Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!killer!ames!amdahl!mat From: mat@amdahl.uts.amdahl.com (Mike Taylor) Newsgroups: comp.arch Subject: Re: Mainframe channels vs. DMA vs. NeXT Message-ID: Date: 28 Oct 88 04:42:58 GMT References: <10778@reed.UUCP> Organization: Amdahl Corporation, Sunnyvale CA Lines: 45 In article <10778@reed.UUCP>, mdr@reed.UUCP (Mike Rutenberg) writes: > > Mainframe channels?? > ================== > I equate channels with special IO coprocessors that can request things > from memory without "main" processor interaction. I believe the > IBM 3033 uses 801s as channel processors. Actually, the 3033 channel director is a recycled 370/158 processor as far as I know. The 3090 uses an 801, though. How is this all set up? > Does the system have multiported memory so that the 370 can get at > it from one end while the channel controllers get at it from the > other? Yes, it does. Since the 801 is a general purpose processor, that would > suggest that the channel controller will have customized programs > for the different peripherals that it feeds. > Only in a certain sense. The channel program is device dependent and consists of a common set of commands (such as Transfer in Channel, TIC, which is a conditional branch) and device-dependent commands (such as Seek). Each channel can concurrently execute 256 channel programs, each of which is controlling a device. Channel programs are generally executed interpretively by the channel controller. The channel controller also handles such things as getting programs started, requesting CPU interrupts and performing error recovery, statistics and measurement, path selection ( a device may be accessible through several device controllers and different paths may be used and selected by the channel and device during an operation) queuing of requests. A channel processor is typically as large as a main CPU, although most of the real estate is devoted to multiplexing functions and replicated path logic, not the controller function. The above is an oversimplification. -- Mike Taylor ...!{hplabs,amdcad,sun}!amdahl!mat [ This may not reflect my opinion, let alone anyone else's. ]