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I/O port access from the command line
By Richard W.M. Jones
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This package adds commands to access machine I/O ports directly.  For
example:

  outw 0x443 0xf
  inb 0x278

Note most of the time you should never use these commands unless you
know what you are doing.

Usage
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The commands are pretty much used as you would expect.  For additional
help, use '--help' and/or read the manual page.

You will need to run the commands as root.  Strictly speaking you will
need CAP_SYS_RAWIO capability.

Implementation
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At first glance it would seem that /dev/port would be the best way to
access I/O ports.  However /dev/port under Linux is fundamentally
flawed because it only allows byte-sized access to ports.  For example
if you write a single long (4 bytes) it will turn that into 4
individual outb operations.

Instead we use iopl(2) to raise our I/O privilege level so that we can
do the correct in<size> and out<size> operation as requested.