> In message <Pine.LNX.4.05.9811252223010.11212-100000@tahallah.demon.co.uk>,
> Ale
> x Buell writes:
> +-----
> | On Wed, 25 Nov 1998, Brandon S. Allbery KF8NH wrote:
> | > Linuxers, marred only by small issues such as (IIRC) ATI and S3 video
> | > cards vs. COM4. And it's right smack in the middle of the list of
> |
> | Ah yeah, the infamous COM4 bug with S3 chipsets. Seems some fuckwit used
> | the usual COM4 I/O port as one of its I/O registers. Ahh, that particular
> | chipset designer should have been taken out and made to use Microsoft
> | products in eternity (with a Windows 95 Beta!).
> +--->8
>
> Not exactly.
>
> The ATI and S3 chipsets started out as enhancements of IBM's 8514/A video
> card. But the 8514/A was designed for Microchannel, which addresses the
> entire I/O space. So the 8514/A used several registers with addresses
> 0xX2e8, for several values of X.
>
> The ISA bus only has 10 I/O address lines, so I/O addresses above 0x03FF
> effectively wrap around. The 8514/A ports ended up aliased to 0x02e8....
nope! the ISA bus has _16_ I/O address lines. but there are some (broken!)
serial cards around which only decode 10 address bits and thus cause
address aliases/ghosts and collisions...
> Arguably the bad design decision was to base an ISA card on the 8514/A. It
> couldn't be IBM's decision because (a) there was no wraparound on
> Microchannel and (b) Microchannel is perfectly capable of sharing IRQs, so
> COM4 could be on IRQ3 as originally documented without conflicting with COM2
> (and (c) they never intended 8514/A for use on ISA systems).
it _is_ IBM who made this broken decission! check the schematics in
IBM PC/XT technical handbook for their serial card (`async. communication adapt.'):
they're using a single 74LS30 (8port NAND) to decode AEN and A3...A9
adresses -- A10 to A15 aren't touched at all:(
Harald
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