From: hyc@hanauma.jpl.nasa.gov (Howard Chu)
Subject: Re: WHAT'S GOING ON ?
Organization: SAR Systems Development & Processing, JPL
Date: Sun, 8 Nov 1992 00:00:13 GMT

In article <1deq7qINNp0d@phakt.usc.edu> baffoni@phakt.usc.edu (Juxtaposer) writes:
>	I have to disagree.  A PDS is the only way to go for CPU upgrades or
>other products that require a 0waitstate operation within the machine.  What 
>good would it be to have a 50MHz '040 if _every_ access to main memory incurs
>one or more waits?  I agree that VME is a good bus architecture - 1WS incurred
>very fast, and with the larger bus (6U) it is accesses about everything the CPU
>can.  The point is that not everything is appropriate for an external bus like
>that.  True, you don't want to put your graphics adapter in a PDS (unless, like
>the Falcon it is the only one you have :( ) but you don't want the '040 in the
>VME either.

I certainly wouldn't put a processor upgrade out on the VMEbus. Nor would I
put it in a PDS; I'd pull out the old processor and put the new one in the
vacated socket... The PDS is completely useless for this application anyway
(if we're talking about the Falcon) since it only provides a 68000-style
A24/D16 interface. (Of course, no matter what you had, it would be pretty
tough to upgrade a <=68030 system to 68040 since the '040 doesn't support
an asynchronous bus...)
>
>>VMEbus is not very different from what you'd have to cram into a processor-
>>direct slot. The difference in bus control/arbitration additions is negligible
>>now since you can get single-chip VME bus interface controllers and such.
>
>	One waitstate (80ns or less) doesn't sound like much...until you try
>to access memory.  It is questionable whether a VME based DSP/sound sytem would
>do well, besides lacking the ability to bus master (on the Atari, not the spec)
>so that sound DMA to the DSP or other chips (like DAC/ADC) would be impossible.
>I am not saying that you couldn't transfer to those chips very quickly - 
>bandwidth to the 16-bit/8chnl ADC/DAC would only be max 800k/s, but that is not
>insignificant even on the TTbus, let alone the MegaSTe bus.  However, a PDS 
>based DSP/sound system would definately work (if the PDS supported bus 
>mastering, all data/address/interrupts ).  Too bad the MegaSTe and the TT
>don't have a (decent) PDS.

Like you say, nothing rules out putting a decent DMA engine on a VME device.
Also, you seem to be overlooking the possibility of block-mode transfers
when you keep referring to that 1 wait-state.
>
>>Of course, you don't need to put *everything* on the VMEbus. It still makes
>>sense for system RAM and ROM to live on a private bus, just like the way STs
>>and TTs already work. But you could stick myriad enhancements on the VME -
>>graphics boards, custom coprocessors, ethernet network controllers, serial
>>port multiplexors, etc. etc. etc... VME is definitely the best way to go.
>
>	Agreed.  For those (relatively) low bandwidth applications.
>
>>  -- Howard Chu @ Jet Propulsion Laboratory, Pasadena, CA
>>
>>All true wisdom is conveyed in one-line witticisms.
>
>-Mike


-- 
  -- Howard Chu @ Jet Propulsion Laboratory, Pasadena, CA

All true wisdom is conveyed in one-line witticisms.