adding raw asm files for creating a bootloader

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README.md

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+# sarOS
+
+Following [3zanders](http://3zanders.co.uk/2017/10/13/writing-a-bootloader/) & [Phil Opp](https://os.phil-opp.com/) tutorial on writing an OS
+
+## Deps
+- NASM
+- QEMU

asm/32bit.asm

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+bits 16
+org 0x7c00
+
+boot:
+	mov ax, 0x2401
+	int 0x15 ; enable A20 bit
+
+	mov ax, 0x3
+	int 0x10 ; set vga text mode 3
+
+	cli
+	lgdt [gdt_pointer] ; load the gdt table
+	mov eax, cr0
+	or eax,0x1  ; set the protected mode bit on special CPU reg cr0
+	mov cr0, eax
+	jmp CODE_SEG:boot2 ; long jump to the code segment
+gdt_start:
+	dq 0x0
+gdt_code:
+	dw 0xFFFF
+	dw 0x0
+	db 0x0
+	db 10011010b
+	db 11001111b
+	db 0x0
+gdt_data:
+	dw 0xFFFF
+	dw 0x0
+	db 0x0
+	db 10010010b
+	db 11001111b
+	db 0x0
+gdt_end:
+gdt_pointer:
+	dw gdt_end - gdt_start
+	dd gdt_start
+
+CODE_SEG equ gdt_code - gdt_start
+DATA_SEG equ gdt_data - gdt_start
+
+bits 32
+boot2:
+	mov ax, DATA_SEG
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+	mov esi,hello
+	mov ebx,0xb8000
+.loop:
+	lodsb
+	or al,al
+	jz halt
+	or eax,0x0100
+	mov word [ebx], ax
+	add ebx,2
+	jmp .loop
+halt:
+	cli
+	hlt
+hello: db "Hello world!",0
+
+times 510 - ($-$$) db 0
+dw 0xaa55

asm/boot3.asm

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+bits 16
+org 0x7c00
+
+boot:
+	mov ax, 0x2401
+	int 0x15
+
+	mov ax, 0x3
+	int 0x10
+
+	mov [disk],dl ; The disk number is implicitly placed into dl by the BIOS on startup. Earlier on we stashed it into memory with mov [disk], dl.
+
+	mov ah, 0x2    ;read sectors
+	mov al, 1      ;sectors to read
+	mov ch, 0      ;cylinder idx
+	mov dh, 0      ;head idx
+	mov cl, 2      ;sector idx
+	mov dl, [disk] ;disk idx
+	mov bx, copy_target;target pointer
+	int 0x13
+
+	cli
+	lgdt [gdt_pointer]
+	mov eax, cr0
+	or eax,0x1
+	mov cr0, eax
+
+	mov ax, DATA_SEG
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+	jmp CODE_SEG:boot2
+gdt_start:
+	dq 0x0
+gdt_code:
+	dw 0xFFFF
+	dw 0x0
+	db 0x0
+	db 10011010b
+	db 11001111b
+	db 0x0
+gdt_data:
+	dw 0xFFFF
+	dw 0x0
+	db 0x0
+	db 10010010b
+	db 11001111b
+	db 0x0
+gdt_end:
+gdt_pointer:
+	dw gdt_end - gdt_start
+	dd gdt_start
+disk:
+	db 0x0
+CODE_SEG equ gdt_code - gdt_start
+DATA_SEG equ gdt_data - gdt_start
+
+times 510 - ($-$$) db 0
+dw 0xaa55
+copy_target:
+bits 32
+	hello: db "Hello more than 512 bytes world!!",0
+boot2:
+	mov esi,hello
+	mov ebx,0xb8000
+.loop:
+	lodsb
+	or al,al
+	jz halt
+	or eax,0x0F00
+	mov word [ebx], ax
+	add ebx,2
+	jmp .loop
+halt:
+	cli
+	hlt
+
+times 1024 - ($-$$) db 0

asm/boot4.asm

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+section .boot
+bits 16
+global boot
+boot:
+	mov ax, 0x2401
+	int 0x15
+
+	mov ax, 0x3
+	int 0x10
+
+	mov [disk],dl ; The disk number is implicitly placed into dl by the BIOS on startup. Earlier on we stashed it into memory with mov [disk], dl.
+
+	mov ah, 0x2    ;read sectors
+	mov al, 6      ;sectors to read
+	mov ch, 0      ;cylinder idx
+	mov dh, 0      ;head idx
+	mov cl, 2      ;sector idx
+	mov dl, [disk] ;disk idx
+	mov bx, copy_target;target pointer
+	int 0x13
+
+	cli
+	lgdt [gdt_pointer]
+	mov eax, cr0
+	or eax,0x1
+	mov cr0, eax
+
+	mov ax, DATA_SEG
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+	jmp CODE_SEG:boot2
+gdt_start:
+	dq 0x0
+gdt_code:
+	dw 0xFFFF
+	dw 0x0
+	db 0x0
+	db 10011010b
+	db 11001111b
+	db 0x0
+gdt_data:
+	dw 0xFFFF
+	dw 0x0
+	db 0x0
+	db 10010010b
+	db 11001111b
+	db 0x0
+gdt_end:
+gdt_pointer:
+	dw gdt_end - gdt_start
+	dd gdt_start
+disk:
+	db 0x0
+CODE_SEG equ gdt_code - gdt_start
+DATA_SEG equ gdt_data - gdt_start
+
+times 510 - ($-$$) db 0
+dw 0xaa55
+copy_target:
+bits 32
+	hello: db "Hello more than 512 bytes world!!",0
+boot2:
+	mov esi,hello
+	mov ebx,0xb8000
+.loop:
+	lodsb
+	or al,al
+	jz halt
+	or eax,0x0F00
+	mov word [ebx], ax
+	add ebx,2
+	jmp .loop
+halt:
+	mov esp,kernel_stack_top
+	extern kmain
+	call kmain
+	cli
+	hlt
+
+section .bss
+align 4
+kernel_stack_bottom: equ $
+	resb 16384 ; 16 KB
+kernel_stack_top:

asm/hello_world.asm

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+bits 16 ; tell NASM this is 16 bit code
+org 0x7c00 ; tell NASM to start outputting stuff at offset 0x7c00
+boot:
+    mov si,hello ; point si register to hello label memory location
+    mov ah,0x0e ; 0x0e means 'Write Character in TTY mode'
+.loop:
+    lodsb
+    or al,al ; is al == 0 ?
+    jz halt  ; if (al == 0) jump to halt label
+    int 0x10 ; runs BIOS interrupt 0x10 - Video Services
+    jmp .loop
+halt:
+    cli ; clear interrupt flag
+    hlt ; halt execution
+hello: db "Hello world!",0
+
+times 510 - ($-$$) db 0 ; pad remaining 510 bytes with zeroes
+dw 0xaa55 ; magic bootloader magic - marks this 512 byte sector bootable!

asm/makefile

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+
+hello_world.bin: hello_world.asm
+	nasm -f bin hello_world.asm -o hello_world.bin
+
+run_hello_world: hello_world.bin
+	qemu-system-x86_64 -fda hello_world.bin
+
+32bit.bin: 32bit.asm
+	nasm -f bin 32bit.asm -o 32bit.bin
+
+run_32bit: 32bit.bin
+	qemu-system-x86_64 -fda 32bit.bin
+
+boot3.bin: boot3.asm
+	nasm -f bin boot3.asm -o boot3.bin
+
+run3: boot3.bin
+	qemu-system-x86_64 -fda boot3.bin
+
+clean:
+	rm -f *.bin