30 Apr
2018
30 Apr
'18
6:43 p.m.
On 4/30/18, Noel Chiappa <jnc(a)mercury.lcs.mit.edu> wrote:
There's also the System 360 approach, where processes share a single
address
space (physical memory - no virtual memory on them!), but it uses
protection
keys on memory 'chunks' (not sure of the correct IBM term) to ensure that
one
process can't tromp on another's memory.
IBM always used the term "storage" rather than "memory". The Storage
Protection feature for System/360 was optional on some models, and
provided a 4-bit protection key for each 2048-byte block (IBM's term)
of physical storage, allowing for up to 15 processes to be executing
simultaneously (key value 0 disabled storage protection).
The System/370 operating systems DOS/VS, OS/VS1, and OS/VS2 SVS all
had a single, demand-paged virtual address space, usually a few times
larger than the physical memory. For DOS/VS the virtual address space
was partitioned into a space (mapped virtual=physical address)
starting at address 0 for the OS, then up to five user program
partitions. Programs were linked to run in one of these partitions.
OS/VS1 (successor to OS/360 MFT) also had a single virtual address
space, but it had more partitions and the program loader could
dynamically relocate applications to run in any of the partitions.
OS/VS2 SVS (first successor to OS/360 MVT) had a single virtual
address space, but dynamically allocated partitions as needed. OS/VS2
MVS had processes in the modern OS sense--each executing program got
its own virtual address space.
-Paul W.