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Objectives

- Know how numbers are represented in computers
- Be introduced to the circuits used to perform arithmetic operations
- Be aware of performance issues in large circuits
- Know VHDL to specify arithmetic circuits

Variables

- Variables to represent the state of a switch or other condition
- 010 convenient to think of as 2 but really means switches 1 and 3 off and switch 2 on
- Variables to represent numbers

Basic Concepts

- Bit – One Binary Digit
- Nibble – Four Bits
- Byte – Eight Bits
- LSB – Least Significant Bit
- MSB – Most Significant Bit
- Octal – 1101 1001 = 331 octal
- Hexadecimal 1101 1001 = D9 hex

1011 1001

Integers

- Unsigned
- Positional number representation
- Convert

positive integer decimal to

unsigned binary by

repetitively dividing by 2

- If remainder is 1 bit is 1
- Else bit is 0

Convert 857 to Binary

857/2 = 428 r1 1 LSB

428/2 = 214 r0 0

214/2 = 107 r0 0

107/2 = 53 r1 1

53/2 = 26 r1 1

26/2 = 13 r0 0

13/2 = 6 r1 1

6/2 = 3 r0 0

3/2 = 1 r1 1

1/2 = 0 r1 1 MSB

1101011001 base 2

Addition of Unsigned Binary

Carry 1 1 1 0

X = 0 1 1 1 1

Y = 0 1 0 1 0

Sum = 1 1 0 0 1

- Truth Table

A B Sum Carry

0 0 0 0

0 1 1 0

1 0 1 0

1 1 0 1

Design Logic to Add 2 5 bit Binary Numbers

- Truth Table
- x4 x3 x2 x1 x0 y4 y3 y2 y1 y0 Sum
- Method is to Consider each pair separately

XOR

2 Input Truth Table

A B XOR

0 0 0

0 1 1

1 0 1

1 1 0

- 3 Input Truth Table
- A B C XOR
- 0 0 0
- 0 0 1
- 0 1 0
- 0 1 1
- 1 0 0
- 1 0 1
- 1 1 0
- 1 1 1

0

1

1

0

1

0

0

1

XOR Gate

- Generates Modulo-2 sum of its inputs
- Output is equal to
- 1 if an odd number of inputs have the value of 1s
- 0 otherwise
- Sometimes referred to as the ODD function

Signed Binary Integers

- Sign and Magnitude
- MSB becomes the sign bit
- MSB of number is n-1
- Not well suited for use in binary computers

1’s Complement

- Negative numbers created by subtraction
- N bit negative number K is generated by subtracting its equivalent positive number P from 2n-1
- K = (2n-1) – P
- -5 1111 – 0101 = 1010
- -3 1111 – 0011 = 1100
- Basically complement the absolute value of the number
- Has some drawbacks too

2’s Complement

- Negative numbers created by subtracting from 2n Negative number K is obtained by subtracting its equivalent positive number P from 2n
- K = 2n – P
- -5 10000 – 0101 = 1011
- -3 10000 – 0011 = 1101
- Add 1 to number’s 1’s complement
- 2’s complement number are obtained in this manner
- Examine bits from right, copy all bits that are 0 and the first bit that is 1, complement the rest

2’s Complement

- - 5 0101 = 1011
- -3 0011 = 1101
- -6 0110 = 1010

Fixed Point

- B = bn-1bn-2…b1b0.b-1b-2…b-k
- 1011.101
- 1x23 + 0x22 + 1x21 + 1x20 + 1x2-1 + 0x2-2 + 1x2-3
- 8+2+1+0.5+0.125
- 11.625

S

E

M

Sign

8-bit

23 bits of mantissa

+

0 denotes

excess-127

–

1 denotes

exponent

(a) Single precision

64 bits

S

M

E

Sign

11-bit excess-1023

52 bits of mantissa

exponent

(b) Double precision

Floating Point- Sign, Mantissa, and Exponent

Single Precision Floating-Point Format

- Exponent in excess-127 format
- Exponent = E – 127
- Exponent always positive integer
- E = 0 is zero
- E = 255 is infinity
- Normal range of E –126 to 127 or an
- E of 1 to 254
- Mantissa field 23 bits
- Value +/-1.M x 2E-127

