Enable the A20 Address Line

Enabling the A20 address line

The A20 gate controls CPU address bit 20. With A20 off, accesses to 0x100000 alias to 0x00000, breaking loaders that place buffers in extended memory. For example, wrapping causes a stack at 0x90000 to corrupt the IVT when addresses roll over the 1 MB boundary.

Common enable methods

1. BIOS INT 0x15, AX=0x2401: Ask firmware to enable A20 (easiest in boot sectors)
2. Keyboard controller: Write 0xD1 then 0xDF to port 0x64/0x60 (slow, legacy)
3. Fast A20 port 0x92: Set bit 2 on many chipsets

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• Test with a wrap test: store different values at 0x00000 and 0x100000 and compare
• Protected-mode kernels assume A20 is on before touching memory above 1 MB
• QEMU usually enables A20 via the fast port, but real hardware may not

Order relative to PM

Enable A20 before switching to protected mode if the kernel or stage-two loader sits above 1 MB. Some tutorials test wrapping by writing to physical 0x000000 and 0x100000 and comparing; in QEMU the fast port 0x92 often already has bit 2 set. Document which method you used because keyboard controller sequencing can hang if the controller is not ready.

Why for this exercise

You will enable A20 and verify memory no longer wraps at the megabyte boundary. This exercise requires using at least one enable method and a simple test that fails if wrapping persists.