t>(chr) | static_cast<uint16_t>(vgaEntryColor(color) << 8);
+}
+
+size_t gTerminalRow = 0;
+size_t gTerminalColumn = 0;
+VgaDoubleColor gTerminalColor = VgaDoubleColor();
+uint16_t* gTerminalBuffer = reinterpret_cast<uint16_t*>(0xB8000);
+}
+
+size_t strlen(const char* str) // TODO: move to standard lib
+{
+    size_t len = 0;
+    while (str[len] != '\0') {
+        ++len;
+    }
+    return len;
+}
+
+void initialize()
+{
+    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;
+            gTerminalBuffer[index] = vgaEntry(' ', gTerminalColor);
+        }
+    }
+}
+
+void setColor(VgaDoubleColor color)
+{
+    gTerminalColor = color;
+}
+
+void putEntryAt(char chr, VgaDoubleColor color, size_t posX, size_t posY)
+{
+    const size_t index = posY * VGA_WIDTH + posX;
+    gTerminalBuffer[index] = vgaEntry(chr, color);
+}
+
+void putChar(char chr)
+{
+    putEntryAt(chr, gTerminalColor, gTerminalColumn, gTerminalRow);
+    if (++gTerminalColumn == VGA_WIDTH)
+    {
+        gTerminalColumn = 0;
+        if (++gTerminalRow == VGA_HEIGHT)
+        {
+            gTerminalRow = 0;
+        }
+    }
+}
+
+void write(const char* data, size_t size)
+{
+    for (size_t idx = 0; idx < size; idx++)
+    {
+        putChar(data[idx]);
+    }
+}
+}