T
Tom Del Rosso
- Jan 1, 1970
- 0
John Larkin said:I loved the 286 ("brain damaged CPU" to quote Bill Gates) trick to get
out of protected mode back into real mode. The CPU designers forgot to
allow an instruction to do this, so somebody patented the idea of
sending a command to the keyboard controller to reset the CPU. I think
early versions of Windoze actually used this technique.
Well, sure they used it. IBM designed the AT to do that, with the
keyboard controller connected to drive CPU reset, and programmed to
assert reset on I/O command, so of course Windows used it. They were
indended to.
I don't think anybody forgot to include an instruction. They just
assumed that if any system used virtual memory mode then that system
would only have to switch modes one way, during initialization. They
just didn't figure on running legacy 8086 code that was aware of
segments but assumed they were consecutive in real memory. Maybe it
would have worked in protected mode running 8080 code that wasn't even
aware of segments.