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Number Representation and Logic Design CS 3220 Fall 2014 Hadi Esmaeilzadeh Georgia Institute of Technology Some slides adopted from Prof. Milos Prvulovic

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Computing with digital technology Physical Layer – Low voltage (0 V) – High voltage (5 V, then 3.3 V, 1.1 V, and now is 0.9 V or lower) Abstraction (we do not deal with voltage levels) – ‘0’ – ‘1’ Groups of binary values construct words, numbers, pixels, audio signals, … 2

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What does this binary value represent? = 0100_ _ _ _1001 3

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What does this binary value represent? = 0100_ _ _ _1001 Hadi … 4

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Terminology Physical: – Bit: one binary digit Abstract: – Nibble: four binary digits – Byte: eight binary digits = two nibbles – Word: Usually 32 binary digits = 4 bytes 5

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Number Representation Positional notation Same as base-10 but now it’s base 2: – In base 10, we have 9807 = 9* * * *10 0 – In base 2, we have 1011 = 6

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Signed numbers Easy: one bit for sign, then absolute value – E.g (- 011) is actually -3 How do we add two such numbers? – First check the sign bits – If both are 1 or both 0, add the absolute values and retain the same sign bit – If one is 1 and one is 0, compare the two absolute values, then subtract the smaller from the larger and use the sign of the larger number Lots of circuitry needed for all this! Also we have to representation for zero: (-0, +0) We need a better way! 7

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Two’s complement OK, let’s say we want 4-bit signed numbers but – Want to just add the numbers as if they were unsigned – Want to quickly tell if number is positive or negative So if we add 1 to -1 we should get 0 – 0 is 0000, 1 is 0001, so -1 has to be 1111 Now, if we add 1 to -2, we should get -1 – So -2 has to be 1110 We can still tell if positive or negative But add, subtract, etc. is much simpler now 8

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Two’s complement 9 What is the range?

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Two’s complement Quickly negate a 2’s number: – 1011 – Invert(all bits) + ‘1’ – Start from right, copy until the first ‘1’, then invert the remaining Sign extension: – Store 1011 in a byte 10

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Note on Number Representation Digital logic still operates on binary signals 2’s complement vs. sign-and-value is all about how we choose to represent numbers using the underlying binary signals If four wires have values of 1, 0, 1, 1, then – If sign-and-value, it represents -3 – If 2’s complement, it is -5 – If unsigned number, it is 11 (eleven) – May not even be a number! 11

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Hexadecimal Notation Writing binary numbers is inconvenient – More than 3 times as many digit as decimal notation So we also use hexadecimal (base 16) notation – Fewer digits needed than in binary (or even decimal) – Each hex digit represents exactly 4 binary digits, so it is easy to convert back-and-forth – Example: Hexadecimal E04C is in binary: Note: no actual “hexadecimal” hardware – Hardware still operates in binary – Hexadecimal notation is only for our convenience 12

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Digital Logic Implemented using MOS transistors 13 P-type substrate N-type Source Gate Drain Channel - - -

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MOS transistor 14 +V

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Inverter (NOT gate) 15 +V

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NOR 16 +V

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NAND 17 +V

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How do we get AND and OR gates? 18

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XOR? 19

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Gates with more inputs 20 ?

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1-bit add 21 A B OUT Carry OUT

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Full Adder 22 A B OUT Carry OUTCarry IN

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Full adder 23 A B Cin S (Ouptut) A B Cout

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Full Adder Truth Table 24

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Full Adder Karnaugh Map 25

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3-bit add? 26 1-bit Full Adder Cin B0A0 S0 Cout 1-bit Full Adder Cin B1A1 S1 Cout 1-bit Full Adder Cin B2A2 S2 Cout Data dependence serializes the additions!

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Keeping State 27 We use latches and flip-flops – Here is an SR latch

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D latch 28 When E is 1 – When D=1, make the S signal be 0 (OUT -> 1) – When D=0, make the R signal be 0 (OUT -> 0) – When E is 0, both S and R are 1 (OUT unchanged) S R OUT D(ata) E(nable) D (inverted D input)

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Flip-Flop? Essentially two latches in series: Latch 1 has CLK connected to its “Enable” – Keeps latching changes in input value while clock is 0 – When clock becomes 0, it keeps what it had Latch 2 has CLK connected to its enable – Keep latching the output of Latch 1 while clock is 1 – Keeps same output value when clock is 0 While clock is 0, Latch 2 outputs the stored bit When clock becomes 1, Latch 2 outputs 29

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How the flip-flop works While clock is 0 – Output of Latch 1 follows the input – But Latch 2 outputs the stored bit When clock changes from 0 to 1 – Latch 1 stops following the input – Latch 2 now outputs what Latch 1 is outputting – Result: FF output == FF input when clock went 0->1 When clock changes back to 0 – Latch 1 starts following the input again – But Latch 2 now keeps what it had – Result: FF output unchanged until clock goes 0->1 30

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