Alan Perry <alanp at snowmoose.com> wrote: The A-series were direct successors to the B5000/6000/7000 series and used the same architecture. Unisys still sells the ClearPath MCP systems, now using Intel processors and running the MCP operating system (under emulation I think). The architecture has so-called display registers, each of which pointed to a stack frame for a lexicographical level in the code, i.e. the procedure call level. Data is addressed as an address couple consisting of a display register number and an offset, stored in a so-called Indirect Reference Word (IRW). Data outside of the stack can be addressed by means of descriptors. Data in another program's stack is addressed by means of Stuffed Indirect Reference Words (SIRW), which include a stack number and an offset. The operating system keeps track of individual program stacks by means of a tree of pointers to job stacks. Also, to keep track of the procedure calling and return linkage, Mark Stack Control Words (MSCW) are created whenever a procedure is called. The Display Registers point to the MSCWs. All this is designed to support block-structured high level languages, in fact all the operating system software is (was) written in various ALGOL dialects. There is quite a lot of information about this on the net. A book which describes it is Computer Systems Organization by Elliott Organick (ACM Monograph Series, 1973. LCN: 72-88334). -Jonas