Hence the "erroneous _impression_" (emphasis added). I'm curious as to how they decided which models to run which editions on. Although V4 _ran_ on the /45, split I+D wasn't supported - for user or kernel - until V6. (I'm assuming a number of things - both in the kernel, and applications - started hitting the 64KB limit, which led to its support.) Speaking of split I+D, there's an interesting little mystery in V6 that at one point in time I thought involved split I+D - but now that I look closely, apparently not. The mystery involves a 'tombstone' in the V6 buf.h: #define B_RELOC 0200 /* no longer used */ I had created (in my mind) an explanation what this is all about - but now that I look, it's probably all wrong! My explanation involves the slightly odd layout of the kernel in physical memory, with split I+D; data below the code, at physical 0. This actually makes a lot of sense; it means the virtual address of any data (e.g. a buffer) is the same as its physical address (needed for DMA). It does require the oddness of 'sysfix', to invert the order of code+data in the system binary, plus odd little quirks in the assembler startup (e.g. copying the code up to make room for BSS). So I thought that B_RELOC was a hangover from a time, at the start of split I+D, when data _wasn't_ at physical 0, so a buffer's virtual and phsyical addresses differed. But that must be wrong (at least in any simple way). B_RELOC was in buf.h as of V4 - the first kernel version in C - with no split I+D. So my theory has to be wrong. However, I am unable to find any code in the V4 kernel which uses it! So unless someone who remembers the very early PDP-11 kernel can enlighten us, its purpose will always remain a mystery! Noel