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Master the fundamental concepts of boot sector development through this focused micro-challenge.
You have read the whole brief, and the concepts above stay free on every task. Writing and running the code needs a plan.
Three hints are available for this task, revealed one at a time inside the code workspace so you can struggle productively before seeing them.
Every task includes starter code, theory, and hidden tests so you can implement and verify locally in the browser.
How it worksThe 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.
0xD1 then 0xDF to port 0x64/0x60 (slow, legacy)0x92: Set bit 2 on many chipsetsnasmLoading…
0x00000 and 0x100000 and compareEnable 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.
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.
On the 8086, addresses wrap at 1 MB. To stay compatible, the PC kept address line 20 gated off at boot, so code must open the A20 gate before using memory above 1 MB. Simulate the gate and the ways a boot sector controls it.
The gate starts off, and the last value read from port 0x92 starts at 0. Commands, one per line, until the end of input:
| Command | What happens | Output |
|---|---|---|
read92 V | port 0x92 (System Control Port A) reads V (hex); A20 follows its bit 1 | port 0x92 = 0x02, A20 on |
fast | "fast A20": set bit 1 of the last value read and clear bit 0, because writing 1 to bit 0 resets the CPU; skip the write if bit 1 is already set | fast: out 0x92, 0x02 or fast: A20 already enabled, no write |
kbc | the 8042 keyboard controller method: command 0xD1 (write output port) to port 0x64, then 0xDF (bit 1 set) to port 0x60 | kbc: out 0x64, 0xD1 and kbc: out 0x60, 0xDF |
disable | clear bits 0 and 1 of the last value read and write it back; A20 off | disable: out 0x92, 0x00 |
addr SEG OFF | translate a real-mode address (hex): SEG * 16 + OFF, masked to 20 bits while the gate is off | FFFF:0010 -> 0x00000 |
test | the standard A20 check: do 0000:0500 and FFFF:0510 reach the same byte? | test: 0000:0500 -> 0x00500, FFFF:0510 -> 0x00500: A20 OFF (aliased) or ...: A20 ON (distinct) |
fast and kbc both leave the gate on. Port values are printed as 2 uppercase hex digits, and addresses as SEG:OFF in 4 digits each plus a physical address of at least 5 digits.
Commands as above, one per line.
One or two lines per command, as in the table.