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Date: Tue, 23 Jul 1996 18:21:16 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199607230821.SAA24711@godzilla.zeta.org.au>
To: hardware@freebsd.com, schofiel@xs4all.nl
Subject: Re: The multiple COM ports discussion
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>3) There are two versions of the PIC, the original type and the 
>   enhanced "A" variant. The original does not allow for level/edge
>   selection, the enhanced does. The original does not allow individual
>   IRQ operation selection (ie. all interrupt lines must operate with the
>   same mode), te enhanced does allow individual mode selection.

The "A" variant was used in the original PC.  I don't think the original
version has been used in any "ISA" PC.  The "A" version doesn't allow
different modes for different lines either.

>6) Individual interrupt lines can not be programmed "off" at the PIC, only
>   in the calling device.

They can be masked.  This is equivalent to "off" for the purpose of
generating interrupts.  Masked interrupts can still be detected by reading
the Interrupt Request Register.

Bruce