Charles Kime & Thomas Kaminski © 2008 Pearson Education, Inc. (Hyperlinks are active in View Show mode) Chapter 4 – Arithmetic Functions Logic and Computer Design Fundamentals

Chapter 4 2 Overview Iterative combinational circuits Binary adders Half and full adders Ripple carry adders Binary subtraction Signed binary numbers Signed binary addition and subtraction Binary adder- subtractor Overflow Binary multiplication Other arithmetic functions Design by contraction

Chapter 4 3 Iterative Combinational Circuits Arithmetic functions Operate on binary vectors Use the same subfunction in each bit position Can design functional block for subfunction and repeat to obtain functional block for overall function Cell - subfunction block Iterative array - a array of interconnected cells An iterative array can be in a single dimension (1D) or multiple dimensions

Chapter 4 4 Block Diagram of a 1D Iterative Array Example: n = 32 Number of inputs = ? Truth table rows = ? Equations with up to ? input variables Equations with huge number of terms Design impractical! Iterative array takes advantage of the regularity to make design feasible

Chapter 4 5 Functional Blocks: Addition Binary addition used frequently Addition Development: Half-Adder (HA), a 2-input bit-wise addition functional block, Full-Adder (FA), a 3-input bit-wise addition functional block, Ripple Carry Adder, an iterative array to perform binary addition.

Chapter 4 6 Functional Block: Half-Adder A 2-input, 1-bit width binary adder that performs the following computations: A half adder adds two bits to produce a two-bit sum The sum is expressed as a sum bit, S and a carry bit, C The half adder can be specified as a truth table for S and C X Y C S X Y C S

The K-Map for S, C is given below This is a pretty trivial map! Chapter 4 7 Logic Simplification: Half-Adder Y X S Y X C YXYXYXS YXC

Chapter 4 8 Five Different Implementations for Half-Adder We can derive following sets of equations for a half- adder: (a), (b), and (e) are SOP, POS, and XOR implementations for S. In (c), the C function is used as a term in the AND- NOR implementation of S, and in (d), the function is used in a POS term for S. YXC )(S)c( YXC )YX()YX(S)b( YXC YXYXS)a( YX C YXC YXS)e( )YX(C C)YX(S)d( C

Chapter 4 9 Implementations: Half-Adder The most common half adder implementation is: (e) A NAND only implementation is: YXC YXS )(C C)YX(S )YX( X Y C S X Y C S

Chapter 4 10 Functional Block: Full-Adder A full adder is similar to a half adder, but includes a carry-in bit from lower stages. Like the half-adder, it computes a sum bit, S and a carry bit, C. For a carry-in (Z) of 0, it is the same as the half-adder: For a carry- in (Z) of 1: Z0000 X Y C S Z1111 X Y C S

Chapter 4 11 Logic Optimization: Full-Adder Full-Adder Truth Table: Full-Adder K-Map: XYZCS X Y Z S X Y Z C

Chapter 4 12 Equations: Full-Adder From the K-Map, we get: The S function is the three-bit XOR function (Odd Function): The Carry bit C is 1 if both X and Y are 1 (the sum is 2), or if the sum is 1 and a carry-in (Z) occurs. ZYZXYXC ZYXZYXZYXZYXS ZYXS

Chapter 4 13 Implementation: Full Adder Full Adder Schematic

Chapter 4 14 Binary Adders To add multiple operands, we bundle logical signals together into vectors and use functional blocks that operate on the vectors Example: 4-bit ripple carry adder: Adds input vectors A(3:0) and B(3:0) to get a sum vector S(3:0) Note: carry out of cell i becomes carry in of cell i + 1 Description Subscript Name Carry In CiCi Augend AiAi Addend BiBi Sum SiSi Carry out C i+1

Chapter bit Ripple-Carry Binary Adder A four-bit Ripple Carry Adder made from four 1-bit Full Adders: C0 is assumed to be zero, or we can use a half adder for A 0 and B 0

Chapter 4 16 Binary Subtraction: Unsigned Numbers Unsigned numbers are all positive integers including zero Algorithm: Subtract the subtrahend N from the minuend M; result is n bits If no end borrow occurs, then M N, and the result is a non- negative number and correct. If an end borrow occurs, the N > M and the difference M N + 2 n is subtracted from 2 n, and a minus sign is appended to the result. Examples: 0 1 End Borrow ( ) 0011

Chapter 4 17 Binary Subtraction: Complements Two complements: Diminished Radix Complement of N (r 1)s complement for radix r 1s complement for radix 2 Defined as (r n Radix Complement rs complement for radix r 2s complement in binary Defined as r n N n is the number of digits in the number. The use of complements is very useful in implementing subtraction in computers.

Chapter 4 18 Binary 1's Complement For r = 2, N = , n = 8 (8 digits): (r n – 1) = = or The 1's complement of is then: – Since the 2 n – 1 factor consists of all 1's and since 1 – 0 = 1 and 1 – 1 = 0, the one's complement is obtained by complementing each individual bit (bitwise NOT).

