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‎os/part1/Makefile

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.PHONY: clean, .force-rebuild
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all: bootloader.bin
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bootloader.bin: os.asm .force-rebuild
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nasm -fbin os.asm -o os.bin
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clean:
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rm *.bin

‎os/part1/kernel/kernel.asm

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; Dummy file
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times 512 db 0

‎os/part1/os.asm

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stage1_start:
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times 90 db 0 ; BPB (BIOS Parameter Block) will go here
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%include "stage1/bootstage1.asm"
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stage1_end:
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stage2_start:
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%include "stage2/bootstage2.asm"
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align 512, db 0
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stage2_end:
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kernel_start:
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%include "kernel/kernel.asm"
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align 512, db 0
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kernel_end:

‎os/part1/os.bin

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Binary file not shown.

‎os/part1/stage1/bootstage1.asm

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BITS 16 ; On the x86, the BIOS (and consequently the bootloader) runs in 16-bit Real Mode.
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ORG 0x7C00 ; We are loaded/booted by BIOS into this memory address.
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Stage1_entrypoint: ; Main entry point where BIOS leaves us. Some BIOS may load us at 0x0000:0x7C00 while others at 0x07C0:0x0000.
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jmp 0x0000:.setup_segments ; We do a far jump to accommodate for this issue (CS is reloaded to 0x0000).
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.setup_segments: ; Next, we set all segment registers to zero.
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xor ax, ax
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mov ss, ax
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov sp, Stage1_entrypoint ; We set up a temporary stack so that it starts growing below Stage1_entrypoint (i.e. the stack base will be located at 0:0x7c00).
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cld ; Clear the direction flag (i.e. go forward in memory when using instructions like lodsb).
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; Loading stage 2 from disk into RAM
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mov [disk], dl ; Storing disk number. BIOS loads into dl the "drive number" of the booted device.
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mov ax, (stage2_start-stage1_start)/512 ; ax: start sector
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mov cx, (kernel_end-stage2_start)/512 ; cx: number of sectors (512 bytes) to read
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mov bx, stage2_start ; bx: offset of buffer
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xor dx, dx ; dx: segment of buffer
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call Real_mode_read_disk
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mov si, stage1_message
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call Real_mode_println
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; Jump to the entry point of stage 2 (commented out for now)
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;;; jmp Stage2_entrypoint
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.halt: hlt
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jmp .halt ; Infinite loop (it prevents us from going off and executing other junk in memory).
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; Include
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%include "stage1/disk.asm"
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%include "stage1/print.asm"
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times 510-($-$$) db 0 ; Padding
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dw 0xAA55 ; The last two bytes of the boot sector should have the 0xAA55 signature.
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; Otherwise, we'll get an error message from BIOS that it didn't find a bootable disk.
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; This signature is represented in binary as 1010101001010101. The alternating bit
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; pattern was thought to be a protection against certain (drive or controller) failures.

‎os/part1/stage1/disk.asm

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BITS 16
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;---Initialized data------------------------------------------------------------
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disk db 0x80
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disk_error_message dw 11
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db 'Disk error!'
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DAP:
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;*******************************************************************************;
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; Disk Address Packet ;
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;-------------------------------------------------------------------------------;
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; Offset Size Description ;
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; 0 1 size of packet (16 bytes) ;
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; 1 1 always 0 ;
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; 2 2 number of sectors to load (max = 127 on some BIOS) ;
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; 4 2 16-bit offset of target buffer ;
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; 4 2 16-bit segment of target buffer ;
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; 8 4 lower 32 bits of 48-bit starting LBA ;
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; 12 4 upper 32 bits of 48-bit starting LBA ;
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;*******************************************************************************;
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db 0x10 ; size of packet = 16 bytes
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db 0 ; always 0
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.num_sectors: dw 127 ; number of sectors to load (max = 127 on some BIOS)
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.buf_offset: dw 0x0 ; 16-bit offset of target buffer
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.buf_segment: dw 0x0 ; 16-bit segment of target buffer
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.LBA_lower: dd 0x0 ; lower 32 bits of 48-bit starting LBA
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.LBA_upper: dd 0x0 ; upper 32 bits of 48-bit starting LBA
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;---Code------------------------------------------------------------------------
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Real_mode_read_disk:
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;**********************************************************;
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; Load disk sectors to memory (int 13h, function code 42h) ;
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;----------------------------------------------------------;
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; ax: start sector ;
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; cx: number of sectors (512 bytes) to read ;
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; bx: offset of buffer ;
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; dx: segment of buffer ;
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;**********************************************************;
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.start:
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cmp cx, 127 ; (max sectors to read in one call = 127)
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jbe .good_size
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pusha
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mov cx, 127
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call Real_mode_read_disk
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popa
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add eax, 127
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add dx, 127 * 512 / 16
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sub cx, 127
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jmp .start
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.good_size:
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mov [DAP.LBA_lower], ax
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mov [DAP.num_sectors], cx
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mov [DAP.buf_segment], dx
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mov [DAP.buf_offset], bx
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mov dl, [disk]
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mov si, DAP
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mov ah, 0x42
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int 0x13
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jc .print_error
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ret
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.print_error:
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mov si, disk_error_message
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call Real_mode_println
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.halt: hlt
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jmp .halt; Infinite loop. We cannot recover from disk error.

‎os/part1/stage1/print.asm

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BITS 16
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;---Initialized data------------------------------------------------------------
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newline dw 2
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db 13,10 ; \r\n
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stage1_message dw 17
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db 'Stage 1 finished.'
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;---Code------------------------------------------------------------------------
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Real_mode_print:
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;*********************************************************************************;
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; Prints a string (in real mode) ;
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;---------------------------------------------------------------------------------;
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; si: pointer to string (first 16 bits = the number of characters in the string.) ;
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;*********************************************************************************;
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push ax
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push cx
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push si
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mov cx, word [si] ; first 16 bits = the number of characters in the string
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add si, 2
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.string_loop: ; print all the characters in the string
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lodsb
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mov ah, 0eh
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int 10h
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loop .string_loop, cx
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pop si
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pop cx
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pop ax
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ret
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Real_mode_println:
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;***********************************************************;
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; Prints a string (in real mode) and a newline (\r\n) ;
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;-----------------------------------------------------------;
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; si: pointer to string ;
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; (first 16 bits = the number of characters in the string.) ;
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;***********************************************************;
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push si
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call Real_mode_print
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mov si, newline
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call Real_mode_print
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pop si
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ret

‎os/part1/stage2/bootstage2.asm

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; Dummy file
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times 512 db 0

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