Build your own operating system Part #6 (Savitar Os)
7 min readAug 27, 2021
Hello all !
This is the sixth post in our blog series on deploying operating systems. This week I will explain to you how to easily run a small program in kernel mode.
Kernel
The kernel is the core of the operating system. It is the kernel that provides basic services for all other parts of the operating system. It is the main layer between the operating system and the hardware. Helps with process and memory management, file system, device control and networks. But the kernel shouldn’t do the application logic on its own. It is left to the application.
Unlike kernel mode, user mode is the environment in which user programs run. The privilege of this environment is less than that of the kernel, which can prevent user programs from interfering with other programs or with the kernel. So here we will run a little kernel-mode program.
Loading an External Program
To load the code we want to execute into memory we will use a feature in GRUB called modules to load the program.
GRUB Modules
GRUB can load arbitrary files into memory from the ISO image, and these files are usually referred to as modules. To make GRUB load a module, add the following line at the end of the “menu.lst” file which is located in “iso/boot/grub/”.
module /modules/programTo store modules, we need to create the “modules” directory under the “iso” directory. We can do it using this command.
mkdir -p iso/modulesNow we need to update the “loader.s” file as follows to instruct GRUB on how to load our modules.
global loader ; the entry symbol for ELF
extern kmain
KERNEL_STACK_SIZE equ 4096 ; size of stack in bytes
MAGIC_NUMBER equ 0x1BADB002 ; define the magic number constant
ALIGN_MODULES equ 0x00000001 ; tell GRUB to align modules
; calculate the checksum (all options + checksum should equal 0)
CHECKSUM equ -(MAGIC_NUMBER + ALIGN_MODULES)
section .bss
align 4 ; align at 4 bytes
kernel_stack: ; label points to beginning of memory
resb KERNEL_STACK_SIZE ; reserve stack for the kernel
section .text ; start of the text (code) section
align 4 ; the code must be 4 byte aligned
dd MAGIC_NUMBER ; write the magic number
dd ALIGN_MODULES ; write the align modules instruction
dd CHECKSUM ; write the checksum
mov esp, kernel_stack + KERNEL_STACK_SIZE ; point esp to the start of the
; stack (end of memory area)
loader: ; the loader label (defined as entry point in linker script)
mov eax, 0xCAFEBABE ; place the number 0xCAFEBABE in the register eax
add esp, 4
push ebx ; multiboot info in ebx
call kmain
.loop:
jmp .loop ; loop foreverExecuting a Program
The program written at this stage can only perform a few actions. Therefore, we need to store a very simple program in the “modules” directory. This is a simple procedure.
bits 32
start:
mov eax, 0xDEADBEEF
; enter infinite loop, nothing more to do
jmp startWe need to compile the code into a flat binary using this command.
nasm -f bin program.s -o programNow we have both “program.s” and “program” files in the “modules” directory.
Finding the Program in Memory
We must find where our program resides in memory which we can entirely do from C. We are going to pass the contents of “ebx” as an argument to “kmain”. To do it we need to create a header file called “multiboot.h”.
/* multiboot.h - Multiboot header file. */
/* Copyright (C) 1999,2003,2007,2008,2009,2010 Free Software Foundation, Inc.