Chapter 4 19 Binary 2's Complement For r = 2, N = , n = 8 (8 digits), we have: (r n ) = or The 2's complement of is then: – Note the result is the 1's complement plus 1, a fact that can be used in designing hardware

Chapter 4 20 Alternate 2s Complement Method Given: an n-bit binary number, beginning at the least significant bit and proceeding upward: Copy all least significant 0s Copy the first 1 Complement all bits thereafter. 2s Complement Example: Copy underlined bits: 100 and complement bits to the left:

Chapter 4 21 Unsigned Subtraction with 2s Compleme nt For n-digit, unsigned numbers M and N, find M N in base 2: Add the 2's complement of the subtrahend N to the minuend M: M + (2 n N) = M N + 2 n If M N, the sum produces end carry 2 n which is discarded; from above, M N remains. If M < N, the sum does not produce an end carry and, from above, is equal to 2 n ( N M ), the 2's complement of ( N M ). To obtain the result (N – M), take the 2's complement of the sum and place a to its left.

Chapter 4 22 Unsigned 2s Complement Subtraction Example 1 Find – – The carry of 1 indicates that no correction of the result is required. 1 2s comp

Chapter 4 23 Unsigned 2s Complement Subtraction Example 2 Find – – The carry of 0 indicates that a correction of the result is required. Result = – ( ) 0 2s comp

Chapter 4 24 Signed Integers Positive numbers and zero can be represented by unsigned n-digit, radix r numbers. We need a representation for negative numbers. To represent a sign (+ or –) we need exactly one more bit of information (1 binary digit gives 2 1 = 2 elements which is exactly what is needed). Since computers use binary numbers, by convention, the most significant bit is interpreted as a sign bit: s a n–2 a 2 a 1 a 0 where: s = 0 for Positive numbers s = 1 for Negative numbers and a i = 0 or 1 represent the magnitude in some form.

Chapter 4 25 Signed Integer Representations Signed-Magnitude – here the n – 1 digits are interpreted as a positive magnitude. Signed-Complement – here the digits are interpreted as the rest of the complement of the number. There are two possibilities here: Signed 1's Complement Uses 1's Complement Arithmetic Signed 2's Complement Uses 2's Complement Arithmetic

Chapter 4 26 Signed Integer Representation Example r =2, n=3 We need a sign bit with 1s and 2s complements to distinguish between positive and negative numbers.

Chapter 4 27 Signed 2s Complement Arithmetic Addition: 1. Add the numbers including the sign bits, discarding a carry out of the sign bits 2. If the sign bits were the same for both numbers and the sign of the result is different, an overflow has occurred. 3. The sign of the result is computed in step 1. Subtraction: Form the complement of the number you are subtracting and follow the rules for addition.

Chapter 4 28 Example 1: Example 2: Signed 2s Complement Examples

Chapter s Complement Adder/Subtractor Subtraction can be done by addition of the 2's Complement. 1. Complement each bit (1's Complement.) 2. Add 1 to the result. The circuit shown computes A + B and A – B: For S = 1, subtract, the 2s complement of B is formed by using XORs to form the 1s comp and adding the 1 applied to C 0. For S = 0, add, B is passed through unchanged

Chapter 4 30 Overflow Detection Overflow occurs if n + 1 bits are required to contain the result from an n-bit addition or subtraction Overflow can occur for: Addition of two operands with the same sign Subtraction of operands with different signs 1 Signed number overflow cases with correct result sign Detection can be performed by examining the result signs which should match the signs of the top operand

Chapter 4 31 Overflow Detection Signed number cases with carries C n and C n shown for correct result signs: Signed number cases with carries shown for erroneous result signs (indicating overflow): Simplest way to implement overflow V = C n C n -

Chapter 4 32 Other Arithmetic Functions Convenient to design the functional blocks by contraction - removal of redundancy from circuit to which input fixing has been applied Functions Incrementing Decrementing Multiplication by Constant Zero Fill and Extension

Chapter 4 33 Design by Contraction Contraction is a technique for simplifying the logic in a functional block to implement a different function The new function must be realizable from the original function by applying rudimentary functions to its inputs Contraction is treated here only for application of 0s and 1s (not for X and X) After application of 0s and 1s, equations or the logic diagram are simplified by using rules given on pages of the text.

Chapter 4 34 Design by Contraction Example Contraction of a ripple carry adder to incrementer for n = 3 Set B = 001 The middle cell can be repeated to make an incrementer with n > 3.

Chapter 4 35 Incrementing & Decrementing Incrementing Adding a fixed value to an arithmetic variable Fixed value is often 1, called counting (up) Examples: A + 1, B + 4 Functional block is called incrementer Decrementing Subtracting a fixed value from an arithmetic variable Fixed value is often 1, called counting (down) Examples: A 1, B 4 Functional block is called decrementer

Chapter 4 36 Multiplication/Division by 2 n (a) Multiplication by 100 Shift left by 2 (b) Division by 100 Shift right by 2 Remainder preserved B 0 B 1 B 2 B 3 C 0 C 1 00 C 2 C 3 C 4 C 5 (a) B 0 B 1 B 2 B 3 C 0 C 2 1 C 2 2 C 1 C 2 00 C 3 (b)

Chapter 4 37 Multiplication by a Constant Multiplication of B(3:0) by 101 B 1 B 2 B B 0 B 1 B 2 B 3 Carry output 4-bit Adder Sum B 0 C 0 C 1 C 2 C 3 C 4 C 5 C 6

Chapter 4 38 Zero Fill Zero fill - filling an m-bit operand with 0s to become an n-bit operand with n > m Filling usually is applied to the MSB end of the operand, but can also be done on the LSB end Example: filled to 16 bits MSB end: LSB end:

Chapter 4 39 Extension Extension - increase in the number of bits at the MSB end of an operand by using a complement representation Copies the MSB of the operand into the new positions Positive operand example extended to 16 bits: Negative operand example extended to 16 bits: