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POLinux x86 bootloader
 

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  1. POLinux x86 bootloader
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    <p>I am trying to build a simple x86 Linux bootloader in nasm.</p> <p>The Linux bzImage is stored on disk partition sda1 starting from the first sector.</p> <p>I read the real mode code from the bzImage (15 sectors) into memory starting from 0x7E00. However when i jump into the code it just hangs, nothing happens.</p> <p>I have created code for the master boot record on sda. I's probably best if i just attach the whole thing. I would like to know why it just hangs after the far jump instruction.</p> <pre><code>[BITS 16] %define BOOTSEG 0x7C0 %define BOOTADDR (BOOTSEG * 0x10) %define HDRSEG (BOOTSEG + 0x20) %define HDRADDR (HDRSEG * 0x10) %define KERNSEG (HDRSEG + 0x20) [ORG BOOTADDR] entry_section: cli jmp start start: ; Clear segments xor ax, ax mov ds, ax mov es, ax mov gs, ax mov fs, ax mov ss, ax mov sp, BOOTADDR ; Lots of room for it to grow down from here ; Read all 15 sectors of realmode code in the kernel mov ah, 0x42 mov si, dap mov dl, 0x80 int 0x13 jc bad ; Test magic number of kernel header mov eax, dword [HDRADDR + 0x202] cmp eax, 'HdrS' jne bad ; Test jump instruction is there mov al, byte [KERNSEG * 16] cmp al, 0xEB jne bad xor ax, ax ; Kernel entry code will set ds = ax xor bx, bx ; Will also set ss = dx jmp dword KERNSEG:0 ; Simple function to report an error and halt bad: mov al, "B" call putc jmp halt ; Param: char in al putc: mov ah, 0X0E mov bh, 0x0F xor bl, bl int 0x10 ret halt: hlt jmp halt ; Begin data section dap: ; Disk address packet db 0x10 ; Size of dap in bytes db 0 ; Unused dw 15 ; Number of sectors to read dw 0 ; Offset where to place data dw HDRSEG ; Segment where to place data dd 0x3F ; Low order of start addres in sectors dd 0 ; High order of start address in sectors ; End data section times 446-($-$$) db 0 ; Padding to make the MBR 512 bytes ; Hardcoded partition entries part_boot: dw 0x0180, 0x0001, 0xFE83, 0x3c3f, 0x003F, 0x0000, 0xF3BE, 0x000E part_sda2: dw 0x0000, 0x3D01, 0xFE83, 0xFFFF, 0xF3FD, 0x000E, 0x5AF0, 0x01B3 part_sda3: dw 0xFE00, 0xFFFF, 0xFE83, 0xFFFF, 0x4EED, 0x01C2, 0xb113, 0x001D part_sda4: dw 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000 dw 0xAA55 ; Magic number at relative address 510 mbrend: ; Relative address 512 </code></pre>
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    1. USSteven Penny
    1. USDruesukker
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    1. CODid you study the implementations of GRUB and LILO?
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      1. USBasile Starynkevitch
    2. COI have looked at the implementation of LILO and it seems like they jump into the Linux code the same way. It's kind of hard to read their code though.
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      1. USDruesukker
    3. CODid you use a debugger to step through the code and see the reason for the hang?
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      1. USJester
 

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