Where should the type information be: in tags and descriptors 423

snip decimal discussion

Prime 50-series machines are multics-like segmented machines. They have several addressing modes; but in all cases you have to handle segments of 64k 16-bit words. There are a maximum of 4096 such segments, but the OS takes the first 1024 of them (the OS is always mapped). Standard division is OS, "highseg" or manually shared code, user code and stack with 1024 segments each.

The addressing modes are R,S (16-bit modes, needs longjumps to access other segments, but have pretty compact code; and becomes an nightmare to link if the code exceeds 64k words), I and V. For C they also modified the I mode to IX, where there is better pointer traversal support. There are two c compilers, CC and CI. The first one suffered from a FORTRAN-based memory model(V-mode), and has efficiency problems with large structures and pointer traversal. The CI compiler is designed for IX mode; where these problems are solved; but linking of V-mode code can become interesting; and weird casting is needed to present the V-mode programs (the OS included) with the correct data types.

All V and IX mode pointers have segment and offset, and the (0,0) location has some magic properties. The NULL therefore maps to some unmapped segment; ISTR it is (07777,0). '

-- mrr