# 1240893538 - Overview of Lecture Binary number...

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Computer Organization CDA 3103 Dr. Hassan Foroosh Dept. of Computer Science UCF © Copyright Hassan Foroosh 2004 Overview of Lecture ± Binary number representation and 2’s complement ± Addition in 2’s complement ± overflow ± ALU and adder ± full adder ± logical operations ± carry lookahead ± Multiplication Computer System Organization Processor Computer Control Datapath Memory Devices Input Output Arithmetic is here Arithmetic is here Binary Binary Decimal 0 0000 1 0001 2 0010 3 0011 Decimal 4 0100 5 0101 6 0110 7 0111 0011 0010 + 0101 1 + 0110 11 Introduction to Binary Numbers ± Value of digit i (LSB is digit 0) = digit x 2 i ± Addition examples: 3 + 2 = 5 3 + 3 = 6 ± Consider a 4-bit unsigned binary number

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2’s Complement Binary Decimal 0 0000 1 0001 2 0010 3 0011 0000 1111 1110 1101 Decimal 0 -1 -2 -3 Bitwise Inverse 1111 1110 1101 1100 4 0100 5 0101 6 0110 7 0111 1100 1011 1010 1001 -4 -5 -6 -7 1011 1010 1001 1000 1000 -8 0111 8 1000 Not a Positive Number! Two’s Complement Representation ± 2’s complement representation of negative numbers ± Bitwise inverse and add 1 ± MSB is always “1” for negative number => sign bit ± Biggest 4-bit number : 7 (2 n–1 –1) ± Smallest 4-bit number: -8 (–2 n-1 ) Two’s Complement ± Value of number (assuming 32-bits = b 31 b 30 .…b 0 ): -b 31 x 2 31 + b 30 x 2 30 +... +b 1 x 2 1 + b 0 x 2 0 ± 2’s complement ± to negate an n-bit number x : 2 n ± invert all bits and add 1 ± + ~ = -1 (represented by 11…1) => x + ~ 1 = 0 and therefore ~ + 1 = - ± Alternative integer representations ± sign-magnitude: sign bit + absolute value ± 1’s complement: invert each bit ± Disadvantages of alternatives ± +0, –0 ± arithmetic not as simple ± Advantage of alternatives ± max positive = max negative ± Two’s complement is the standard Two’s Complement Arithmetic ± Examples: 7 + (- 6) = 1 3 + (- 5) = -2 2’s Complement Binary Decimal 0 0000 1 0001 2 0010 3 0011 0000 1111 1110 1101 Decimal 0 -1 -2 -3 4 0100 5 0101 6 0110 7 0111 1100 1011 1010 1001 -4 -5 -6 -7 1000 -8 0111 1010 + 0001 1 0011 1011 + 1110 11 1 1 2’s Complement: Subtraction/Overflow ± Subtraction: add 2’s complement 5 – 7 = 5 +(–7) 7 – (–2) = 7 + (– – 2) = 7 + 2 ± Overflow => sum is too large to represent in precision ± Addition overflow ± sign of operands the same, and sign of result differs ± Subtraction overflow: A – B ± sign of operands different, and ± A positive: sign of result negative, or ± A negative: sign of result positive ± Interesting case: 0 – max negative
Requirements for MIPS Integer ALU ± ALU = Arithmetic Logical Unit ± implements integer arithmetic (2’s complement) and logical functions ± MIPS instructions (representative subset) ± add, subtract ± AND, OR ± set-less-than ± overflow detection ± Zero detect: useful for branches: ± beq = subtract followed by 0 test ± ALU = basic logic + control ± control actually determines what the ALU does ± Also need multiply, divide, shift (not covered) Functional Specification of the ALU ± ALU Control Lines (ALUop) Function 000 And 001 Or 010 Add 110 Subtract 111 Set-on-less-than ALU N N N A B Result Overflow Zero 3 ALUop COut A One-Bit ALU ± This 1-bit ALU will perform AND, OR, and ADD A B 1-bit Full Adder COut CIn Mux Result Full adder Inputs: Ai, Bi, Carry in Outputs: Sum, Carry out ALUop Select

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## 1240893538 - Overview of Lecture Binary number...

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