How Computers Calculate – the ALU: Crash Course Computer Science #5

How Computers Calculate – the ALU: Crash Course Computer Science #5

Take the 2017 PBS Digital Studios Survey: http://surveymonkey.com/r/pbsds2017. Today we’re going to talk about a fundamental part of all modern computers. The thing that basically everything else uses – the Arithmetic and Logic Unit (or the ALU). The ALU may not have to most exciting name, but it is the mathematical brain of a computer and is responsible for all the calculations your computer does! And it’s actually not that complicated. So today we’re going to use the binary and logic gates we learned in previous episodes to build one from scratch, and then we’ll use our newly minted ALU when we construct the heart of a computer, the CPU, in episode 7.

*CORRECTION*

We got our wires crossed with the Intel 4004, which we discuss later. The 74181 was introduced by Texas Instruments in 1970 but appeared in technical manuals around 1969. The design of the 74181, like most of the 74xx/74xxx series, was an open design which was manufactured by many other companies – Fairchild was one such manufacturer. They produced a chip, the Fairchild 9341, which was pin-for-pin compatible with the 74181.

Fairchild was the first to prototype an ALU, building the Fairchild 4711 in 1968 – a one-off device not optimized for scale manufacturing. In 1969, Signetics came out with the 8260, which they marketed in a very limited sense (it was attached, AFAICT, to one particular computer, the Data General SUPERNOVA). TI follows afterwards (March 1970) with the 74181, coupled with the 9341 from Fairchild. The 74181 became the standard number for this part, and was available from many manufacturers (back in those days, chip makers cross-licensed designs all over the place in order to provide assurance that their part could be sourced from multiple manufacturers).

Produced in collaboration with PBS Digital Studios: http://youtube.com/pbsdigitalstudios

The Latest from PBS Digital Studios: https://www.youtube.com/playlist?list…

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50 Comments

  1. Patrick Kim on October 11, 2019 at 3:05 pm

    fat head got liquidated again?????



  2. Mazapan Putrefacto on October 11, 2019 at 3:05 pm

    Damn as someone whos seeing this for the first time its really overwhelming, kinda discouraged to keep on learning at this point lol



  3. Taripar on October 11, 2019 at 3:05 pm

    I kinda wish this video had gone into more detail on the circuitry for the opcodes, as they’re hardwired individually for each CPU. I would’ve liked to see a schematic for how the opcode bits interact with the ALU (i.e. exactly HOW the opcode 1000 causes the chain reaction of switches to add the two 8-bit numbers vs. subtract/increment/etc.), as well as how the ALU calculates the negative flag (the Check Zero flag was mentioned and the carry bit is clearly the Overflow flag, but the negative one confuses me).

    Perhaps this will be answered in part later on, but I wanted more info on that bit (HA) specifically.



  4. Patrick Kim on October 11, 2019 at 3:06 pm

    GOT FATTER bobby? not showing ur face anymore? eating all that instant cup millioniare???



  5. Patrick Kim on October 11, 2019 at 3:07 pm

    anyone can make 10 predicitons….. stay silent mostly, and BAM when one is remotely close to what you mentioned. thats ur TA???



  6. Mohammad Mainuddin on October 11, 2019 at 3:07 pm

    waoo! What a great approaches for me as an apprentice learner. I am really impressed. Thanks for the nice contents for us.



  7. Patrick Kim on October 11, 2019 at 3:08 pm

    four, five, seven, 10-20 times!! your range is so fake LOL. just like u bought bitcoin 5, 8, 9 years ago. just make up and exxaggerate as you go Rob!!!



  8. Patrick Kim on October 11, 2019 at 3:08 pm

    flip a coin and gamble with your mom’s money, stop losing client money on leverage scammer



  9. 01001100 IVE on October 11, 2019 at 3:13 pm

    Me: It’s simple as 1+1!
    ALU: Is it really?



  10. Patrick Kim on October 11, 2019 at 3:13 pm

    cool, calm, collected to "fu** off liar!" LOL



  11. Justin McCown on October 11, 2019 at 3:14 pm

    CREEPA



  12. gdenn on October 11, 2019 at 3:15 pm

    amazing video



  13. Dapper Don on October 11, 2019 at 3:15 pm

    why cant computers just use the decimal system. this seems so complicated for no reason. like a temporary hotfix. can someone explain? imean cant we use different voltages for representing numbers 0-9?



  14. Michael Novello on October 11, 2019 at 3:16 pm

    I wish these videos were a little slower. They talk so quickly, and slowing the video down makes the sound distorted. It would be no matter for people who want to speed it up to just speed it up without distorting anything. Otherwise this video would get a thumbs up from me.



  15. Patrick Kim on October 11, 2019 at 3:17 pm

    u get 1 call remotely correctly, 50 bams! 1 bad call??? SILENCE lol



  16. Patrick Kim on October 11, 2019 at 3:17 pm

    ur like a lonely housewife that nags her husband on the side. "see, look" "bam" "i told u so" "u will need to follow us" "im gonna need a bam" u little cu**



  17. miguelfinland on October 11, 2019 at 3:18 pm

    Nice video, good content and teacher is very clever and nice (thanks ๐Ÿ™‚ but, like so often in nowdays, too fast speaking. Why is it so common in teaching videos today? Tip: It is not bad if you remember breathing and stops between thr sentences, and finally – we will not leave you if you give us short breaks. Stop running if you can walk! ๐Ÿ™‚



  18. Tomรกลก Heller on October 11, 2019 at 3:19 pm

    So..half adder, full adder, Blackadder



  19. david jaki on October 11, 2019 at 3:19 pm

    the street name of the ALU now is "the foxi"



