Article 71 of comp.sys.cdc: Path: matra.meer.net!news.neumedia.net!uunet!in3.uu.net!207.172.3.52!feed1.news.erols.com!news-peer.sprintlink.net!news.sprintlink.net!Sprint!howland.erols.net!vixen.cso.uiuc.edu!avenir!walker From: walker@avenir.cerl.uiuc.edu (Michael W. Walker) Newsgroups: comp.lang.fortran,comp.sys.cdc Subject: Re: Reminiscing About the Cyber 6600 Date: 5 May 1997 14:31:26 GMT Organization: University of Illinois at Urbana-Champaign Lines: 38 Distribution: inet Message-ID: <5kkqvu$mka@vixen.cso.uiuc.edu> References: <01bc567d$25cfb000$7a59e1cc@default> <336DD429.74BA@rsn.hp.com> <336DEB2D.2FCD@worldnet.att.net> NNTP-Posting-Host: avenir.cso.uiuc.edu Xref: matra.meer.net comp.lang.fortran:50212 comp.sys.cdc:71 In article <336DEB2D.2FCD@worldnet.att.net>, Dick Hendrickson wrote: >Joel Williamson wrote: > >> Here's a bit of trivia. Each time we trained a new systems programmer, >> about the time they thought they knew it all, we'd challenge them to >> save the contents of all the registers to memory _without_ using the >> exchange jump instruction. Anybody remember how to do it? >> >Gee, this sure brings back happy memories. To do a store the hardware >required placing an address into either register B6 or B7 and that >"automatically" stored the contents of X6 or X7 to the memory address. >So, the only hard part is saving either B6 or B7 before putting an >address into it. I don't remember the op code sequence, but the trick >is that a "return jump" does code modification. So, the code looks >something like: > Jump if B6 negative to Label-2 > return jump to Label-1 >Label-1 zero >Label-2 SB6 B6+B6 > >The net effect is that the contnts of Label-1 will be zero if >the sign bit of B6 is positive and non-zero if B6 is negative. >The add at Label-1 shifts B6 left 1 place (the 6600 was a >one's (or two's? I forget) complement machine, so any overflow >was ignored.) Repeat 23 more times and you wind up with a >sequence of 24 memory words that are zero or not depending on the >value of the associated B6 bit. After that, it's easy to store X6 >and all of the other registers. > Close, but not exactly right. Two minor problems: it is A6 or A7 that you have to modify to store the corresponding X register. One has to first save a B register in order to have a place to save either A6 or A7. A B register is 18 bits, not 24. -- Michael W. Walker mwwalker@uiuc.edu University of Illinois at Urbana-Champaign