daveb@geaclib.UUCP (David Collier-Brown) (10/02/88)
In article <2550@sultra.UUCP>, dtynan@sultra.UUCP (Der Tynan) writes: | Rather than have a standard U/S bit in the CPU status register, it might make | more sense to assign it to each I-page in a paged-MMU system.... From article <936@raspail.UUCP>, by bga@raspail.UUCP (Bruce Albrecht): | This scheme has been around for a long time. One of the first systems that | I'm aware of that used this was Multics, which has been described in a book | by Organick (MIT Press, not sure of exact title). Yup, Elliot I. Organick, "The Multics System, An Examination of Its Structure", Cambridge, Mass. (MIT Press), 1972 More recent work has broken this down into "domains of protection" instead of purely nested rings, and may be found in several forms on machines ranging from mainframes (Honeywell-Bull, naturely) to micros (Intel 286/386, of all things!). The overhead usually gets quite high, though: if yiu want decent protection you usually set up another stack, dump all registers, flush the fpp, etc... Probably not a good idea for a Unix-like machine, where people are already complaining about the overhead involved in system calls. --dave (spread thinly) c-b -- David Collier-Brown. | yunexus!lethe!dave Interleaf Canada Inc. | 1550 Enterprise Rd. | HE's so smart he's dumb. Mississauga, Ontario | --Joyce C-B
ron@motmpl.UUCP (Ron Widell) (10/05/88)
In article <3265@geaclib.UUCP> daveb@geaclib.UUCP (David Collier-Brown) writes: =In article <2550@sultra.UUCP>, dtynan@sultra.UUCP (Der Tynan) writes: =| Rather than have a standard U/S bit in the CPU status register, it might make =| more sense to assign it to each I-page in a paged-MMU system.... = =From article <936@raspail.UUCP>, by bga@raspail.UUCP (Bruce Albrecht): =instead of purely nested rings, and may be found in several forms on =machines ranging from mainframes (Honeywell-Bull, naturely) to That was the reason for the CALLM and RTM instructions on the 68020 and the access level protection mechanism on the 68851 (at the request of a single customer who, subsequently, never used them). =protection you usually set up another stack, dump all registers, =flush the fpp, etc... Probably not a good idea for a Unix-like =machine, where people are already complaining about the overhead =involved in system calls. This is probably why they were never used (other than having to rewrite an existing OS and all of the applicatios SW :-)). Just a single instruction, but lots of bus cycles :-(. =--dave (spread thinly) c-b -- Ron Widell |UUCP: motmpl!ron Motorola Semiconductor Products, Inc., |Voice:(612)941-6800 9600 W. 76th St., Suite G Eden Prairie, Mn. 55344 -3718 << Usual Disclaimer >>