Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!mtxinu!taniwha!paul From: paul@taniwha.UUCP (Paul Campbell) Newsgroups: comp.sys.mac Subject: Re: Hey Apple Mac engineers, I want an answerO!! Keywords: DMA, coprocessing, improvements Message-ID: <401@taniwha.UUCP> Date: 4 Aug 89 16:43:45 GMT References: <577@studsys.mu.edu> Reply-To: paul@taniwha.UUCP (Paul Campbell) Organization: Taniwha Systems Design, Oakland Lines: 37 In article <577@studsys.mu.edu> stevej@studsys.mu.edu (jovanovic) writes: >impede design progress, don't you. Steve Jobs has got it right. 25MHz >processor, 25MHz NuBus. Let's settle this once and for all the NextBus is a 12.5MHz NuBus that bursts two words/cycle - this means that (for a 68k family motherboard) you have to have your graphics engine at least partly on the motherboard in order to generate write bursts (68030 chips only generate burst reads), also many graphics ops involve read-modify-write cycles which NuBus doesn't burst at all, you would have to burst in a chunk RMW them then burst them out when you are done which means you can't overlap memory ops with graphics ALU ops. If you can't burst then you are stuck with a minimum of 2 12.5MHz cycles per video access (or 160nS - about the right amount for DRAM cycle time anyway). Apart from this misconception (not helped by Next's marketting :-) the NextBus is really a great piece of work, it is not terminated, except by the source driving impedance (ie it bounces exactly once and then is absorbed back where it came from within an easily determined time which can be taken into account and ignored due to having a synchronous and fixed length bus) this really saves on power and lets you drive the bus with CMOS right out of your gate-array rather than through expensive power-hungry drivers. Clock distribution is different of course (it has to be very clean) and there are (25MHz and 12.5MHz) two of them letting one do a faster state machine on a card without having to fall all over the place recovering 20MHz from 10MHz etc With my software hat on I like the fact that every card has an single known location in which to check for a pending interrupt and to clear it. Paul -- Paul Campbell UUCP: ..!mtxinu!taniwha!paul AppleLink: D3213 "Free Market": n. (colloq.) a primitive fertility goddess worshipped by an obscure cult in the late 20th C. It's chief priest 'Dow Jones' was eventually lynched by an enraged populace during an economic downturn (early 21st C).