Just like we have binary multipler circuit in cisc computer so do we have binary divider circuit in cisc computer? Or it is just like we use algothrims instead of having a real divider circuit? (Just like risc don't have multiplier circuit so it use adders circuit instead of multiplier circuit).

  • \$\begingroup\$ Some RISC devices do have a hardware multiplier. "· Advanced RISC Architecture — [...] — On-chip 2-cycle Multiplier" \$\endgroup\$ Mar 13 '18 at 23:29
  • \$\begingroup\$ Wait. A copy of your own question that you accepted an answer to? That's basically an insult to me spending time on your question. How very rude of you, alphabet. \$\endgroup\$ Mar 14 '18 at 1:26
  • \$\begingroup\$ Yeah i am because my post is being put on hold i just help you get more reputation and it help me a parts not all \$\endgroup\$
    – AlphaBetA
    Mar 14 '18 at 2:02
  • \$\begingroup\$ @AlphaBetA you've asked a question again that you marked as "I've gotten an answer to this", and I spent time on that. Obviously the message you're transporting is that our time is worth nothing to you. And believe me, once you've got a couple hundred points of reputation (which happens really fast if you, you know, ask good questions instead of such that get closed!! Or at least, incorporate the feedback you've gotten), you really don't care about the one or two additional upvotes. So, please, don't be rude. \$\endgroup\$ Mar 14 '18 at 11:24
  • \$\begingroup\$ Ok i know but everyone put my post on hold while i edit it again \$\endgroup\$
    – AlphaBetA
    Mar 14 '18 at 13:12

You're confusing things:

CISC or RISC says something about the instruction set.

Nothing about how instructions are implemented. You can have a RISC with barrel shifters, hardware single-clock floating point multipliers and dividers, and you can have a CISC system that implements rotational shift under the hood in multiple single-bit-shifts with a carry register, and can't do any floating point in hardware, but does all floating point in slow fixed-point microcode.

CISC and RISC are unrelated to the question of how (and how fast) things work!

Generally, hardware dividers are rare:

  1. because division is surprisingly seldom useful and
  2. because a hardware divider is humongous to implement. I'll invite you to look at the "small" images in this answer of mine about why division is slower than multiplication in hardware.
  • \$\begingroup\$ I was just about to comment on the instruction set lol. Thank you for saving me the trouble ;) \$\endgroup\$
    – user103380
    Mar 13 '18 at 23:48
  • \$\begingroup\$ @KingDuken this started as a comment. Then it got so long that I ctrl-c/ctrl-v'ed it into an answer. \$\endgroup\$ Mar 13 '18 at 23:49
  • \$\begingroup\$ I was about to provide an example with MIPS since that instruction set is for x86 architectures. From what I remember, MUL and DIV were pseudo instructions. \$\endgroup\$
    – user103380
    Mar 13 '18 at 23:53

RISC versus CISC has nothing to do with which raw machine capabilities are included, like whether there is a multiplier, divider, barrel shifter, etc. A hardware divider is a particulary heavyweight computational block to implement, so fewer processors have them than the more simple multipliers and even simpler adders.

The distinction is more about addressing modes, and what parts of the processor data can be moved between during individual instructions. This distinction was always somewhat blurry, and of course with marketing involved, it got even blurrier. For a while, everything was labeled RISC because the marketers thought it sounded more cool and modern. Neither is true, but some of that has left the impression that RISC is somehow more advanced. That's not true either.

In my opinion, the drive towards RISC or CISC had more to do with the relative speed of main memory compared to the combinatorial logic to implement instruction decoding and data manipulation. Consider that main memory and logic gates are implemented with different technologies. It should be no surprise that the relative speed of each therefore changes as different technologies advance and new ones emerge.

Back when main memory was core and logic was TTL chips, main memory was relatively slower than it generally is today. It therefore made sense to highly encode instructions, then perform lots of logic operations as a result of each, since you'd be waiting around a while for the next instruction anyway. When main memory is relatively faster, then you can fetch instructions more often, or put another way, you have fewer gate delays to do anything with a instruction before you need to process the next one without stalling the pipeline.

Back in the 1980s when core memory as main memory was being phased out in favor of dynamic RAMs, various new machine architectures emerged based on the "RISC" philosophy. This was over-hyped to make RISC sound like some breakthrough, as apposed to the logical evolution of faster main memory relative to gates. The relative speeds of memory and logic continue to waffle back and forth, so there are machines with tradeoffs all along the CISC/RISC spectrum.

Actually, parts of common computers from the 1960s and 1970s were way off at the RISC end of the spectrum, because microcode is in some ways the ultimate RISC architecture. CISC machines of the time had so many logic delays available to them per main memory instruction fetch that the complicated instructions were actually performed by a fast internal RISC machine running from a small (and much more expensive) gate-like memory. These micro-instructions were typically very wide, with individual bits directly controlling buffer enables controlling the source of data onto internal busses, the ALU functions, and the like.

As a result of all this, CISC instruction sets generally have lots of possible addressing modes for the sources and destinations of operands. RISC instruction sets are simpler ("reduced") by generally performing operations only between a limited set of special hardware registers. The source values have to be moved into the registers from memory and the results written back to memory (when these are required) by separate instructions from those performing the computations.

However, back to your question, none of this says whether a machine has native divide hardware, or whether divides must be done in software.

  • \$\begingroup\$ That what i ask for : do we have hardware that can divide in cisc instead of using software like risc (the complex is in software not hardware) \$\endgroup\$
    – AlphaBetA
    Mar 14 '18 at 2:08

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