  20. Humanitarian WATSON on October 11, 2019 at 3:23 pm

    Are you telling a story or teaching ? I’m confused



  21. christian37ism on October 11, 2019 at 3:23 pm

    Informative and thorough video, thank you for this



  22. Saurabhav on October 11, 2019 at 3:24 pm

    I was following along perfectly fine till this video lol



  23. markdelej on October 11, 2019 at 3:25 pm

    I am a biological computer which cannot understand how computers work, mind blown ๐Ÿคฏ



  24. Pen on October 11, 2019 at 3:26 pm

    omg thank you
    I was trying to build this to no success myself



  25. TheOriginalJoeBloggs on October 11, 2019 at 3:27 pm

    More girls should be like you ๐Ÿ˜‰



  26. Silentroar on October 11, 2019 at 3:28 pm

    How is an 8-bit ALU only twice as complex as a 4-bit ALU ๐Ÿ˜›



  27. Patrick Kim on October 11, 2019 at 3:28 pm

    hows that 10x taste FUND MANAGER LOL



  28. Patrick Kim on October 11, 2019 at 3:30 pm

    u bought bitcoin 9 years ago in… 2010 right bob??? right around the time you were doing PLASTIC MOLDING LOL



  29. Waqas Narang on October 11, 2019 at 3:34 pm

    This Video is GOLD. It should have been viewed billions of times. But then i think, the real number is always less.



  30. Patrick Kim on October 11, 2019 at 3:34 pm

    ITS SO pathetic how you hang on to every remote win. u have so few of those. "1 bad cal, 3 correct calls!!" (praying for customers) wen car payment???



  31. Zhaun Fouche on October 11, 2019 at 3:34 pm

    That moment when you explain the ‘8bit ripple carry adder’ it comes across as an "IF" formula within Microsoft Excel. Where you would use other formula within the "IF" to gain a specified response if the outcome were in fact true or false eg. =IF(A1=1,True,False) or further eg. =IF(A1=1,True,IF(B1=1,True,False)), using only 0 and 1 within the example.



  32. Cubebass on October 11, 2019 at 3:36 pm

    you know, I’m actually taking notes on this playlist.



  33. Smokey Brown on October 11, 2019 at 3:36 pm

    Aww man creeper



  34. Patrick Kim on October 11, 2019 at 3:39 pm

    time to meditate again scamming bobby!



  35. Patrick Kim on October 11, 2019 at 3:39 pm

    u hold on to that "may drop" for so long. right after you call a 10x….. SILENT about that call.



  36. Bryan Gui on October 11, 2019 at 3:40 pm

    ALU made in Malaysia… woo hoo!
    We were once the world largest producer of semiconductors before China took over.



  37. riiad on October 11, 2019 at 3:43 pm

    Still in the middle of your series. Still loving it. However, the choice of going up a level in terms of abstraction to explain the full adder was not helpful since as someone new to this, i had to stop and remind myself of how the half adder was made in the first place. You should have kept the logic gates visible to explain the full adder. When you just grasped some concept, you can’t instantly make it an abstraction and elaborate on it and add complexity.



  38. Sami Rahim on October 11, 2019 at 3:44 pm

    You are great teacher and good voice thank you very much



  39. ryans jewel on October 11, 2019 at 3:45 pm

    Thank you for all theses videos this helps my son a lot.



  40. Amanda Purello on October 11, 2019 at 3:46 pm

    aahhhh my brain is melting lol



  41. Patrick Kim on October 11, 2019 at 3:47 pm

    still bullish on COVA scammer???



  42. Patrick Kim on October 11, 2019 at 3:48 pm

    "i’m a dweeb" "BAM" "i’m a lil bit**" "BAM" ur such a fat little GURL



  43. Javier R on October 11, 2019 at 3:49 pm

    The image at 8:20 of the Intel 74181 seems to be a Texas Instruments chip.



  44. Patrick Kim on October 11, 2019 at 3:51 pm

    clients complaining about losses. thats all i see. never ever see clients saying they made money with you….. always complaining of being scammed and 10%, 20%, 80% losses!! LOL!!!! such big range master~~~ LOL



  45. azrul nizam on October 11, 2019 at 3:53 pm

    So all of this crazy transistors setup are added to the instruction set dictionary. How many thousand of words that they have created through x86. Are all of theme being used as often as the other? I wonder.



  46. Patrick Kim on October 11, 2019 at 3:53 pm

    any more excuse now scammer? long right? lol



  47. Bipin Panday on October 11, 2019 at 3:55 pm

    can you bind all the video series into one playlist?



  48. Boggless on October 11, 2019 at 3:58 pm

    I was building an ALU in minecraft and I ran into the problem that the carry was delayed. If I overflowed, or hit zero when subtracting, I would get a clock with the carry going through each adder. I’ve got to look into this carry lookahead adder.



  49. ื“ื•ื“ ื›ื”ืŸ on October 11, 2019 at 4:01 pm

    ื”ืชืจื’ื•ื ื‘ืขื‘ืจื™ืช ื˜ื•ื‘ ืื‘ืœ ืœื ืžื•ืฉืœื



  50. Caroline Nilsson on October 11, 2019 at 4:02 pm

    I watched this series when it first came out, before I started uni. I loved it. I was so exciting to learn about these magical things that goes on inside of computers. Now I just started my second year of computer science engineering, partly thanks to this course. It’s so wonderful to go back an rematch the episodes with a whole new understanding of the topics. This is truly a wonderfully produced series!