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Band Map Question

To: <ct-user@contesting.com>
Subject: Band Map Question
From: n6tv@VNET.IBM.COM (Robert A. Wilson)
Date: Thu, 14 Jul 94 09:52:48 PDT
Ref:  Your note of Wed, 13 Jul 94 23:40:14 EDT (attached)

Hi Walt,

I just had a note from KJ6TC explaining this.  The original IBM PC used
8-bit cards that could be used for IRQ 0-7 only.  When the AT-bus was
introuced, they "cascaded" another 8-bit interrupt controller to handle
IRQs 8-15.  This controller was attached to the old controllers's IRQ2

In order not to obsolete any 8-bit boards that had to use IRQ2, they
set aside IRQ 9 on the 2nd controller.  You can specify IRQ 9 on your
software driver and it will work with an 8-bit card that is jumpered
for IRQ 2.  On an AT-bus, IRQ 9 is the way to get to IRQ 2.

Bottom line, you can have an 8-bit card configured for IRQ 2, or
a 16-bit card configured for IRQ 9, but not both.  Some older VGA
cards were hard wired to use IRQ 9, but apparently that is no longer

Bob, N6TV
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To: <ct-user@contesting.com>
Date:     Wed, 13 Jul 94 23:40:14 EDT
To: ct-user@sttng.mlo.dec.com
Subject:  Re:  Band Map Question
Message-Id:  <9407132340.aa14794@COR5.PICA.ARMY.MIL>
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Reply-To: ct-user@sttng.mlo.dec.com

Hi Bob. Thanx for the clarification. What is a cascade interrupt. I have
always been puzzled by this. For some programs setting COM3 to IRQ2 does
the same thing - makes my device communicate with the computer. I have
never fully understood why this is so and why specifying one or the
other interrupt works some times and not other.
To beat an old phrase to death: "Curious minds want to know."
73 de Walt - K2WK
                     73 de Walt Kornienko - K2WK (FRC)
waltk@pica.army.mil  K2WK > W2JT or K2WK@N2ERH.NJ.USA.NOAM

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