Thank you Marko and Patrick: On Thu, 2010-10-28 at 21:36 +0100, Marko Vojinovic wrote: > On Thursday, October 28, 2010 19:27:16 William Case wrote: > > How does the cpu search and find stuff? > > > > There is a huge amount of searching and finding of text in > > memory, conditional statements requiring comparisons, and the use of > > entry points but not exact addresses from within both kernel space and > > user space. It has occurred to me that a there is necessarily a lot of > > physical or bit comparing going on. Too much, I would think, to keep > > dumping a search criteria into a cpu register and then replacing the > > contents of a second register from a block of memory until one matches. > > Believe it or not, in a nutshell that's exactly what is happening. When a > processor needs to "search" for something in memory, it typically does a > conditional loop --- load the number from memory into the register (the MOV > assembly instruction), compare it to the wanted value in the second register > (CMP), if they are the same break the loop (LOOPNE), otherwise load the next > number and repeat. > > Why do you think there is "too much" of that comparison going on? > I didn't think there was too much from my point of view. I thought there might be too much from an engineer's perspective. As I have dug into the inner workings of computers over the last few years, I have become enthralled with the ingenuity, if not down right genius, of how computers have been designed and improved over the years -- starting with basic circuits and instruction sets. I thought I might have missed something -- missed an aha! moment. In my mind. there was a good chance that some engineer somewhere had seen the repetition involved in a search and comparison and developed something quite cunning to shorten the process. As I said, I always look for the eureka's. > > Is there a special unit within the cpu or memory were rapid comparison, > > or partial comparisons are made during a search, before a criteria is > > noted as found and moved into the cpu registers? In a generalized way, > > at the hardware level, how are searched for criteria found? > > There is no special unit, AFAIK. You load the two numbers (to be compared) > into the processor registers, and do a CMP instruction. The processor > basically subtracts the two numbers, and modifies the flag register according to > whether the result is positive, negative or zero. The next instruction (say, > LOOPNE) inspects the flag register and decides what will be the following > instruction, based on the contents of the flags. This "deciding" amounts to > changing the value of the IP (instruction pointer) register to one or the > other predefined value. > > You might want to learn a bit of assembly language, if you wish to understand > basic low-level operations of the processor. And if you want to understand the > concepts of how processors actually work and what is a computer program in > principle, you can start from here > > http://en.wikipedia.org/wiki/Turing_machine I have looked at a Turing Machine and assembly language. Enough to have a comfortable overview without developing a strong desire or need to dig deeper. Even spent a couple of afternoons with IA-32 just to take away the mystery. > > but don't curse me later for pointing you to that page. You asked for it! ;-) > > > I have googled for an answer and found nothing helpful. That usually > > means I have somehow mis-posed the question, am working from wrong > > assumptions or lack the correct terminology. Help chasing the > > 'searching' mechanism would be appreciated. > > Searching "mechanism" is a software algorithm that I described above --- pick > a value from memory, compare it to the one you search for. If they are the > same, you found it. If not, pick the next value from memory and repeat. > > There is nothing about searching (per se) that is implemented in hardware. The > processor knows how to move numbers (to registers and memory), how to add > them, subtract, multiply, divide, compare and jump to the next instruction. > And that's about it. Everything else is built from that, through various > algorithms. > > Note that this instruction set I described above is not minimal --- for > example, you can perform multiplication by looping addition (3x2 = 2+2+2). > Depending how the processor is designed, it can have more or less of these > non-essential instructions implemented in hardware. You can read on CISC > versus RISC philosophy here: > > http://en.wikipedia.org/wiki/RISC > > But I've never heard of a processor that has a "search" instruction > implemented in hardware. :-) > > HTH, :-) > Marko > -- Regards Bill Fedora 13, Gnome 2.30.2 Evo.2.20.2, Emacs 23.2.1 -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: https://admin.fedoraproject.org/mailman/listinfo/users Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines
