PDP 11/23 help needed

On Apr 22, 16:08, Philip.Belben(a)powertech.co.uk wrote: An 11/23 only supports 18/22-bit addressing if it has the MMU chip fitted, which almost all do, though it was, strictly speaking, an option (at least, for most of the 23's life). Early KDF11-A's (Rev.A) only support 18-bit, most (Rev.C) support 22-bit addressing -- iff they have the MMU. Otherwise, they can only access 16-bits of address space. I don't recall any setting to change that, you just only get 18 bits in an 18-bit backplane. If you try to access beyond that range, you get a bus error. ISTR that the ODT still works (always 18-bit) without the MMU, though. I can't easily check as the only one I have running ATM is a 22-bit system. A number of KDF11-A's were fitted to 11/03's as upgrades, and those had 18-bit backplanes. The 11/03, however, only had a 16-bit address range, as did early core, MOS RAM, and ROM boards. I wasn't referring to the bus as 16-bit, but to the address range. For slightly different reasons, you can't use an MSV11-D (or several other options) in a 22-bit system. It will fit in the backplane, and work fine, but it effectively turns the whole system into 18-bit, because it doesn't decode BAL18-21, and therefore responds to sixteen blocks of addresses. For similar reasons, the RKV11-D is not often used in 18-bit or 22-bit systems, since it can only perform DMA with 16-bit addressing (you can access the registers in an 18- or 22-bit system, of course, because it responds to the BBS7 signal). There are lots of other I/O options with similar restrictions. -- Pete Peter Turnbull Dept. of Computer Science University of York