6502 illegal opcodes questions
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From: Michael J. Mahon on 19 May 2006 02:56 Jim Brain wrote: > Michael J. Mahon wrote: > >> What do you know! The decoding *is* done with a PLA after all! > > > I don't think a PLA allows X (don't care) states. The large logic > matrix at the bottom of the diagram is a wired-or matrix, which is why > the illops exist. In the area of processor design these days, a wired ROM decoder is often referred to as a PLA, since it is an array and it is "programmable" by wiring choices at design time. Wire-ORing is common to many logic configurations. -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."
From: Michael J. Mahon on 19 May 2006 03:01 mdj wrote: > John Selck wrote: > > >>Best case: 21 vs 11 clock cycles (almost 2x speed with illegal) >>Worst case: 25 vs 15 clock cycles (still 1.67x speed) > > > Nice... It would be amusing to see a game title on the Apple II+ > running faster than on the IIe :-) > > Of course, the point remains though that your would never do this on > the Apple II series, as you'd limit your target market to the earliest > machine, rather than just the lowest common denominator of all machines > (64k, 6502 code that's documented and unbuggy) > > If I were coding on a different 6502 platform, I might consider using > these. > > But then, I'm very much a 65C02 coder these days, as pretty much > everything I write is intended for a IIe/IIc and wouldn't run on an > older Apple II anyway. > > Michael will probably shoot me down for saying this, but when I mean > BRA, I write BRA. It's clearer code, and I really wish that anyone who > writes code specifically for the earlier processors would comment their > unconditional branches accordingly. No shooting from here. ;-) I agree that "unconditional" branches should always be commented so, even when they are written as conditional. I have found only a very small fraction of unconditional jumps that cannot be perfectly safely with the right conditional jump. -michael Parallel computing for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."
From: Eric Smith on 19 May 2006 03:13 "Michael J. Mahon" <mjmahon(a)aol.com> writes: > The 6502 was already a much less expensive processor than its > competitors, because of a mask retouching technique MOS Technology > developed that saved mask iterations. I saw that claim on Wikipedia. Is there any published reference? Stephen Harris wrote: > With the 6502 essentially not having a microcode level, this > optimisation is exposed to the programmer. The PLA is essentially equivalent to microcode. The difference is that it is addressed by the instruction register and a few state bits, rather than a micro-PC. Eric
From: Eric Smith on 19 May 2006 03:43 Michael J. Mahon > True, but the decoding in the 6502 is handled by a kind of PLA, so > it would likely not be very expensive (in real estate) to trap or > NOP the invalid combinations. Bruce Tomlin wrote: > Nope, it's handled by good old random logic. A PLA is designed to be a > programmable generic replacement for random logic, and is way too > inefficient for high-volume VLSI. Sorry, but Michael is correct. It is definitely a PLA. You can see it on the die (or a photomicrograph); it's the most regular structure. It is near one edge of the die, takes up an area about 1/6 of the long dimension of the chip by most of the short dimension. If I've counted correctly, the logical size of the product term array is 21 inputs by 137 product terms. 15 of the inputs are from the instruction register; they are the true and complement forms of each bit except bit 1. It appears that two inputs come from the clock generator, and four from a state counter, but I haven't studied it in enough detail to be certain of that. Most of the product terms control gating of signals between registers. MOS PLA design is actually quite space-efficient, and for functions like instruction decode is usually more space-efficicent than equivalent random logic would be. This is especially true of the NMOS 6502, for which the instruction encoding was designed in such a way that they did not need to OR together many (any?) of the product terms. It's not a *field* programmable logic array, if that's what you were thinking of. It's mask programmed by the presence or absence of transistors at specific locations in the array. Many microprocessors in the 1970s, and almost all newer ones use PLAs for instruction decode and other tasks. Even the lowly TMS1000 four-bit microprocessor, the epitome of cheap microprocessors in the mid-1970s, used PLAs. Eric
From: Eric Smith on 19 May 2006 03:47
Jim Brain <brain(a)jbrain.com> writes: > I don't think a PLA allows X (don't care) states. Certainly it does. That's one of the primary distinguishing characteristics of a PLA as opposed to a ROM. Eric |