Multics used segments and Lisp Machines had a single address space. UNIX breaks down quickly without multiple fake single address spaces for each program.
> Programs just have to be compiled relocatable.
Yes, and with unrestricted memory access, one program can crash the entire system.
> You know, like what happens with shared libraries: which are written in C, and get loaded at different addresses in the same space, yet access their own functions and variables just fine.
That is except when one piece manipulates global state in a way with which another piece can't cope, and at best the whole thing crashes. Dynamic linking in UNIX is so bad some believe it can't work, and instead use static linking exclusively.
> Programs just have to be compiled relocatable.
Yes, and with unrestricted memory access, one program can crash the entire system.
> You know, like what happens with shared libraries: which are written in C, and get loaded at different addresses in the same space, yet access their own functions and variables just fine.
That is except when one piece manipulates global state in a way with which another piece can't cope, and at best the whole thing crashes. Dynamic linking in UNIX is so bad some believe it can't work, and instead use static linking exclusively.