Exam1 Review Dr. Bernard Chen Ph.D. University of Central Arkansas

Computer Science at a Crossroads “Power wall” Triple hurdles of maximum power dissipation of air- cooled chips “ILP wall” Little instruction-level parallelism left to exploit efficiently “Memory wall” Almost unchanged memory latency

Computer Science at a Crossroads Old Conventional Wisdom : Uniprocessor performance 2X / 1.5 yrs New Conventional Wisdom : Power Wall + ILP Wall + Memory Wall = Brick Wall Uniprocessor performance now 2X / 5(?) yrs

Computer Science at a Crossroads

Defining Computer Architecture The task of computer designer: Determine what attributes are important for a new computer, then design a computer to maximize performance while staying within cost, power, and availability constrains

Defining Computer Architecture In the past, the term computer architecture often referred only to instruction set design Other aspects of computer design were called implementation, often assuming that implementation is uninteresting or less challenging Of course, it is wrong for today’s trend Architect’s job much more than instruction set design; technical hurdles today more challenging than those in instruction set design

Instruction Set Architecture (ISA) The instruction set architecture serves as the boundary between the software and hardware. We will have a complete introduction to this part. (Some examples in the next two slides)

Outline Decoder Encoder MUX

2-to-4 Decoder

Decoder Expansion

How about 4-16 decoder Use how many 3-8 decoder? Use how many 2-4 decoder?

Encoders Perform the inverse operation of a decoder 2 n (or less) input lines and n output lines

Encoders

Accepts multiple values and encodes them Works when more than one input is active Consists of: Inputs (2 n ) Outputs when more than one output is active, sets output to correspond to highest input V (indicates whether any of the inputs are active) Selectors / Enable (active high or active low) Priority Encoder

4 to 1 line multiplexer S1S1 S0S0 F 00I0 01I1 10I2 11I3 4 to 1 line multiplexer 2 n MUX to 1 n for this MUX is 2 This means 2 selection lines s 0 and s 1

Outline Data Representation Compliments

Conversion Between Number Bases Decimal(base 10) Octal(base 8) Binary(base 2) Hexadecimal (base16) °We normally convert to base 10 because we are naturally used to the decimal number system. °We can also convert to other number systems

Example Convert to a. octal number b. hexadecimal number a. Each 3 bits are converted to octal : (101) (011) (110) (110) (011)      = (53663) 8 b. Each 4 bits are converted to hexadecimal: (0101) (0111) (1011) (0011)     5 7 B = (57B3) 16 Conversion from binary to hexadecimal is similar except that the bits divided into groups of four.

Subtraction using addition Conventional addition (using carry) is easily implemented in digital computers. However; subtraction by borrowing is difficult and inefficient for digital computers. Much more efficient to implement subtraction using ADDITION OF the COMPLEMENTS of numbers.

Complements of numbers (r-1 )’s Complement Given a number N in base r having n digits, the (r- 1)’s complement of N is defined as (r n - 1) - N For decimal numbers the base or r = 10 and r- 1= 9, so the 9’s complement of N is (10 n -1)-N 99999……. - N Digit n Digit n-1 Next digit First digit

l’s complement For binary numbers, r = 2 and r — 1 = 1, r-1’s complement is the l’s complement. The l’s complement of N is (2^n- 1) - N. Digit n Digit n-1 Next digit First digit Bit n-1Bit n-2…….Bit 1Bit 0 -

r’s Complement Given a number N in base r having n digits, the r’s complement of N is defined as r n - N. For decimal numbers the base or r = 10, so the 10’s complement of N is 10 n -N ……. - N Digit n Digit n-1 Next digit First digit

10’s complement Examples Find the 10’s complement of and The 10’s complement of is = and the 10’s complement of is = Notice that it is the same as 9’s complement

For binary numbers, r = 2, r’s complement is the 2’s complement. The 2’s complement of N is 2 n - N. 2’s complement Digit n Digit n-1 Next digit First digit

Subtraction of Unsigned Numbers using r’s complement (1) if M  N, ignore the carry without taking complement of sum. (2) if M < N, take the r’s complement of sum and place negative sign in front of sum. The answer is negative.

Subtract by Summation Subtraction with complement is done with binary numbers in a similar way. Using two binary numbers X= and Y= We perform X-Y and Y-X

X-Y X= ’s com. of Y= Sum= Answer=

Y-X Y= ’s com. of X= Sum= There’s no end carry: answer is negative (2’s complement of )

How To Represent Signed Numbers Plus and minus signs used for decimal numbers: 25 (or +25), -16, etc.. For computers, it is desirable to represent everything as bits. Three types of signed binary number representations: 1. signed magnitude, 2. 1’s complement, and 3. 2’s complement

1. signed magnitude In each case: left-most bit indicates sign: positive (0) or negative (1). Consider 1. signed magnitude: = Sign bitMagnitude = Sign bitMagnitude

2. One’s Complement Representation The one’s complement of a binary number involves inverting all bits. To find negative of 1’s complement number take the 1’s complement of whole number including the sign bit.To find negative of 1’s complement number take the 1’s complement of whole number including the sign bit = Sign bitMagnitude = Sign bit1’complement

3. Two’s Complement Representation The two’s complement of a binary number involves inverting all bits and adding 1. To find the negative of a signed number take the 2’s the 2’s complement of the positive number including the sign bit = Sign bitMagnitude = Sign bit2’s complement

The rule for addition is add the two numbers, including their sign bits, and discard any carry out of the sign (leftmost) bit position. Numerical examples for addition are shown below. Example: In each of the four cases, the operation performed is always addition, including the sign bits. Only one rule for addition, no separate treatment of subtraction. Negative numbers are always represented in 2’s complement. Sign addition in 2’s complement

Overflow Overflow example: = = An overflow may occur if the two numbers added are both either positive or negative.

BINARY ADDER-SUBTRACTOR

Example Extend the previous logic circuit to accommodate XNOR, NAND, NOR, and the complement of the second input. S2S1S0OutputOperation 000 X  Y AND 001 X  Y OR 010 X  Y XOR 011AComplement A 100 (X  Y) NAND 101 (X  Y) NOR 110 (X  Y) XNOR 111 BComplement B

Shift Microoperations Symbolic designation Description R ← shl R Shift-left register R R ← shr R Shift-right register R R ← cil R Circular shift-left register R R ← cir R Circular shift-right register R R ← ashl R Arithmetic shift-left R R ← ashr R Arithmetic shift-right R TABLE 4-7. Shift Microoperations

Logical Shift Example 1. Logical shift: Transfers 0 through the serial input. R1  shl R1 Logical shift-left R2  shr R2 Logical shift-right (Example) Logical shift-left  (Example) Logical shift-right 

Circular Shift Example Circular shift-left Circular shift- right (Example) Circular shift-left is shifted to (Example) Circular shift-right is shifted to

Arithmetic Shift Right Arithmetic Shift Right : Example (4)  0010 (2) Example (-6)  1101 (-3)

Arithmetic Shift Left Arithmetic Shift Left : Example (2)  0100 (4) Example (-2)  1100 (-4) Arithmetic Shift Left : Example (4)  1000 (overflow) Example (-6)  0100 (overflow)