Characteristics of Computer Memory

The key characteristics of computer memory systems are listed below.

Location
Capacity
Unit of Transfer
Performance
Access method
Physical type & characteristics

The internal memory capacity is expressed in terms of bytes or words whereas the external memory capacity is expressed in bytes.

The size of a word is equal to the number of bits used to represent an integer and the instruction length.

The relationship between the length in bits A of an address and the number N of addressable units is 2^A = N.

The Unit of transfer is the number of bits read out of or written into memory at a time for main memory.

The important performance parameters are access time, memory cycle time and transfer rate.

The access time is the time it takes to perform a read or write operation for random access memory and time it takes to position the read–write mechanism at the desired location for non random access memory.

The access time plus any additional time required before a second access can commence is memory cycle time. The memory cycle time is concerned with the system bus, not the processor.

The transfer rate is the rate at which data can be transferred into or out of a memory unit.

For random access memory,

Transfer time = 1/Cycle Time

For non random access memory,

where

T_n - Average time to read or write n bits

T_A - Average access time

n - No of bits

R - Transfer rate in bps

The relation between capacity, access time and cost of a memory is

Faster access time, High cost per bit
Larger capacity, Low cost per bit
Larger capacity, slower access time

Gaussian Elimination - Row reduction Algorithm

Gaussian elimination is a method for solving matrix equations of the form, Ax=b. This method is also known as the row reduction algorithm. Back Substitution Solving the last equation for the variable and then work backward into the first equation to solve it. The fundamental idea is to add multiples of one equation to the others in order to eliminate a variable and to continue this process until only one variable is left. Pivot row The row that is used to perform elimination of a variable from other rows is called the pivot row. Example: Solving a linear equation The augmented matrix for the above equation shall be The equation shall be solved using back substitution. The eliminating the first variable (x1) in the first row (Pivot row) by carrying out the row operation. As the second row become zero, the row will be shifted to bottom by carrying out partial pivoting. Now, the second variable (x2) shall be eliminated by carrying out the row operation again. ...

Decision Tree - Gini Index

The Gini index is used in CART. The Gini index measures the impurity of the data set, where p i - probability that data in the data set, D belong to class, C i and pi = |C i,D |/|D| There are 2 v - 2 possible ways to form two partitions of the data set, D based on a binary split on a attribute. Each of the possible binary splits of the attribute is considered. The subset that gives the minimum Gini index is selected as the splitting subset for discrete valued attribute. The degree of Gini index varies between 0 and 1. The value 0 denotes that all elements belong to a certain class or if there exists only one class, and the value 1 denotes that the elements are randomly distributed across various classes. A Gini Index of 0.5 denotes equally distributed elements into some classes. The Gini index is biased toward multivalued attributes and has difficulty when the number of classes is large.

Data Cleaning Process

Data cleaning attempt to fill in missing values, smooth out noise while identifying outliers, and correct inconsistencies in the data. Data cleaning is performed as an iterative two-step process consisting of discrepancy detection and data transformation. The missing values of the attribute can be addressed by Ignoring the value filling the value manually Using global constant to fill the value Using a measure of central tendency (mean or median) of value Using attribute mean or median belonging to same class Using the most probable value Noise is a random error or variance in a measured variable. The noisy data can be smoothened using following techniques. Binning methods smooth a sorted data value by consulting the nearby values around it. smoothing by bin means - each value in a bin is replaced by the mean value of the bin. smoothing by bin medians - each bin value is replaced by the bin median smoothing by bin boundaries - the minimum and maximum values in a ...

The Data Ilm

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