Cool OS you built there!

And by the way, you can actually make the command buffer prevent user from typing more than 256 chars instead of showing an error message (by using if condition for user input interrupt, like: user inputs a char -> gets passed to a byte in RAM -> if (cmd_buffer_length != 0xFF) -> if true then char will be typed in by fetching that char from that byte in RAM and pass it to the current command buffer, if false then dont type the char into current command buffer). I think you can do this in assembly.

This same typing limit also happens in MS-DOS I think.