Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!usc!ucsd!ogicse!littlei!omepd!omews35!colwell From: colwell@omews35.intel.com (Robert Colwell) Newsgroups: comp.arch Subject: Re: Opcode assignment for RISC processors Message-ID: <6097@omepd.UUCP> Date: 10 Oct 90 13:26:09 GMT References: <39029@ucbvax.BERKELEY.EDU> <4153@bingvaxu.cc.binghamton.edu> <41963@mips.mips.COM> <-S86F-8@xds13.ferranti.com> <41964@mips.mips.COM> Sender: news@omepd.UUCP Reply-To: colwell@omews35.UUCP (Robert Colwell) Organization: Intel Corp., Hillsboro, Oregon Lines: 39 In article ted@nmsu.edu (Ted Dunning) writes: >the worst i have seen was on an old IMSAI-8080 where FF was the trap 7 >instruction, and was also what unused memory locations effectively >contained. the service routine for this instruction was hardwired >into the non-existent high end of memory. the result was that trap 7 >pushed a return address, and jumped to a service routine where it >found another trap 7 instruction, pushed another return address (which >happened to consist of two trap 7 instructions... This reminds me of two memorable hacks that Paul Rodman and I did while finishing a Master's project at CMU back in 1978. We were using a PDP-11/45 as a host for a digital music synthesizer of our own design, and the PDP-11 was getting a tad flaky for want of decent maintenance. One manifestation of this latent senility was that it would occasionally fail its boot diagnostics for memory, putting the failing address up on the front panel lights, and taking a trap to a non-existent trap handler, at which point it would hang. Were one to toggle data into the supposedly bad memory location, it would be remembered just fine. So we began to routinely toggle in a "return-from-interrupt" at the "trap-handler" location just after powering up and before trying to boot, and the machine would boot just fine. (No one else figured out how to do this, so the PDP-11 became our machine after this.) The other hack had to do with the DEC removable disk drives. There are batteries in those things, presumably to provide the wherewithal to retract the head from the disk in the event of a power fail. But we discovered that if those batteries were to fail, one couldn't use the drive at all. So we taped some flashlight batteries together with electrical tape, wired that in place of the internal batteries, and hung the whole thing out of the drive chassis. It looked pretty weird. Almost as bad as the time we had to take a Radio Shack 12.6V transformer and remove some secondary windings because the nominal output was a bit too high for our three-terminal regulator, causing it to shut down completely. But that's another story. Bob Colwell mipon2!colwell@intel.com 503-696-4550 Intel Corp. JF1-19 5200 NE Elam Young Parkway Hillsboro, Oregon 97124