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Insights Why Can't My Computer Do Simple Arithmetic? - Comments

  1. Jan 29, 2016 #1

    Mark44

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  2. jcsd
  3. Jan 29, 2016 #2

    BvU

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    Last edited: Jan 29, 2016
  4. Jan 29, 2016 #3
    I learned a lot with this Insight!
     
  5. Jan 29, 2016 #4
    ##6.022 × 10^{23}## right? I fixed it.
     
  6. Jan 29, 2016 #5
    The same problem happens in decimal, for example if you used 1/3 instead. You could use rational numbers instead but this is why you should not do if (a == 1.0) because it may be pretty close to 1.0 but not actually 1. So you need to do if (abs(a - 1.0) < 0.00001), I wish there was an approximately equals function built in. The other problem with floats is that once you get above 16 million you can only store even values, then multiples of 4 past 32 million, etc.
     
  7. Jan 29, 2016 #6
    Great!! Just learnt something new today. Thanks for sharing!!
     
  8. Jan 29, 2016 #7

    anorlunda

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    Good Insight. It brings back memories of the good old days (bad old days?) when we had no printers, screens or keyboards. I had to learn to read and enter 24 bit floating point numbers in binary, using 24 little lights and 24 buttons. It was made easier by the fact that almost all the numbers were close to 1.0.
     
  9. Jan 29, 2016 #8

    jim mcnamara

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    Great post, Mark. I was thinking of doing this very thing. Saved me from making a noodle of myself.... Thanks for that. FWIW decimal libraries from Intel correctly do floating point decimal arithmetic. The z9 power PC also has a chip that supports IEEE-754-2008 (decimal floating point) as does the new Fujitsu SPARC64 M10 with "software on a chip". Cool idea.
    Cool post, too.
     
  10. Jan 29, 2016 #9
    Last edited by a moderator: Jan 29, 2016
  11. Jan 29, 2016 #10

    Mark44

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    The first programming class I took was in 1972, using a language named PL-C. I think the 'C' meant it was a compact subset of PL-1. Anyway, you wrote your program and then used a keypunch machine to punch holes in a Hollerith (AKA 'IBM') card for each line of code, and added a few extra cards for the job control language (JCL). Then you would drop your card deck in a box, and one of the computer techs would eventually put all the card decks into a card reader to be transcribed onto a tape that would then be mounted on the IBM mainframe computer. Turnaround was usually a day, and most of my programs came back (on 17" wide fanfold paper) with several pages of what to me was gibberish, a core dump, as my program didn't work right. With alll this rigamarole, programming didn't hold much interest for me.

    Even as late as 1980, when I took a class in Fortran at the Univ. of Washington, we were still using keypunch machines. It was when personal computers started to really get big, and compilers could compile and run your program in about a minute or so, that I could see there was somthing to this programming thing.
    Thanks, Jim, glad you enjoyed it. Do you have any links to the Intel libraries, or a name? I'd like to look into that more.
     
  12. Jan 29, 2016 #11
    I certainly remember the days when you'd expect an integer and get "12.00000001". I think Algodoo still has that "glitch"! I can't even remember if my first computer had floating point operations in the CPU, it was a Tandy (radio shack) color computer 2 64kb 8086 1 Mhz processor. When I graduated to 80386 the floating point wasn't precise enough so I devised a method to use 2 bytes for the interger (+/- 32,767) and 2 bytes for the fractions (1/65,536ths). It was limited in flexibility but exact and quite fast!
     
  13. Jan 29, 2016 #12
    My first computer was so slow I would crunch the code for routines in machine code in my head and just type a data string of values to poke into memory through basic because even the compiler was slow and buggy... its the object oriented programming platforms that was when I really saw the "next level something" for programming.
     
  14. Jan 29, 2016 #13

    Mark44

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    I don't think it had an Intel 8086 cpu. According to this wiki article, https://en.wikipedia.org/wiki/TRS-80_Color_Computer#Color_Computer_2_.281983.E2.80.931986.29, the Coco 2 had a Motorola MC6809 processor. I'm 99% sure it didn't have hardware support for floating point operations.
    You must have had the optional Intel 80387 math processing unit or one of its competitors (Cyrix and another I can't remember). The 80386 didn't have any hardware floating point instructions.
     
  15. Jan 29, 2016 #14
    That's what I thought at first 68B09E was the coco3 was the last one now that I remember, second guessed myself.
    It was a pentium whatever number I forget but I was only programming up to the 386 instruction set at that time... I printed out the instruction set on that same old fanfold paper stack and just started coding.
     
  16. Jan 29, 2016 #15

    Mark44

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    The Pentium would have been 80586, but I guess the marketing people got involved and changed to Pentium. If it was the first model, that was the one that had the division problem where some divisions gave an incorrect answer out in the 6th or so decimal place. It cost Intel about $1 billion to recall and replace those bad chips. I think that was in '94, not sure.
     
  17. Jan 29, 2016 #16
    What C will do with that is do the computations as doubles but store the result in a float. Floats save no time. They are an old-fashioned thing in there to save space.

    Numerical analysis programmers learn to be experts in dealing with floating point roundoff error. Hey, you can't expect a computer to store an infinite series, which is the definition of a real number.

    There are packages that do calculations with rational numbers, but these are too slow for number crunching.
     
  18. Jan 29, 2016 #17
    I think that was about the same time, I think it was a pentium 1, 25 Mhz intel chip with no math co-pro but it had a spot on the board for it. Perhaps it was just a 80486... can't even remember the computer model!
     
    Last edited: Jan 29, 2016
  19. Jan 29, 2016 #18
    Found this on wikipedia: "The i486 does not have the usual 80-prefix because of a court ruling that prohibits trademarking numbers (such as 80486). Later, with the introduction of the Pentium brand, Intel began branding its chips with words rather than numbers."
     
  20. Jan 29, 2016 #19

    jim mcnamara

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    Last edited: Jan 30, 2016
  21. Jan 29, 2016 #20

    jim mcnamara

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    As an addendum - somebody may want to consider how to compare as equal 2 floating point numbers. Not just FLT_EPSILON or DBL_EPSILON which are nothing like a general solution.
     
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