42 lines
1.6 KiB
Markdown
42 lines
1.6 KiB
Markdown
# lec11
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At this point I'l mention that just reading isn't going to get you anywhere, you have to try things, and give it a real earnest attempt.
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__ALU:__ Arithmetic Logic Unit
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## Building a 1-bit ALU
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First we'll create an example _ALU_ which implements choosing between an `and`, `or`, `xor`, or `add`.
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Whether or not our amazing _ALU_ is useful doesn't matter so we'll go one function at a time(besides `and/or`).
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First recognize that we need to choose between `and` or `or` against our two inputs A/B.
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This means we have two inputs and/or, and we need to select between them.
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_Try to do this on your own first!_
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Next we'll add on the `xor`.
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AGAIN: try to do this on your own, the main hint I'll give here is: the current mux needs to be changed.
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Finally we'll add the ability to add and subtract.
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You may have also noted that we can subtract two things to see if they are the same dhowever, we can also `not` the result of the `xor` and get the same result.
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At this point our _ALU_ can `and`, `or`, `xor`, and `add`/`sub`.
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The mux will choose one which logic block to use; the carry-in line will tell the `add` logic block whether to add or subtract.
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Finally the A-invert and B-invert line allow us to determine if we want to invert either A or B (inputs).
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## N-bit ALU
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For sanity we'll use the following block for our new ALU.
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Note that we are chaining the carry-in's to the carry-out's just like a ripple adder.
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also each ALU just works with `1` bit from our given 4-bit input.
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