8-bit IDE

From: Richard Erlacher <edick_at_idcomm.com>
Date: Mon Apr 17 23:56:37 2000

please see embedded comments below.


----- Original Message -----
From: allisonp <allisonp_at_world.std.com>
To: <classiccmp_at_classiccmp.org>
Sent: Monday, April 17, 2000 7:07 PM
Subject: Re: 8-bit IDE

> I'd agree from my experience. I'd add the 2.5" drive are less likely too.
> >> willing to believe that the interface will not assert IOCS16- if the
> drive's
> >> interface is programmed to operate in 8-bit mode. I would expect that
> I
> >
> >The term 'interface' is ambiguous here. IOCS16/ is asserted by the drive,
> >not by the bus adapter card.
What it is is an I/O channel controlled by the host. The host provides an
address and a select, in addition to which it provides a read or write
strobe, and the interface, which is implemented at the target device,
interprets the signals on the channel and responds accordingly. The signal
name is inherited from the 1003-WAH, since it had to assert that signal to
force the ISA bus to transfer a whole word at once. But for that signal,
the AT bus would have steered the bytes separately from two byte transfers.
> True! I built the interface on the assumption it would be asserted to
> save logic.
> >The bus adapter simply buffers the signal (most of the time) and sends it
> >to the ISA slot. In particular, note that the address decoder logic on
> >the bus adapter is not what asserts IOCS16/
... and that's EXACTLY why I'd expect the thing to decline to assert IOCS16-
when it's conditioned for 8-bit operation as the standard suggests. I am
beginning to see that there's a lot of doubt that drive manufacturers
actually implemented this mode, and I certainly see the logic behind that
> Correct, those assert CS lines.
> >AFAIK, a standard IDE drive only asserts IOCS16/ for accesses to the data
> >register. Not for accesses to any of the control/status registers. Which
> >means all of those are seen as 8-bit registers.
> Yep. That mas a 8<>16 adaptor simpler as it's only reads and writes to
> address that need folding.
> >> MSI-implemented IDE channels don't have the means to operate in that
> mode.
> >
> >See above. The bus adapter should do the right thing provided the drive
> >asserts IOCS16/ at the right time. The TTL bus adapter cards are just a
> >couple of buffers and an address decoder (normally a PAL, but it doesn't
> >have to be).
> Actually this is only important to obscure hardware as the software knows
> a data register read/write is 16bit (word) and that all other registers
> byte.
> I think the idea was that IOCS16 was to notify byte oriented logic to
> the 16bit transfer (ISA-8).
> Allison
> >
> >-tony
> >
Received on Mon Apr 17 2000 - 23:56:37 BST

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