minow@decvax.UUCP (Martin Minow) (09/07/85)
nvuxr!rdm2 gave some slightly incorrect information about PDP11 trap/emt instructions, of which there are four: EMT xxx interrupt through the vector at location 30. TRAP xxx interrupt through the vector at location 34. IOT interrupt through the vector at location 20. BPT interrupt through the vector at location 14. The earliest discussion of these instructions that I have (PDP-11 Handbook, 2nd edition, 1969) describes them as follows: "Trap instructions provide for calls to emulators, I/O monitors, debugging packages, and user-defined interpretors." As originally used, IOT called the IOX I/O executive (paper-tape software), BPT called the debugger, EMT was unused (I think), and TRAP was reserved for user-written programs. The floating-point emulator could be compiled to use either EMT or TRAP (can't remember which). The disk operating system released in the early 1970's used EMT, as did subsequent operating systems (RSX, RT11, RSTS), to call executive services. IOT has also been used for executive service calls. Hope this clears up any confusion. Martin Minow decvax!minow