Subject: Re: [stella] "Illegal" opcodes From: Russ Perry Jr <slapdash@xxxxxxxxxxxx> Date: Sun, 18 Jun 2000 22:44:17 -0500 |
At 9:42 AM -0400 6/17/00, Ruffin Bailey wrote: >> Can anyone modify DASM so that it accepts illegal opcodes or recommend >> a PC assembler that accepts them? >Okay, if these instructions not only work, but in this case do something >useful _and_ more efficiently, why are they called "illegal" opcodes? As others have pointed out, they're not really illegal, but merely undocumented. But IIRC the reason they used a stronger-sounding term was because they're not guaranteed to be there in new revisions of the chip, or in the next generation with the same architecture. At 12:50 PM -0400 6/17/00, Matthew W. Miller added: >That they work at all is a side effect of the way the 65xx's microcode >is lashed together. Great, careless coding even on that level... I've always wondered why they didn't just tie off any used opcodes as NOPs to actually MAKE them unuseable. At 10:08 AM -0400 6/18/00, Ruffin Bailey wrote: >Forgive my ignorance and curiousity, but who "officially" defined the >instruction set As Erik Mooney answered, it's the designers who define which are official opcodes; anything is is "illegal", because they only guarantee that what's documented will work as described. In general, illegal opcodes should be used only if they've been tested for side effects. For instance, even though an illegal opcode may do something useful, there's a chance that it will also do something unexpected, like set flags incorrectly. -- //*================================================================++ || Russ Perry Jr 2175 S Tonne Dr #105 Arlington Hts IL 60005 || || 847-952-9729 slapdash@xxxxxxxxxxxx VIDEOGAME COLLECTOR! || ++================================================================*// -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
Current Thread |
---|
|
<- Previous | Index | Next -> |
---|---|---|
Re: [stella] "Illegal" opcodes, Erik Mooney | Thread | Re: [stella] "Illegal" opcodes, Erik Mooney |
Re: [stella] "Illegal" opcodes, Julian Squires | Date | Re: [stella] "Illegal" opcodes, Erik Mooney |
Month |