On Thu, 7 Nov 2002 10:46:30 -0600, Linas Vepstas <[EMAIL PROTECTED]> wrote:
>On Wed, Nov 06, 2002 at 10:36:40AM +0800, John Summerfield was heard to remark: >> On Wed, 6 Nov 2002 05:45, you wrote: >> >> > >> > The core idea is actually so simple, its painful. Today, most CPU's >> > define two memory spaces: the one that the kernel lives in, and the >> > one that the user-space lives in. When properly designed, there is >> > nothing a user-space program can do to corrupt kernel memory. One >> > 'switches' between these memory spaces by making system calls, i.e. >> > by the SVC instruction. >> > >> > The 390 arch has not two, but 16 memory spaces (a 4-bit key) with >> > this type of protection. (When I did the i370 port, I put the >> > kernel in space 0 or 1 or someething like that, and user space >> > programs ran in one of the others.) The partitioning between them >> > is absolute, and is just like the kernel-space/user-space division >> > in other archs. The mechanism is independent/orthogonal to the >> > VM/TLB subsystem (you can have/use virtual memory in any of the >> > spaces.) I'm coming late to the party, but this information should be corrected. Th 4 bit key is not a "memory space" but an attribute of a block of memory (4K or 2K) within a continous address space. The PSW also contains a protect key and the CAW (Channel Address Word) two. Key zero typically allows access to all memory. When a PSW is set to a non-zero key, it can typically read any storage but it can only write to its own key. [The storage key byte has other items like a read-only bit.] Thus the storage key allows for memory isolation between programs running in the same address space, with key zero being used for the supervisor. That was MVT/MFT days... Nowadays 2K storage keys have vanished. In MVS, program isolation is by address space. Most of kernel memory is shared with the application program space. Key 8 is for programs and key zero is for kernel. Subsystems like VTAM can take over a storage key. And of course there are all sorts of multi-address space stuff now - access registers, primary/secondary address space instructions, etc etc. But the storage key was originally used for program isolation. It was a pretty nice advance for its time. john alvord
