thakurvivek · October 28, 2016 06:50
diff --git a/Kernel 101 - Custom Kernel start point b/Kernel 101 - Custom Kernel start point
 Notes from http://arjunsreedharan.org/post/82710718100/kernel-101-lets-write-a-kernel

 Overview:

 EIP(Instruction Pointer) with value 0xFFFF FFF0, also known as Reset Vector (RV)
 RV's Value points to a memory location in BIOS
 Shadowing begins: Copy BIOS to RAM for fsater access
 Execute copied BIOS code
  Search for Bootable devices (flag: 0xAA55 in byte 512 and 511 of the first sector)
  Copy Bootable device's first sector starting from 0x7C00 to RAM, -> Bootloader
  Execute Bootloader code
    Load kernel from 0x100000 (start address for all x86 based kernels) [Kernel entry point]
    
    
 Build commands:
 nasm -f elf32 kernel.asm -o kasm.o
 gcc -m32 -c kernel.c -o kc.o          //Dont link using -c flag
 ld -m elf_i386 -T link.ld -o kernel kasm.o kc.o

diff --git a/kernel.asm b/kernel.asm
 ;;kernel.asm
 bits 32			;nasm directive - 32 bit
 section .text

 global start
 extern kmain	        ;kmain is defined in the c file

 start:
  cli 			;block interrupts
  mov esp, stack_space	;set stack pointer
  call kmain
  hlt		 	;halt the CPU

 section .bss
 resb 8192		;8KB for stack
 stack_space:
diff --git a/kernel.c b/kernel.c
 /*
 *  kernel.c
 */
 void kmain(void)
 {
 	const char *str = "my first kernel";
 	char *vidptr = (char*)0xb8000; 	//video mem begins here.
 	unsigned int i = 0;
 	unsigned int j = 0;

 	/* this loops clears the screen
 	* there are 25 lines each of 80 columns; each element takes 2 bytes */
 	while(j < 80 * 25 * 2) {
 		/* blank character */
 		vidptr[j] = ' ';
 		/* attribute-byte - light grey on black screen */
 		vidptr[j+1] = 0x07; 		
 		j = j + 2;
 	}

 	j = 0;

 	/* this loop writes the string to video memory */
 	while(str[j] != '\0') {
 		/* the character's ascii */
 		vidptr[i] = str[j];
 		/* attribute-byte: give character black bg and light grey fg */
 		vidptr[i+1] = 0x07;
 		++j;
 		i = i + 2;
 	}
 	return;
 }
diff --git a/KernelBuild.jpg b/KernelBuild.jpg
diff --git a/link.ld b/link.ld
 /*
 *  link.ld
 */
 OUTPUT_FORMAT(elf32-i386)
 ENTRY(start)
 SECTIONS
 {
   . = 0x100000;
   .text : { *(.text) }
   .data : { *(.data) }
   .bss  : { *(.bss)  }
 }
	Notes from http://arjunsreedharan.org/post/82710718100/kernel-101-lets-write-a-kernel

	Overview:

	EIP(Instruction Pointer) with value 0xFFFF FFF0, also known as Reset Vector (RV)
	RV's Value points to a memory location in BIOS
	Shadowing begins: Copy BIOS to RAM for fsater access
	Execute copied BIOS code
	Search for Bootable devices (flag: 0xAA55 in byte 512 and 511 of the first sector)
	Copy Bootable device's first sector starting from 0x7C00 to RAM, -> Bootloader
	Execute Bootloader code
	Load kernel from 0x100000 (start address for all x86 based kernels) [Kernel entry point]


	Build commands:
	nasm -f elf32 kernel.asm -o kasm.o
	gcc -m32 -c kernel.c -o kc.o //Dont link using -c flag
	ld -m elf_i386 -T link.ld -o kernel kasm.o kc.o
	;;kernel.asm
	bits 32 ;nasm directive - 32 bit
	section .text

	global start
	extern kmain ;kmain is defined in the c file

	start:
	cli ;block interrupts
	mov esp, stack_space ;set stack pointer
	call kmain
	hlt ;halt the CPU

	section .bss
	resb 8192 ;8KB for stack
	stack_space:
	/*
	* kernel.c
	*/
	void kmain(void)
	{
	const char *str = "my first kernel";
	char vidptr = (char)0xb8000; //video mem begins here.
	unsigned int i = 0;
	unsigned int j = 0;

	/* this loops clears the screen
	* there are 25 lines each of 80 columns; each element takes 2 bytes */
	while(j < 80 * 25 * 2) {
	/* blank character */
	vidptr[j] = ' ';
	/* attribute-byte - light grey on black screen */
	vidptr[j+1] = 0x07;
	j = j + 2;
	}

	j = 0;

	/* this loop writes the string to video memory */
	while(str[j] != '\0') {
	/* the character's ascii */
	vidptr[i] = str[j];
	/* attribute-byte: give character black bg and light grey fg */
	vidptr[i+1] = 0x07;
	++j;
	i = i + 2;
	}
	return;
	}
	/*
	* link.ld
	*/
	OUTPUT_FORMAT(elf32-i386)
	ENTRY(start)
	SECTIONS
	{
	. = 0x100000;
	.text : { *(.text) }
	.data : { *(.data) }
	.bss : { *(.bss) }
	}