Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!cbmvax!daveh From: daveh@cbmvax.commodore.com (Dave Haynie) Newsgroups: comp.sys.m68k Subject: Re: Bus Request & Isolation On The 68000 Message-ID: <22238@cbmvax.commodore.com> Date: 7 Jun 91 07:36:55 GMT References: Reply-To: daveh@cbmvax.commodore.com (Dave Haynie) Distribution: comp Organization: Commodore, West Chester, PA Lines: 57 In article steveh@tasman.cc.utas.edu.au (Steven Howell) writes: >The simplicity of replacing the chip at board level crossed my mind, but was >shattered when I realised I had to divide a heap of timing signals to ensure >compatibility with the SCC, Video, Sound and HD/Floppy disk Interfaces. That's true of most systems. My suggestion would be to get one of the 68000 socket upgrades available commercially. There are a number of these puppies on the market. They generally replace your 68000 with a fast 68020 or 68030. One company producing these is Computer Systems Associates. They sell these in the Amiga and maybe Macintosh markets. I don't have their address handy, but they do advertise in most Amiga magazines. >I wish to send a Bus request (-BR) signal to processor, so that it will release >the CPU, to another CPU. Namely the faster 68000 or 68020. You could do that, but... >I understand that (according to my motorola reference manuals) if i send a >(-BR), it will accept, and issue a bus grant signal (-BG), indicating the cpu >has released the bus to the next bus master requesting it.(when it finishes its >last cycle). The 68000 arbiter works like like this. Your alternate bus master asserts BR*. When it's ready to relinquish the bus, the 68000 returns BG*. As soon your alternate master has BG* and AS*, DS*, and DTACK* are negated, it can assert BGACK*, negate BR*, and proceed to drive the main 68000 bus. This mechanism doesn't let you replace any clocks. >What actually happens to CPU when this condition has been met. Does it go high >impedance, and can another processor replace all signals except clock in this >condition. Basically. >Can another CPU replace all current I/O and operate without any clashes at a >higher speeds. That all depends on what's on your 68000 bus. In general, you can't run things faster, because most systems are designed around a known clock speed. You certainly can't supply a faster clock. You may, depending on the system, be able to run a cycle in fewer clocks than the 68000 normally would (four minimum), but again, that's very system dependent. >Can i remain permanantly in this state after invoking it. You can. However, if your system has other potential bus masters, you'll block them with your speed up board if you do. Unless your system is specifically designed to accept some kind of speedup board, it's probably best to physically replace the 68000 with a socket tower , rather than logically replace the 68000 as a bus master. -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy "This is my mistake. Let me make it good." -R.E.M.