On 2014-05-20 21:39, Dave McGuire wrote: On 05/20/2014 11:04 AM, Johnny Billquist wrote: M+ do not need split I/D-space. But you do need 512K of memory. But the 11/23 and 11/24 are supported CPUs. I have 1M of memory. Sure you're not confusing the 11/23 with the 11/23+? I was poking at 4.6M+'s SYSGEN and it wouldn't accept 11/23 as an answer. The difference between the 11/23 and 11/23+ is the amount of memory. The original 11/23 could not go above 256K... If you have more, then you actually have an 11/23+. In practice, though, the vast majority of systems out there that say "PDP-11/23" (not -PLUS) on the front are in fact capable of 22-bit addressing; you've gotta look at the etch rev of the CPU board. See my other email. Aha. Thanks for the information, Dave. I knew that very few 11/23 systems really were limited to 18 bits, but I didn't know exactly which ones. So even the M8186 boards are mostly 22-bit. Really nice to know. Johnny