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4.1.1

Binary addition has four basic rules, the first 3 of which are pretty obvious: 0 + 0 = 0 Example 1:
If you are adding in columns and you have to do 1 + 1 + carry then the result will be 1 carry 1 to the next col umn:
Confirm that these additions are correct by converting the numbers to base 10. Try these:
What happened in the last example? This is known as "overflow ". Imagine now that the three examples given above are in two's complement form (the last value is the same magnitude but negative  ie 128). Convert to decimal and see what the examples tell you. Comment on the last example again. We conclude that when subtracting two numbers by adding a two's complement form positive number to a two's complement form negative number, that any overflow beyond the MSB can be ignored . This is not true, of course, for addition. Consider two hexadecimal numbers:
When we add the first column 9 + A we clearly go beyond F  the largest hex symbol we can have. I got three by starting my lefthand thumb at A and then counting round 9 fingers (not forgetting 0) to get the 3. Then of course we need to carry 1 (place value 16) to the next column. This is a bit tricky if you have to add, say C and D  you need more than 10 fingers (unless you can "wrap around" of course. The easiest way (I find) is to write down the numbers on a piece of paper (which you can do in the exam  remembering you won't have a calculator with you), and count around: For example to add C to D: 0 1 2 3 4 5 6 7 8 9 A B C D E F Now, count around 1 on D, 2 on E, 3 on F, 4 on 0 and so on, until you count to D; you should end up on 9.
Try these  no calculators allowed! (Except for your poor old teacher, that is):
related: [ Topic 4 home  next: real numbers ] 
The IB Exam does not allow calculators so you will have to learn these the way you did maths in elementary school. 


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