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Understand Bootloader Chainloading

medium8/8

Understand the Initial RAM Filesystem (initramfs)Next

Language

Compiler

2.0s Limit 256MB

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Implementation

Simulate bootloader chainloading in C.

Requirements:

Define Bootloader structure with name, entry_point, and setup function
Implement chainload_mbr() that loads sector to 0x7C00 and jumps
Implement chainload_uefi() that loads and starts an EFI application
Implement grub_chainload() showing GRUB-style chainloading
Show register setup for different bootloaders
Show drive number being passed
Show memory layout before and after chainload
Implement a simple multi-boot menu
Show chainloading to Windows bootloader
Show direct Linux kernel boot

Test:

Show menu displayed
Show MBR chainload
Show UEFI chainload
Show registers set
Show control transferred

Why This Matters

GRUB's `chainloader +1` command and rEFInd's boot-picker menu both rely on exactly this mechanism to hand control from one bootloader to Windows' BOOTMGR or another OS's loader, which is how millions of dual-boot Windows/Linux machines work today. Getting the register handoff wrong (wrong drive number in DL, wrong segment) is why chainloading a Windows partition from GRUB sometimes hangs instead of booting.

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Validation Suite

main.c

Workspace 1.0

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