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https://github.com/RGBCube/serenity
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Bootloader: Locate the kernel's data segment and clear it.
This was a constant source of stupid bugs and I kept postponing it because I wasn't in the mood to write assembly code. Until now! :^)
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22 changed files with 54 additions and 83 deletions
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@ -25,9 +25,13 @@ boot:
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mov ax, 0x2401
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int 0x15
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mov bx, 0x1000
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; HACK: Load the ELF kernel at 0xf000. Assuming that the first
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; LOAD header has a file offset of 0x1000, this puts _start
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; at 0x10000 which we jump to later.
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; This is all quite rickety.
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mov bx, 0xf00
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mov es, bx
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xor bx, bx ; Load kernel @ 0x10000
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xor bx, bx
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mov cx, word [cur_lba]
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.sector_loop:
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@ -65,6 +69,53 @@ boot:
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xor al, al
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out dx, al
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; Let's look at the ELF header.
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mov bx, 0xf00
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mov fs, bx
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cmp [fs:0], dword 0x464c457f ; ELF magic: { 0x7f "ELF" }
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jne fug
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cmp [fs:24], dword 0x10000 ; Entry should be 0x10000
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jne fug
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mov ebx, dword [fs:28] ; EBX <- program header table
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mov ecx, dword [fs:44] ; ECX <- program header count
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; Let's find the BSS and clear it.
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parse_program_header:
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cmp [fs:ebx], dword 0x1 ; Is Load segment?
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jne .next
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cmp [fs:ebx+24], dword 0x6 ; Is read+write but not execute?
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jne .next
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mov edi, [fs:ebx+8] ; EDI <- p_vaddr
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add edi, [fs:ebx+16] ; skip over 'p_filesz' bytes (leave them intact)
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push ecx
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sub edi, [fs:ebx+16] ; skip over 'p_filesz' bytes (see above)
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; Since we're in 16-bit real mode, create a segment address.
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mov eax, edi
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shr eax, 4
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mov es, ax
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and edi, 0xf
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mov ecx, [fs:ebx+20] ; ECX <- p_memsz
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xor al, al
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rep stosb
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pop ecx
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.next:
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add ebx, 32
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loop parse_program_header
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; Okay we're all set to go!
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lets_go:
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lgdt [cs:test_gdt_ptr]
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mov eax, cr0
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@ -182,7 +233,7 @@ convert_lba_to_chs:
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ret
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cur_lba:
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dw 9
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dw 1
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sectors_per_track:
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dw 18
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heads:
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