Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site peora.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!vax135!petsd!peora!jer From: jer@peora.UUCP (J. Eric Roskos) Newsgroups: net.micro.mac Subject: Re: Harmonic Benchmark - 8086 vs Mac Message-ID: <1032@peora.UUCP> Date: Fri, 7-Jun-85 09:14:04 EDT Article-I.D.: peora.1032 Posted: Fri Jun 7 09:14:04 1985 Date-Received: Sat, 8-Jun-85 03:56:44 EDT References: <1030032@acf4.UUCP> <777@rayssd.UUCP> <211@uwvax.UUCP> Organization: Perkin-Elmer SDC, Orlando, Fl. Lines: 38 >> It doesn't have to mean that at all. It probably means that the Mac is >> slower because half of it's clock cycles are given up to the video hardware. >> The effective 68000 clock rate is only 4Mhz. > >People seem to like to say "The effective 68000 clock rate is only x MHz" >where the x is replaced by some number from 3 to 7 depending on the >person's opinion of the mac. But there's really nothing wrong with saying what the effective clock rate is; it should have nothing to do with your opinion of the MAC. What it has to do with is how much of your program executes in ROM and how much in RAM. The original statement above is definitely wrong, first of all. The video "steals" cycles from the CPU only for part of the time (roughly proportional to the ratio of the time the CRT beam is writing on the screen to the sum of that time plus the time the beam is doing horizontal and vertical retraces). Nowhere near "half" the cycles are given up to the video hardware. Furthermore, this only happens when the CPU tries to access dynamic RAM while the display is accessing it, so when the CPU is executing from ROM, the delays don't occur. There's something else you have to consider, too, though, in comparing to the IBM PC (which the referenced message later did). The IBM PC has its DMA controller programmed to periodically do a dummy DMA transfer to refresh the dynamic RAM. The length of the transfer is sufficient to scan all the combinations of the [row or column, I forget which] addresses in the memory array. It does this fairly often, and takes a good bit of time. On the Mac, the video RAM accessing does this for you, so you don't have the dummy DMAs taking up time. (However, the IBM PC's refresh time takes significantly less time than the Mac's video-accessing time). -- Full-Name: J. Eric Roskos UUCP: ..!{decvax,ucbvax,ihnp4}!vax135!petsd!peora!jer US Mail: MS 795; Perkin-Elmer SDC; 2486 Sand Lake Road, Orlando, FL 32809-7642 "Zl FB vf n xvyyre junyr."