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
line.
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
common.
73,
Bob, N6TV
----------------------------- Note follows ------------------------------
Received: from inet-gw-1.pa.dec.com by vnet.IBM.COM (IBM VM SMTP V2R2) with TCP;
Wed, 13 Jul 94 23:55:07 EDT
Received: from sttng.mlo.dec.com by inet-gw-1.pa.dec.com (5.65/27May94)
id AA08839; Wed, 13 Jul 94 20:53:49 -0700
Received: by sttng.mlo.dec.com; (5.65/1.1.8.2/06Apr94-0505PM)
id AA28973; Wed, 13 Jul 1994 23:50:45 -0400
Received: from inet-gw-1.pa.dec.com by sttng.mlo.dec.com;
(5.65/1.1.8.2/06Apr94-0505PM)
id AA28967; Wed, 13 Jul 1994 23:50:42 -0400
Received: from cor5.pica.army.mil by inet-gw-1.pa.dec.com (5.65/27May94)
id AA08630; Wed, 13 Jul 94 20:47:44 -0700
To: <ct-user@contesting.com>
Date: Wed, 13 Jul 94 23:40:14 EDT
From: Waltk@PICA.ARMY.MIL
To: ct-user@sttng.mlo.dec.com
Subject: Re: Band Map Question
Message-Id: <9407132340.aa14794@COR5.PICA.ARMY.MIL>
Sender: owner-ct-user@sttng.mlo.dec.com
Precedence: bulk
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
____________________________________________________________________________
|