On Jul 23, 1:34, <rogersda(a)cox.net> wrote: It's true that the I/O page always appears *to software* at *logical* addresses 160000-177777. This is fine for the processor, which has logic to deal with it. Nevertheless, the *hardware* addresses which appear on the bus need to be 160000-177777, 760000-777777, or 17760000-17777777 depending on whether the system uses 16, 18, or 22-bit addressing. To make life easier, DEC provide a "Bus Bank Select 7" signal, which most I/O devices use to detect access to the I/O page. What Jerome is referring to, however, is the DMA operation. An RXV21 or RLV11 (and some other devices) can only perform data transfers to 18-bit addresses. Data is transferred only by DMA; you can't transfer data from the device controller, only commands and status. Most DEC operating systems expect data transfers to be able to load/save from data buffers at arbitrary (physical) addresses; therefore drivers in some operating systems have the option to use a bounce buffer mapped to low memory, for 18-bit devices. They therefore transfer the data in two stages: under program control between the bounce buffer and the required address, and letting the DMA transfer between bounce buffer and actual device. -- Pete Peter Turnbull Network Manager University of York