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A comment on The 640K memory limit of MS-DOS. Parent comment

Re:The 640K memory limit of MS-DOS by tzs on Saturday June 09, 2018 @06:14PM (#17275318)
> But Extenders like DOS/16M[0] did this transparently fast

Transparent, yes. Fast, no. To get back to real mode they had to reset the CPU.

There were a couple ways to do this. There was the triple fault technique that phicoh mentioned. I'm not sure how portable that was. A triple fault shuts down the processor. Whether or not that reboots or just sits there halted would depend on the motherboard design.

The official way on the PC AT was to ask the keyboard controller to do it. On the PC AT, the keyboard controller was connected to the reset signal, and had a command to tell it to reset the computer. (The keyboard controller was also used to control the A20 line, to workaround another 286 annoyance).

AT compatibles generally included that behavior in their keyboard controllers for compatibility.

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Re:The 640K memory limit of MS-DOS by DrScump on Saturday June 09, 2018 @06:24PM (#17275358) 

  Transparent, yes. Fast, no. To get back to real mode they had to reset the CPU
But from the user perspective (remember when the user perspective used to matter?), it just worked and gave them power and functionality they'd never had before.

A modern adware-laden web page on gigabit-speed Internet is dog slow in comparison.

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