OT: A call to arms (sort of)

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sat Jul 3 22:04:47 1999

I don't think we disagree that much, but take a look at my comments embedded
in your reply quoted below.


-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
Date: Saturday, July 03, 1999 8:39 PM
Subject: Re: OT: A call to arms (sort of)

>Well, IMHO there is _if you want to call the bus ISA_.
>There's no reason not to use this type of DMA on a homebrew system.
>There's no reason not to use cards that have the same form factor and
>same connectors and ISA cards.
I've seen little reason to use DMA at all when processors generally have the
capacity to move data at the bus bandwidth with block transfer instructions.
It's not a religious issue for me to call the bus whatever seems
appropriate. ISA is the "standard" developed around the PC. The signals
are, for the most part, the obvious ones for ANY microprocessor. The
interrupts are the exception, in that they use the ancient and stupid intel
method, namely positive-going and, as you said, edge sensitive interrupts,
which preclude the more sensible approaches to interrupt management.
>But as soon as you start changing signals, you no longer have an ISA bus.
>Some ISA cards will surely fail to work. That is _all_ I am saying.
Well, pehaps we don't want ISA bus. Perhaps we could benefit from the use
of the cheap and commonly available parts, though. I never used interrupts
or DMA in ten years on the S-100 bus. It was never necessary to do so. I
don't see why you're so worried about interrupts on the few ISA boards one
might actually want to use, e.g. serial I/O and disk adapters, along, maybe,
with video boards. There aren't really any terribly interesting parallel
I/O boards, and if you want to use IEEE488 stuff you'll play hell beating
the WIndows software to help you do it.
>> Leaving off the motherboard doesn't change the BUS to something else.
>> have been systems with multiple processors on passive backplanes for the
>> for years. You don't have to change one signal. Of course, if you leave
>If you want to have active-low interrupts, DMA as I described it above,
>etc then you do have to redefine some of the signals. Maybe not in name,
>but certainly in what they do.
>Suppose you don't have a central DMA controller. What do you propose
>doing with the DRQ/DACK signals? Sure you can make them effectively
>bus request signals. But now the peripheral card has to know to generate
>the address. And no standard ISA card would do that.
Just exactly which boards do you wish to use that do that? If they don't
yet exist I'd say there are some liberties worth taking.
>> off the motherboard, i.e. the circuitry that makes it a PC, then you
>> have to use the otherwise useless 4x-color-burst crystal oscillator
>I don't know what your problem is with that oscillator. It's trivial to
>generate. That really is the least of the problems.
It's just a useless source of noise synchronized approximately with our TV
>> and you don't have to generate that inane 18... Hz interrupt and can use
>> something sensible instead, and you don't have to generate refresh
>> with one DMA channel, and you don't have to use DMA for the floppy which
>Actually, only PCs and XTs used a DMA channel for refresh. ATs and above
>have a separate counter for this.
Yes, but refresh itself doesn't need an address at all, or at least hasn't
since the 64k DRAMs came out with hidden refresh.
>> >> in general, to have DMA, first because the processors used on PC
>> >> motherboards have block transfer operations which operate at the bus
>> >> bandwidth.
>> >
>> >Why do you assume that ISA -> Intel processor? It may be something
>> >totally different, something that doesn't have efficient block transfer
>> >instructions.
>> >
>> Now you're confusing me . . . you just got through saying that the PC has
>> be there, Intel and all, or it's not an ISA bus. Perhaps you spoke (sic)
>You just said '...processors used on PC motherboards...'. Now most PC
>motherboards have an Intel, or Intel-compatible processor on them. So I
>think I was justified in thinking that you were implying an Intel
>processor here.
>> too soon? I made no such assertion! There are lots of processors which
>> have block transfer instructions which operate at the bus bandwidth.
>> the Z80 did that.
>And IIRC the Z80 block moves were ridiculously slow...
Yes, perhaps it was ridiculously slow, but it was the bus bandwidth at the
>> >I see a _lot_ wrong with the ISA signal definitions. For one thing the
>> >IRQs are edge triggered, active high, when any sane designer would make
>> >them level triggered active low (had IBM done this it would have cost
>> >them an extra couple of TTL chips on the PC motherboard. It would also
>> >have allowed the sharing of interrupts). For another thing there's no
>> >proper bus request/grant (multiple masters are almost essential IMHO).
>> >
>> You're certainly right about that! All that would have been needed is
>> they swallow their pride and use a sensible interrupt handler instead of
>> their silly 8259. In fact, they should have left all their LSI's off the
>> MB. The way their counters work, or don't, and the fact they're slow,
>> they're ripple counters so you have to read half of them twice . . .
>> get me started . . .
>Don't get me started either. I sat down with the PC (and later...)
>techrefs, and I went through the schematics. Every so often I would
>exclaim 'They did WHAT???'....
>> which used it, I'd say that's a non-issue. You don't need the schematic,
>> though,since the board you'll be using will be an IDE interface with
>For the <n>th time, the aim is to get 'open hardware'. That means (at
>least to me) available schematics. Not schematics of things that _might_
>work the same (e.g. WD1003 .vs. IDE). It means scheamtics and
>documentation for the hardware that's actually in the machine.
So, you want schematics of the disk drives as well, and the keyboard, and
the floppy drive? . . . and when you have them, how are you going to stick
your 'scope probe into that IC, and how are you going to fix it when it's
broken. It's a custom IC, after all, and they will cost 10x what a new disk
drive costs if you try to buy just one. If "OPEN" means to you that you
have access and rights to all the intellectual property contained in your
computer, you might as well give up right now. If what you want is enough
information to program and use the devices you can buy, that's quite
possible as we sit here.
>> FDC. Those (FDC's) are well characterized and all you need to know about
>> the 1003-WAH is the command set, since IDE still uses it.
>> The little IDE interface boards with 5 TTL's on them are easy enough to
>> out and understand. Data on the LSI's is easy enough to get, though you
>I've found it _very_ hard to get data on the typical ASICs that you find
>on modern PC motherboards and I/O cards. In fact I've not managed to do
>it in a lot of cases.
That's the reason, precisely, why you don't use them.
>I don't mean the 82xx chips that are on the original PC motherboards. I
>have the data sheets on those. I mean the all-in-one chip on a multi-IO
>card that does FDC, IDE, 2 serial ports, parallel port, etc.
There's no reason to use one, even if you could. The little 5-TTL circuit
works fine and just as quickly, for disk I/O. Now, since you hate the PC
MOBO so much, why would you want to use one of the PC-MOBO specific IC's?
Received on Sat Jul 03 1999 - 22:04:47 BST

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