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to
* deal in the Software without restriction, including without limitation the
* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
* sell copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL ANY
* DEVELOPER OR DISTRIBUTOR BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
* IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
#ifndef MULTIBOOT_HEADER
#define MULTIBOOT_HEADER 1
/* How many bytes from the start of the file we search for the header. */
#define MULTIBOOT_SEARCH 8192
#define MULTIBOOT_HEADER_ALIGN 4
/* The magic field should contain this. */
#define MULTIBOOT_HEADER_MAGIC 0x1BADB002
/* This should be in %eax. */
#define MULTIBOOT_BOOTLOADER_MAGIC 0x2BADB002
/* Alignment of multiboot modules. */
#define MULTIBOOT_MOD_ALIGN 0x00001000
/* Alignment of the multiboot info structure. */
#define MULTIBOOT_INFO_ALIGN 0x00000004
/* Flags set in the ’flags’ member of the multiboot header. */
/* Align all boot modules on i386 page (4KB) boundaries. */
#define MULTIBOOT_PAGE_ALIGN 0x00000001
/* Must pass memory information to OS. */
#define MULTIBOOT_MEMORY_INFO 0x00000002
/* Must pass video information to OS. */
#define MULTIBOOT_VIDEO_MODE 0x00000004
/* This flag indicates the use of the address fields in the header. */
#define MULTIBOOT_AOUT_KLUDGE 0x00010000
/* Flags to be set in the ’flags’ member of the multiboot info structure. */
/* is there basic lower/upper memory information? */
#define MULTIBOOT_INFO_MEMORY 0x00000001
/* is there a boot device set? */
#define MULTIBOOT_INFO_BOOTDEV 0x00000002
/* is the command-line defined? */
#define MULTIBOOT_INFO_CMDLINE 0x00000004
/* are there modules to do something with? */
#define MULTIBOOT_INFO_MODS 0x00000008
/* These next two are mutually exclusive */
/* is there a symbol table loaded? */
#define MULTIBOOT_INFO_AOUT_SYMS 0x00000010
/* is there an ELF section header table? */
#define MULTIBOOT_INFO_ELF_SHDR 0X00000020
/* is there a full memory map? */
#define MULTIBOOT_INFO_MEM_MAP 0x00000040
/* Is there drive info? */
#define MULTIBOOT_INFO_DRIVE_INFO 0x00000080
/* Is there a config table? */
#define MULTIBOOT_INFO_CONFIG_TABLE 0x00000100
/* Is there a boot loader name? */
#define MULTIBOOT_INFO_BOOT_LOADER_NAME 0x00000200
/* Is there a APM table? */
#define MULTIBOOT_INFO_APM_TABLE 0x00000400
/* Is there video information? */
#define MULTIBOOT_INFO_VBE_INFO 0x00000800
#define MULTIBOOT_INFO_FRAMEBUFFER_INFO 0x00001000
#ifndef ASM_FILE
typedef unsigned char multiboot_uint8_t;
typedef unsigned short multiboot_uint16_t;
typedef unsigned int multiboot_uint32_t;
typedef unsigned long long multiboot_uint64_t;
struct multiboot_header
{
/* Must be MULTIBOOT_MAGIC - see above. */
multiboot_uint32_t magic;
/* Feature flags. */
multiboot_uint32_t flags;
/* The above fields plus this one must equal 0 mod 2^32. */
multiboot_uint32_t checksum;
/* These are only valid if MULTIBOOT_AOUT_KLUDGE is set. */
multiboot_uint32_t header_addr;
multiboot_uint32_t load_addr;
multiboot_uint32_t load_end_addr;
multiboot_uint32_t bss_end_addr;
multiboot_uint32_t entry_addr;
/* These are only valid if MULTIBOOT_VIDEO_MODE is set. */
multiboot_uint32_t mode_type;
multiboot_uint32_t width;
multiboot_uint32_t height;
multiboot_uint32_t depth;
};
/* The symbol table for a.out. */
struct multiboot_aout_symbol_table
{
multiboot_uint32_t tabsize;
multiboot_uint32_t strsize;
multiboot_uint32_t addr;
multiboot_uint32_t reserved;
};
typedef struct multiboot_aout_symbol_table multiboot_aout_symbol_table_t;
/* The section header table for ELF. */
struct multiboot_elf_section_header_table
{
multiboot_uint32_t num;
multiboot_uint32_t size;
multiboot_uint32_t addr;
multiboot_uint32_t shndx;
};
typedef struct multiboot_elf_section_header_table multiboot_elf_section_header_table_t;
struct multiboot_info
{
/* Multiboot info version number */
multiboot_uint32_t flags;
/* Available memory from BIOS */
multiboot_uint32_t mem_lower;
multiboot_uint32_t mem_upper;
/* "root" partition */
multiboot_uint32_t boot_device;
