a20-gate

Your mission

Build a real-mode address from a segment and an offset, then apply the gate.

today's brief

The IBM PC AT, 1984. The 286 could address more memory than the 8088 it replaced, so programs that counted on addresses wrapping around at one megabyte broke the moment they ran on it. IBM answered with a wire. Hold address line 20 low through the keyboard controller and the old wrap comes back, and that gate outlived the controller by decades. Compute the address that reaches the bus.

See it in action

phys_address(65535, 16, 1)

Expected result

Your checklist

For your own tracking. Tests grade your code.

seg and off are both between 0 and 65535.
The address is the segment shifted left by 4 bit positions, plus the offset.
When a20 is 0 the gate holds address line 20 low, so only the bottom 20 bits reach the bus.
When a20 is 1 the address reaches the bus untouched. Return the address on the bus.

More examples

phys_address(65535, 16, 0)

Expected result

phys_address(4096, 0, 0)

Expected result

Constraints

Expected files

a20.c

Allowed functions

None. Write every helper yourself.

Back to compete

Solutions

a20-gate

Your mission

Build a real-mode address from a segment and an offset, then apply the gate.

today's brief

See it in action

phys_address(65535, 16, 1)

Expected result

Your checklist

For your own tracking. Tests grade your code.

seg and off are both between 0 and 65535.
The address is the segment shifted left by 4 bit positions, plus the offset.
When a20 is 0 the gate holds address line 20 low, so only the bottom 20 bits reach the bus.
When a20 is 1 the address reaches the bus untouched. Return the address on the bus.

More examples

phys_address(65535, 16, 0)

Expected result

phys_address(4096, 0, 0)

Expected result

Constraints

Expected files

a20.c

Allowed functions

None. Write every helper yourself.

Back to compete

Solutions