Aunc.1647 net.unix-wizards utzoo!decvax!duke!unc!lynn Wed Jan 6 11:16:22 1982 DH-11 on a VAX 11/780 The DEC DH11 only partly works on a VAX 11/780. It cannot do DMA over a buffered data path, because it is a byte at a time DMA device. When it requests the seventh byte of the eight in the buffered data path, the BDP cleverly issues the prefetch, clobbering the eighth byte before the DH can see it. (Due to a second piece of braindamage, the BDP only looks at bits 3-18 of the Unibus address when it is empty or prefetching, so it can't tell if you are doing non-consecutive DMA requests.) The obvious "fix" is to run the DH over the direct data path, but this puts a very heavy load on the Unibus because the direct data path ties up the Unibus until the transaction is complete, and VAX 11/780 memory is actually rather slow when used this way. If you only use your Unibus for terminals and printers you can probably get away with it, but tapes and disks may have data late problems with a busy DH on the bus. The ABLE and Emulex multiplexers are do not suffer from these problems, and are cheaper anyway if you are buying new stuff. Lynn TenEyck unc!lynn (919) 966-5045 ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.