/* Kernel command line */
multiboot_uint32_t cmdline;
/* Boot-Module list */
multiboot_uint32_t mods_count;
multiboot_uint32_t mods_addr;
union
{
multiboot_aout_symbol_table_t aout_sym;
multiboot_elf_section_header_table_t elf_sec;
} u;
/* Memory Mapping buffer */
multiboot_uint32_t mmap_length;
multiboot_uint32_t mmap_addr;
/* Drive Info buffer */
multiboot_uint32_t drives_length;
multiboot_uint32_t drives_addr;
/* ROM configuration table */
multiboot_uint32_t config_table;
/* Boot Loader Name */
multiboot_uint32_t boot_loader_name;
/* APM table */
multiboot_uint32_t apm_table;
/* Video */
multiboot_uint32_t vbe_control_info;
multiboot_uint32_t vbe_mode_info;
multiboot_uint16_t vbe_mode;
multiboot_uint16_t vbe_interface_seg;
multiboot_uint16_t vbe_interface_off;
multiboot_uint16_t vbe_interface_len;
multiboot_uint64_t framebuffer_addr;
multiboot_uint32_t framebuffer_pitch;
multiboot_uint32_t framebuffer_width;
multiboot_uint32_t framebuffer_height;
multiboot_uint8_t framebuffer_bpp;
#define MULTIBOOT_FRAMEBUFFER_TYPE_INDEXED 0
#define MULTIBOOT_FRAMEBUFFER_TYPE_RGB 1
#define MULTIBOOT_FRAMEBUFFER_TYPE_EGA_TEXT 2
multiboot_uint8_t framebuffer_type;
union
{
struct
{
multiboot_uint32_t framebuffer_palette_addr;
multiboot_uint16_t framebuffer_palette_num_colors;
};
struct
{
multiboot_uint8_t framebuffer_red_field_position;
multiboot_uint8_t framebuffer_red_mask_size;
multiboot_uint8_t framebuffer_green_field_position;
multiboot_uint8_t framebuffer_green_mask_size;
multiboot_uint8_t framebuffer_blue_field_position;
multiboot_uint8_t framebuffer_blue_mask_size;
};
};
};
typedef struct multiboot_info multiboot_info_t;
struct multiboot_color
{
multiboot_uint8_t red;
multiboot_uint8_t green;
multiboot_uint8_t blue;
};
struct multiboot_mmap_entry
{
multiboot_uint32_t size;
multiboot_uint64_t addr;
multiboot_uint64_t len;
#define MULTIBOOT_MEMORY_AVAILABLE 1
#define MULTIBOOT_MEMORY_RESERVED 2
#define MULTIBOOT_MEMORY_ACPI_RECLAIMABLE 3
#define MULTIBOOT_MEMORY_NVS 4
#define MULTIBOOT_MEMORY_BADRAM 5
multiboot_uint32_t type;
} __attribute__((packed));
typedef struct multiboot_mmap_entry multiboot_memory_map_t;
struct multiboot_mod_list
{
/* the memory used goes from bytes ’mod_start’ to ’mod_end-1’ inclusive */
multiboot_uint32_t mod_start;
multiboot_uint32_t mod_end;
/* Module command line */
multiboot_uint32_t cmdline;
/* padding to take it to 16 bytes (must be zero) */
multiboot_uint32_t pad;
};
typedef struct multiboot_mod_list multiboot_module_t;
/* APM BIOS info. */
struct multiboot_apm_info
{
multiboot_uint16_t version;
multiboot_uint16_t cseg;
multiboot_uint32_t offset;
multiboot_uint16_t cseg_16;
multiboot_uint16_t dseg;
multiboot_uint16_t flags;
multiboot_uint16_t cseg_len;
multiboot_uint16_t cseg_16_len;
multiboot_uint16_t dseg_len;
};
#endif /* ! ASM_FILE */
#endif /* ! MULTIBOOT_HEADER */Finally, we need to call the necessary functions within the “kmain” to run the simple program. So, you can do it like this.
#include "drivers/frame_buffer.h"
#include "drivers/serial_port.h"
#include "drivers/io.h"
#include "segmentation/segments.h"
#include "interrupts/interrupts.h"
#include "interrupts/keyboard.h"
#include "interrupts/pic.h"
#include "multiboot.h"
int kmain(unsigned int ebx)
{
segments_install_gdt();
interrupts_install_idt();
multiboot_info_t *mbinfo = (multiboot_info_t *) ebx;
multiboot_module_t* modules = (multiboot_module_t*) mbinfo->mods_addr;
unsigned int address_of_module = modules->mod_start;
if((mbinfo->mods_count) == 1){
char str[] = "Module successfully loaded";
serial_write(str,sizeof(str));
typedef void (*call_module_t)(void);
call_module_t start_program = (call_module_t) address_of_module;
start_program();
}
else{
char str[] = "Multiple modules loaded";
serial_write(str,sizeof(str));
}
return 0;
}Now we can run the OS using the “make run” command. If you succeed then you can see EAX=deadbeef in “bochslog.txt”.
I think you all get a good idea about user mode.
Thank you for reading and I will be back next week with part 07 of this article series.