Re: what's a segment, 80286 protected mode

John Levine <johnl@taugh.com> writes:

What you're describing is multi-level page tables. Every virtual
memory system has them. Sometimes the operating systems make the
higher level tables visible to applications, sometimes they don't. For
example, in IBM mainframes the second level page table entries, which
they call segments, can be shared between applications.

initial adding virtual memory to all IBM 370s was similar to 24bit
360/67 but had options for 16 1mbyte segments or 256 64kbyte segments
and either 4kbyte or 2kbyte pages. Initial mapping of 360 MVT to VS2/SVS
was single 16mbyte address space ... very similar to running MVT in a
CP/67 16mbyte virtual machine.

The upgrade to VS2/MVS gave each region its own 16mbyte virtual address
space. However, OS/360 MVT API heritage was pointer passing API ...  so
they mapped a common 8mbyte image of the "MVS" kernel into every 16mbyte
virtual address space (leaving 8mbytes for application code), kernel API
call code could still directly access user code API parameters
(basically same code from MVT days).

However, MVT subsystems were also moved into their separate 16mbyte
virtual address space ... making it harder to access application API
calling parameters. So they defined a common segment area (CSA), 1mbyte
segment mapped into every 16mbyte virtual address space, application
code would get space in the CSA for API parameter information calling
subsystesm.

Problem was the requirement for subsystem API parameter (CSA) space was
proportional to number of concurrent applications plus number of
subsystems and quickly exceed 1mbyte ... and it morphs into
multi-megabyte common system area. By the end of the 70s, CSAs were
running 5-6mbytes (leaving 2-3mbytes for programs) and threatening to
become 8mbytes (leaving zero mbytes for programs)... part of the mad
rush to XA/370 and 31-bit virtual addressing (as well as access
registers, and multiple concurrent virtual address spaces ... "Program
Call" instruction had a table of MVS/XA address space pointers for
subsystems, the PC instruction whould move the caller's address space
pointer to secondary and load the subsystem address space pointer into
primary ... program return instruction reversed the processes and moved
the secondary pointer back to primary).

--
virtualization experience starting Jan1968, online at home since Mar1970