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Enable the A20 Address Line

hard4/8

Load and Execute a Second-Stage Bootloader Get Memory Map via BIOS INT 0x15 E820

Language

Compiler

2.0s Limit 256MB

Submit and pass all tests to enable saving to GitHub.

Implementation

Implement A20 line enabling in boot sector code.

Requirements:

Try BIOS method first (INT 0x15, AX=0x2401)
Implement keyboard controller method (port 0x64/0x60)
Implement fast A20 method (port 0x92)
Write an A20 test that confirms wrap-around vs. no wrap-around
Print which method succeeded
Handle failures gracefully with error messages
Show memory before and after A20 enable
Include all three methods with comments

Test:

Show A20 test before enabling
Show method selection
Show A20 enabled confirmation
Show memory no longer aliases

Why This Matters

Linux's own arch/x86/boot/a20.c tries the BIOS INT 0x15/AX=0x2401 call, then the 8042 keyboard controller, then fast A20 port 0x92 in exactly the fallback order you implement here, because no single method is guaranteed to work across all real chipsets. Forgetting to enable A20 before entering protected mode is a classic bug where memory writes above 1MB silently alias back down, corrupting kernel data in ways that are brutal to debug.

Hints

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Validation Suite

main.c

Workspace 1.0

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