Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-lcc!rutgers!husc6!panda!genrad!decvax!decwrl!sun!gorodish!guy From: guy%gorodish@Sun.COM (Guy Harris) Newsgroups: comp.unix.wizards Subject: Re: Squeezing more memory into a process on a pdp 11/73 - HELP ! Message-ID: <11058@sun.uucp> Date: Wed, 7-Jan-87 14:47:36 EST Article-I.D.: sun.11058 Posted: Wed Jan 7 14:47:36 1987 Date-Received: Thu, 8-Jan-87 00:00:31 EST References: <1116@natmlab.dms.oz> <1416@blia.BLI.COM> Sender: news@sun.uucp Lines: 18 Keywords: memory pdp 11/73 help fruit orange Summary: Done for Commercial Union Leasing Corp's version of DEC's FORTRAN IV-PLUS for UNIX? > V7 (PDP11) Unix had some support for process text overlays. Of course > none of it was documented and we were unable to determine the intentions > of ld and had to rewrite it for our needs. As I recall we had to modify > adb and put a hook or two in the kernel to debug the stuff nicely. If I remember correctly, some Bell Labs memorandum said this was originally stuck in to support the overlay mechanism used by the port of DEC's FORTRAN IV-PLUS compiler to UNIX. (It certainly worked the same way that the overlaying stuff in the Harvard Science Center PDP-11 UNIX did.) However, that port also included a linker that understood DEC's object file format; the support for this format from the standard UNIX linker looked pretty primitive. The idea was that each path through the overlay tree got an executable image of its own. The root segment was the main 410/411 image, and the others were 405 images. When it came time to move to another path through the tree, the overlay manager would "exec" the appropriate 405 image. A 405 image would replace the text but leave the data and stack unchanged.