Moving buttembler programs above the line 511

another aspect is that the 360 genre was pretty much pointer pbutting ... as opposed to value pbutting. in the transition from SVS to MVS ... they still found that they were crowding the 16mbyte address spaces ... so they had to push some of the services into separate address spaces. this created a paradigm problem for pointer pbutting. this was initially addressed with "common segment" ... started out as a one mbyte area that appeared in all address spaces ... and applications could push stuff into and pbutt a pointer when calling a service. as the number of services and applications grew the contention for "common segment" space grew and you were starting to see installations with 4 and 5 mbyte "common segments". Now this is in each virtual 16mbyte address space ... where the kernel already occupied 8mbyte of every virtual address space; with 4-5 mbyte "common segment" also occupying every address space ... that just left only 3-4mbytes for an application.

the 3033 started to address this with dual-address space support ... where a service (running in a different virtual address space) could reach into the virtual address space of the calling application. Note however, the call from the application to the service still had to still make a kernel call (as compared to previously where some service calls were simple branch&link).

with 370-xa (711 architecture) on the 3081 ... besides introducing 31-bit virtual addressing ... PC (program call) and access registers were also introduced. PC referenced a supervisor mapping table (in some sense analogous to supervisor virtual memory tables) that supported direct transfer between a calling application (in one address space) and a service (in another address space) w-o having to pbutt thru the kernel. access registers generalized the dual-address space support to multiple virtual address spaces.

some discussion of access registers

discussion of program call

various past postings mentioning dual address space support:

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