54394gt@hocda.UUCP (01/09/84)
You do not get 56K bytes in the PDP-11 kernel data space. The bottom segment is physical address 0, where UNIX boots itself and where traps go; they must work with or without memory management operating. In addition, another data segmentation register points to the user process structure for the currrently running user process. At least the kernel borrows trap/interrupt stacks from the user, so there is no stack segment. That is 48K counting the bottom seg. I don't know whether your version of UNIX allows the 'kernel' to use supervisor as well as kernel addressing modes; ours does; that gives more address space (I haven't looked closely enough at the source to tell if it uses both instruction and data space in supervisor mode).