Parse and Load an ELF Binary from Disk

Loading ELF from a boot sector

Kernels often ship as ELF binaries with program headers describing loadable segments. A bootloader reads each PT_LOAD segment from disk to its p_paddr or p_vaddr and jumps to e_entry. For example, a ET_EXEC kernel at LBA 100 might place .text at 0x100000 and entry at 0x101000.

Parse steps

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• e_phoff / e_phnum: Walk program headers
• p_filesz vs p_memsz: Zero BSS tail if memsz larger
• Stay in real mode for BIOS reads; switch modes before jumping to high addresses
• Page-align destinations if enabling paging immediately after entry

ELF32 header checks

Verify e_ident[EI_MAG] bytes and e_machine == EM_386 before trusting headers. p_paddr may be zero for some objects; fall back to p_vaddr when loading a linked kernel expecting low memory. Read sectors in chunks if your DAP cannot span the whole segment in one BIOS call. Clear the BSS portion when p_memsz > p_filesz so global variables start at zero.

Why for this exercise

You will parse ELF headers and load at least one PT_LOAD segment from disk. This exercise requires computing file offsets, physical addresses, and the entry point before transfer of control.