🚀 go-pugleaf

> Is there anything else to know about port 20H? Can anyone direct me
> (exact pointers please) to find a little detail about this and related 
> subjects.

Page 2-23 in the IBM PC Technical Reference Manual shows an I/O
address map.  This map indicates that I/O ports 20h and 21h are
assigned to the 8259A Programmable Interrupt Controller Chip.
Since this part is made by Intel, the best way to get information
on it is through Intel data sheets.

In the Intel 8259A data sheet, a full explanation is given of the
different modes the PIC can be operated in.  The least significant
bit of the I/O address is directly fed to the A0 line on the 8259A,
i.e. I/O port 20h corresponds to A0=0; 21h to A0=1.  The A0
line, in combination with some of the data lines, is used to
determine if an Initialization Command Word (ICW), or an Operation
Command Word (OCW) is used.

Writing a 20h to I/O port 20h corresponds to a non-specific EOI
command for interrupt level 0.  For more information, refer to
the Intel 8259A data sheet.  Your local Intel representative should
be able to get you a copy.  If not, try contacting the Intel Literature
Department, 3065 Bowers Avenue, Santa Clara,  CA  95051,
(800) 538-1876.

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