Single Precision Floating-Point Format

- 00101011101110011011110010011000
- 0 = Positive number
- 01010111 = 87 -> 87 - 127 = -40
- 01110011011110010011000 = 3783832
- 1.3783832 x 2-40

Double Precision Floating-Point Format

- Exponent in excess-1023 format
- Exponent = E – 1023
- Exponent always positive integer
- E = 0 is zero
- E = 2047 is infinity
- Normal range of E –1022 to 1023 or an
- E of 1 to 2046
- Mantissa field 52 bits
- Value +/-1.M x 2E-127

BCD – Binary Coded Decimal

- Only 0-9 are used
- 8 + 2 results in a carry out

USE ieee.std_logic_1164.all ;

USE ieee.std_logic_unsigned.all ;

ENTITY BCD IS

PORT ( X, Y : IN STD_LOGIC_VECTOR(3 DOWNTO 0) ;

S : OUT STD_LOGIC_VECTOR(4 DOWNTO 0) ) ;

END BCD ;

ARCHITECTURE Behavior OF BCD IS

SIGNAL Z : STD_LOGIC_VECTOR(4 DOWNTO 0) ;

SIGNAL Adjust : STD_LOGIC ;

BEGIN

Z <= (\'0\' & X) + Y ;

Adjust <= \'1\' WHEN Z > 9 ELSE \'0\' ;

S <= Z WHEN (Adjust = \'0\') ELSE Z + 6 ;

END Behavior ;

Parity

- Used for error checking
- Include extra bit called parity bit
- Even parity – parity bit is adjusted such that the number of 1s is even
- Odd parity – parity bit is adjusted such that the number of 1s is odd

Parity

- 1011 0110
- If
- Even parity, parity bit = 1
- Odd parity, parity bit = 0
- Transmitted string
- Even parity 1 1011 0110
- Odd parity 0 1011 0110

Overflow

- Result of arithmetic operation must fit in bits used to represent number if result does not fit an arithmetic overflow has occurred

No Overflow Overflow

0110 0110

+0010 +1010

1000 10000

Multiplication

- Binary multiplication by 2s is a shift left
- Other than 2s – Shift and Add
- Other techniques exist
- Consult V.C. Hamacher, Z.G. Vranesic and S.G. Zaky, Computer Organization, 5th ed. (McGraw-Hill: New York, 2002)

Numbers in VHDL Code

- SIGNAL C : STD_LOGIC_VECTOR(1 TO 3)
- MSB = C(1), LSB = C(3)
- C <= “100” then C(1) = 1, C(3) = 0
- Appropriate for signals grouped together not numbers
- SIGNAL Z : STD_LOGIC_VECTOR(1 DOWNTO 3)
- MSB = Z(3), LSB = Z(1)
- Z <= “100” then C(1) = 0, C(3) = 1

Arithmetic in VHDL

- SIGNAL X, Y, S : STD_LOGIC_VECTOR(15 DOWNTO 0);
- S <= X + Y REPRESENTS a 16-bit adder
- Must add
- USE ieee.std_logic_signed.all;

PORT ( Cin : IN STD_LOGIC;

X, Y : IN STD_LOGIC_VECTOR(15 DOWNTO 0);

S : OUT STD_LOGIC_VECTOR(15 DOWNTO 0);

Cout, Overflow : OUT STD_LOGIC

);

END adder16;

ARCHITECTURE Behavior OF adder16 IS

SIGNAL Sum: STD_LOGIC_VECTOR(16 DOWNTO 0);

BEGIN

Sum <= (‘0’ & X) + Y + Cin;

S <= Sum(15 DOWNTO 0);

Cout <= Sum(16);

Overflow <= Sum(16) XOR X(15) XOR Y(15) XOR Sum(15);

END Behavior;

Solution to S <= X + Y not including carry-in, carry-out, or overflow

& in VHDL means concatenate

One operand must have same number of bits as result – SIGNAL

Using only part of a variable S <= Sum(15 DOWNTO 0);

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