Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!apple!altos!vsi1!zorch!amiga0!mykes From: mykes@amiga0.SF-Bay.ORG (Mike Schwartz) Newsgroups: comp.sys.amiga.advocacy Subject: Re: Blitter vs. 040 (was: Computer Architecture question Message-ID: Date: 14 May 91 00:18:48 GMT References: <3310.tnews@templar.actrix.gen.nz> Organization: Amiga makes it possible Lines: 54 In article melling@cs.psu.edu (Michael D Mellinger) writes: > >In article mykes@amiga0.SF-Bay.ORG (Mike Schwartz) writes: > > The blitter provides parallel computing power. You can decide whether to > use the 68040's cycles to render graphics or to do heavy calcuations, whichever > the application requires. Another way to describe how this works: > > The blitter has a shared bus with the CPU. The CPU has 3 buses (at least on > an A2500) - CHIP (shared with blitter), ZORRO (slots), and FAST memory bus. > While the Blitter blits, the CPU can be accessing either of the other 2 buses > to its heart's content. > >How fast are the Amiga buses? If you use the 040 to render graphics, >it must access the chip RAM correct? So there will be bus contention? >Wouldn't it be better to let the 68040 just have the bus? How much >memory does video RAM take up in the 680x400(4096 colors) graphics >mode or a better graphics mode. When you fill up chip RAM you have a >problem. Do all the current Amiga's ship with the 2MB RAM chip set? > >-Mike Again, if you use the 040 to do graphics instead of a blitter, you lose the ability to do things in PARALLEL with the CPU. The 040 has a limited number of Cycles (yes gobs of them), and using them for rendering ovbiously gives you less of them for doing what work your applications do (or your multitasking OS...). You really should go and get the Amiga Hardware Reference Manual (Addison-Wesley publishing Company, blue cover) and educate yourself a little more... The custom chips in the Amiga can and often do cause the CPU to wait when it accessis CHIP RAM. If the blitter is running in NASTY mode, for example, it will steal every possible cycle from the CPU (on the CHIP BUS only). If you are using some of the higher resolution graphics modes (like 640x400x16 colors), the display hardware steals 100% of the CPU (and blitter) cycles except for horizontal and vertical retrace. The Amiga workbench is 640x200/400 x 2 planes, because it steals no cycles in this mode. Your question about graphics modes shows that you haven't learned much at all about the Amiga (RTFM, please). To get 640x400x4096 colors, you can use a single bitplane of 32,000 bytes, but your graphics mode is seriously limited (you have to do EVERYTHING with the copper). In HAM mode, you can't go 680 horizontal, but you can open 10 (96K each) 4096 (6 plane) 320x400 screens in 1Meg of CHIP RAM. The Amiga does support overscan, so you can get hirhger resolutions (I'm using 704x484). The Amiga 500 doesn't even ship with 1Meg of RAM (the 500P does), and it is fully able to run the entire Amiga OS (multitasking and all), and virtually all applications for the Amiga (except the very few that require 1+ memory). -- **************************************************** * I want games that look like Shadow of the Beast * * but play like Leisure Suit Larry. * ****************************************************