Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!sdd.hp.com!hplabs!hpcc05!hpcuhb!hpcuhe!edwardm From: edwardm@hpcuhe.cup.hp.com (Edward McClanahan) Newsgroups: comp.arch Subject: Re: Re: skip instructions (on 68040) Message-ID: <32580028@hpcuhe.cup.hp.com> Date: 7 May 91 01:03:34 GMT References: <1182@opus.NMSU.Edu> Organization: Hewlett Packard, Cupertino Lines: 41 James Thomas writes: > Is there a reference (or more) describing why SKIP instructions have mostly > disappeared from instruction sets? The 704x and 709x had them, so the > PDP-6 had them (and maybe therefore the PDP-8 inherited them). Now the > HP-PA has them hiding in the guise of the nullify bit. Are conditional > branches generically more useful? HP-PA only has 32 bit instructions, the first 6 bits of which are the "major op-code" (thus ranging from 0 to 63). The so-called "nullify bit" can be applied to many instructions (as well as "condition checks"). I suppose an alternative would be to define the major op-codes to be 7 (or 8 or 9...) bits to include the "nullify bit". The comparison: present HP-PA modified HP-PA meaning ============= ============== ======= BL target,r2 BL target,r2 Branch-and-link, execute following instr. BL,N target,r2 BLN target,r2 Branch-and-link, don't execute following OR r1,r2,r3 OR r1,r2,r3 OR R1 with R2 sticking result in R3 Always execute following instruction OR,TR r1,r2,r3 ORN r1,r2,r3 OR R1 with R2 sticking result in R3 Never execute following instruction OR,< r1,r2,r3 ORLT r1,r2,r3 OR R1 with R2 sticking result in R3 Only execute next instr. if R1