Load and Execute a Second-Stage Bootloader

Two-stage boot design

Stage one fits in 512 bytes: enable A20, load sectors, maybe print status. Stage two holds filesystem logic, ELF parsing, or mode switches. For example, stage one reads sectors 2-17 to 0x7E00 and executes jmp 0x0000:0x7E00 after verifying a magic constant at the buffer start.

Responsibilities split

c
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Pass boot drive in a register or fixed memory cell stage two reads
Leave BSS/stack above both images to avoid overwrite
Stage two may switch to 32-bit code; stage one stays 16-bit real mode
Log strings at each step simplify QEMU debugging

Linker layout concerns

Stage two can be assembled with ORG 0x7E00 so labels match the runtime address. If you use a higher load address, fix up relocations or use position-independent code for data references. Store a magic value at the front of stage two so stage one can detect a failed read before jumping into garbage. Serial debug output in stage one costs bytes but saves hours when LBA math is wrong.

Why for this exercise

You will load and jump to a second-stage image stored contiguously on disk. This exercise requires coordinating load address, entry jump, and register conventions between stages.

Create a two-stage bootloader where Stage 1 loads Stage 2.

Requirements:

Stage 1 (boot sector): print message, load Stage 2 from disk sectors 2-5
Stage 2: print confirmation message, show it is running
Load Stage 2 to memory address 0x7E00
Use proper far jump to enter Stage 2
Stage 2 should reinitialize segment registers
Stage 2 should set up its own stack
Both stages must have boot signatures where needed
Include clear comments distinguishing Stage 1 and Stage 2
Print drive number passed by BIOS

Test:

Show Stage 1 running
Show Stage 2 loaded
Show Stage 2 confirmation message
Show drive number

Two-stage boot design

Responsibilities split

c
Loading…

Pass boot drive in a register or fixed memory cell stage two reads
Leave BSS/stack above both images to avoid overwrite
Stage two may switch to 32-bit code; stage one stays 16-bit real mode
Log strings at each step simplify QEMU debugging

Linker layout concerns

Why for this exercise

You will load and jump to a second-stage image stored contiguously on disk. This exercise requires coordinating load address, entry jump, and register conventions between stages.

Create a two-stage bootloader where Stage 1 loads Stage 2.

Requirements:

Stage 1 (boot sector): print message, load Stage 2 from disk sectors 2-5
Stage 2: print confirmation message, show it is running
Load Stage 2 to memory address 0x7E00
Use proper far jump to enter Stage 2
Stage 2 should reinitialize segment registers
Stage 2 should set up its own stack
Both stages must have boot signatures where needed
Include clear comments distinguishing Stage 1 and Stage 2
Print drive number passed by BIOS

Test:

Show Stage 1 running
Show Stage 2 loaded
Show Stage 2 confirmation message
Show drive number

Topic Overview

Two-stage boot design

Responsibilities split

Linker layout concerns

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Load and Execute a Second-Stage Bootloader

Topic Overview

Two-stage boot design

Responsibilities split

Linker layout concerns

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source