Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!snorkelwacker!paperboy!husc6!contact!ileaf!io!prs From: prs@io.UUCP (Paul Schmidt) Newsgroups: comp.arch Subject: Re: Opcode assignment for RISC processors Message-ID: <2179@io.UUCP> Date: 9 Oct 90 20:38:22 GMT References: <39029@ucbvax.BERKELEY.EDU> <4153@bingvaxu.cc.binghamton.edu> <41963@mips.mips.COM> <-S86F-8@xds13.ferranti.com> <41964@mips.mips.COM> Reply-To: prs@eng.ileaf.com (Paul Schmidt) Organization: Interleaf, Cambridge, MA Lines: 16 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. > ... All of the trap (Int?) instructions called an address at (trap*4) - Trap 0 called location 0, and the trap 7 instruction on the 8080 called location 28 decimal. Since most systems had memory there, it was easy enough to put some sort of appropriate OS handler at location 28, effectively trapping execution of non-existent programs. In your system, I suspect that the instructions located at 28 decimal were effectively garbage.