Someone wrote: >>Assuming DAT on, is the performance hit related to the possibility >> that the following 4096 page is not in virtual memory?
Lynn Wheeler wrote: > way back on 360/67 ... (actually all 360s) TR used to test start & > start+255 (end) address of the table ... which met that if it > crossed a 4k page ... it would catch both ... aka page fault both > pages ... before starting instruction execution. For S/360 models with DAT. > somewhere along the way ... something was raised that TR only > uses that much of the table that the input data-stream might > used ... for instance, if the translation input stream only > had values 0-9 ... and the table was within 256 bytes of the > end of an addressable region ... then the instruction might > fail (with start+256 precheck) ... even tho it otherwise could > succesfully execute. so the TR instruction was > "fixed" ... if the table start is within 256 bytes of the end > of an addressable boundary... it "pre-executes" the instruction > to see if any input stream bytes would index the table > across the boundary. S/360 GA22-6821-8 in the paragraph about logical operations at the beginning of the chapter and describing TR says: "In cases where it is known that not all eight-bit argument values will occur, it may be possible to shorten the list." > this would also theoritically have been a problem with 2k key fetch > protect ... and the table was within 256 bytes of a 2k boundary (with > the next 2k, fetch protected) and the input data stream never indexed > anything (in the table) across the addressable boundary. It seems like it would also be a problem on non-DAT machines at 4K boundaries. It does seem to have been added fairly late in S/370, though. (At least when it got documented.) I first learned about this in Nick Tredennick's book "Microprocessor Logic Design" about the Micro/370. http://en.wikipedia.org/wiki/Nick_Tredennick -- glen ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
