Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!dimacs.rutgers.edu!rutgers!cbmvax!daveh From: daveh@cbmvax.commodore.com (Dave Haynie) Newsgroups: comp.sys.amiga.advocacy Subject: Re: Blitter vs. 040 (was: Computer Architecture question Message-ID: <21610@cbmvax.commodore.com> Date: 15 May 91 18:05:16 GMT References: <1991May10.180908.29565@convex.com> <6o6Hk!=@cs.psu.edu> <48817@ut-emx.uucp> <48829@ut-emx.uucp> Reply-To: daveh@cbmvax.commodore.com (Dave Haynie) Organization: Commodore, West Chester, PA Lines: 50 In article melling@cs.psu.edu (Michael D Mellinger) writes: >I don't think the NeXTstation has double-buffering. The NeXTs very likely use VRAM for their display. This has the advantage of spilling off alot of video fetch overhead from that memory, so the CPU can get at it reasonably fast. On the downside, it makes the display memory a very specific thing, architecturally constrained to a small chunk of RAM. Double buffering can be designed in, but it's nearly impossible to get the kind of N-buffering that can be done on an Amiga. >Someone mentioned that the reason graphics on the Mac are slow is because it >only has a 10MHz NuBus. The new 040 Macs will have a 25MHz bus. That's the reason for most Mac graphics being slow. NuBus uses a 10MHz clock, and a minimal NuBus cycle is two clocks long, so the fastest access to NuBus based resources is 200ns. That's really not all that bad, most DRAM systems on 25MHz 68030s run in 200ns (effectively faster with burst, though). The real problem is one of synchronization. A 68030 cycle must "synch up" to the 10MHz NuBus clock, then "synch down" again to complete the cycle. So in reality, the 68030 may take two or three extra cycles spent synchronizing to the NuBus. And yeah, according to the press I've seen, Apple is leading an effort to support some new, faster, and upward compatible transfer modes on NuBus. This is different that NeXT, which uses NuBus protocols but maintains no compatibility. I haven't seen any technical specs on what Apple is doing with this faster NuBus, but it should be interesting. >NeXT claims to have a 25MHz NuBus, however a few days ago some in comp.arch >mention that it is really only 12.5MHz and the 030(and 040?) burst mode was >added into that. I had previously read that the NeXT bus was 12.5MHz. The main advantage of a 12.5MHz NuBus to a 25MHz 68030/40 computer is that there should be no need to waste time in synchronization delays, since both CPU and bus clock can be based on the same oscillator. Apple may be doing the same thing on their 20MHz IIsi and 20/40MHz IIfx. In any case, you can't run 68030/40 burst cycles on NuBus. NuBus burst cycles are what you call "block transfers". They start on an even multi-word boundary, and require that entire multi-word to be transfereed. So you can burst a quad-word (NuBus word == 68030 longword, of course) in five NuBus cycles, or a 16-word block in 17 NuBus cycles. But the word must be aligned. 68030 bursts are always quad-word transfers, but they start on any word boundary and wrap around to the beginning if necessary. Strangely enough, the Amiga's Zorro III burst mode does support conversion to 68030 or 68040 burst cycles. -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy "That's me in the corner, that's me in the spotlight" -R.E.M.