58 Chapter 2: Computer Organization and Architecture
Table 2.1 | Continued
When the system consists of multiple frequent cases,
where i is the number of frequent cases:
Holds an instruction
that is to be executed
Holds address of
instruction to be
Holds temporary data
Holds input character
Holds output character
Amdahl’s law focused on performance gain after enhancing the system. The performance gain is denoted by
Soverall and ET stands for execution time.
Performance of the system with enhancement
Performance of the system without enhancement
After enhancement, the system consists of two portions:
unenhanced and enhanced portion.
To calculate ETnew, the following two factors are needed:
1. Fractionenhance (F): It indicates how much portion of the old system undergoes enhancement.
2. Speedenhance (S): It indicates how many times the
new portion is running faster than the old portion.
1 ETnew F
1 ETold F
On the basis of the above factor,
Solution: Here S = 2, F = 0.1
2.3 MACHINE INSTRUCTIONS AND
Machine instruction is an individual machine code. The
complete set of all machine codes recognized by a particular processor makes its Instruction Set. Instructions can
be grouped according to the function they perform. The
number of ways by which arguments for these machine
instructions can be specified constitutes the addressing
modes for a processor.
2.3.1 Machine Instructions
An instruction is a command to the microprocessor to
perform a given task. Most computer instructions are
classified as follows:
ETnew = ET of the unenhanced portion
+ ET of enhanced portion
Problem 2.1: Consider a hypothetical processor used
in mathematical model simulation. It consists of two
functional units, floating point and integer. The floating point is enhanced then it runs two times faster,
but only 10% of the instructions are floating point.
What is the speed up?
Soverall = (1 − 0.1) +
2.2.2 Quantitative Principles to Design
Soverall = 1 − ∑ Fi + ∑
ETnew F =
1. Data transfer instructions: These instructions
move data from one place to another in the computer without changing the data content. Example:
LOAD, MOVE, IN, OUT, PUSH, STORE.
2. Data manipulation instructions: These instructions perform arithmetic, logical and shift operations on data. Example: ADD, SUB, MUL, DIV,
INC, AND, XOR, OR, SHR, SHL, ROR, ROL.
3. Program control instructions: These instructions may change the address value in program counter and cause the normal sequential flow to change.
On the basis of the number of address fields in an instruction, they are classified as follows:
Substitute the value of ET in Eq. (2.1):
ETnew F = ETold (1 − F ) +
Let ETold = 1,
1. Three-address instruction: Computer with
three-address instruction format can use each
address field to specify two sources and a destination, which can be either a processor register or a
memory operand. It results in short program but
requires too many bits to specify three addresses.
ETnew F = ETold (1 − F ) +
Chapter 2.indd 58
= (1 − F ) +
(1 − F ) + (F S )
Example: ADD R1, A, B
(R1 ← M[A] + M[B])
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