Other Kinds of Firewalls Packet lters are the crudest kind of rewall: they operate at the network level, and generally look only at TCP, UDP, and IP headers. One can also build rewalls that restrict trafc according to the contents of the data elds; these are known as application-layer rewalls, or application rewalls for short. Application rewalls have some security advantages, because they can enforce more restrictive security policies and because they can transform data on the y. We may come back to the topic of application rewalls later in the course. We've only scratched the surface of the topic in this lecture. For more information on rewalls, the authoritative reference is Cheswick, Bellovin, and Rubin: Firewalls and Internet Security: Repelling the Wily Hacker. Packet ltering software is available for many operating systems: e.g., Linux has iptables, OpenBSD/FreeBSD has PF, and Windows XP has its own rewall. 5 Pr
... Read more »
The Motivation for Firewalls Suppose you are given a machine, and asked to harden it against external attack. How do you do it? One starting point is to look at the network services that this machine is providing to the outside world. If any of its network services are buggy or have security holes, a hacker may be able to penetrate your machine by interacting with that application. As we know, bugs are inevitable, and bugs in security-critical applications often lead to security holes. Thus, the more network services your machine runs, the great
... Read more »
Registers versus Cache In order to devise a coordinated scheme for management of registers and cache, it is first necessary to develop a better understanding of the differences and similarities between these two types of buffer memory. 2.1. Registers 2.1.1. Concepts of Registers Registers, or a "register file”, constitute a relatively small, fast, local memory residing in an address -space distinguished memory. The structure of a is given in Figure 1. from that of main register memory cell name: Figure 1. Structure of Register Memory Cell Since registers are the absolute top of the memory hierarchy (typically with cache just below), register access time is the fastest of all memory systems in a computer and there are typically fewer memory cells in a register file than there are cells in any other level of the memory hierarchy. Each register is usually one word wide, with a total of perhaps 16
... Read more »
Abstract In current computer memory system hierarchy, registers and cache are both used to bridge the reference delay gap between the fast processor(s) and the slow main memory. While registers are managed by the compiler using program flow analysis, cache is mainly controlled by hardware without any program understanding. Due to the lack of coordination in managing these two memory structures, significant loss of system performance results because: 0 Cache space is wasted to hold inaccessible copies of values in registers. 0 Inaccessible copies of values replace those accessible ones from cache. 0 Despite the fact that register allocation has long recognized the benefits of live range analysis, current cache management has completely ignored live range information. In this paper, we propose an unified management of registers and cache using liveness and cache bypass. By using a single model to manage
... Read more »
Data Processing and Descriptive Analysis
I. Today’s class
--review procedures for data processing
--illustrate some of these issues, particularly missing data, with NELS 8th grade survey
--review of Teleform
II. What’s involved in data processing?
--coding data
--entering data
--managing data
--cleaning data
--recoding data
--handling missing data
--creating scales and indexes
--generating descriptive statistics
III. Coding Data
--coding scheme is a set of rules for creating usable data from questionnaire responses
--should be done as early as possible in design stage
--should reflect how data will be used in analysis (e.g., creation of dummy variables for multivariate analysis)
--should provide unique codes for various types of valid and invalid answers as well as non-responses
e.g. NELS88
BYP40 NO. OF TIMES 8TH GRADER CHANGED SCHOOLS
Frequency Percent Valid Percent Cumulative Percent
Valid 0 NONE 9584 39.0 43.3 43.3
1 ONCE 5093 20.7 23.0 66.3
2 TWICE 25
... Read more »
DATA STRUCTURES
ORGANISATION OF DATA
Characters, facts, records, files and databases form an organisation of data. The basic building
block of data is a character. The character consists of upper and lower-case letter, numeric digits
or symbols. Upper and lower-case letters are Aa, Bb, Cc,… Zz. Numeric digits are 0, 1, 2,..,9.
Symbols involve commas (,) quotation mark (?) plus (+) division (/) and so on. Upper and lowercase
letters are called alphabetic character. Numeric digits are called numeric character. Symbol
is called special characters. A combination of the three types is called alphanumeric characters
(#2B, N2.50K). A computer can accept both alphanumeric and numeric and store them in
memory.
Characters are put together to form a fact. A fact is also called a field. A fact or field is a number,
an item, word, name or a combination of characters. Facts are put together to form a record. A
record is a related items of data in a file. An employee record in a
... Read more »
INFORMATION OR DATA REPRESENTATION
There are three ways of representing information or data in computer system. These are as
follows:
Number System
Character codes
Logic gates and Truth Table
4.1 NUMBER SYSTEMS
Number Systems include Binary number, Octal number, Hexadecimal number, Binary Fraction,
Arithmetic operation, Negative numbers, Fixed-point and Floating Point representation and
Errors.
Binary Number – This method is used to represent information with two-state circuits. These
two states are 0 or 1. These two states are used in computer because they are quick, reliable and
take up only small amounts of space and energy. They are called binary digits or bits.
Conversion from Binary to Decimal
e.g. 0110 = 0x23 + 1x22 + 1 x21 + 0 + 20
= 0 + 4 + 2 + 0
01102 = 610
Conversion from Decimal to Binary
2 6
2 3 remainder 0
2 1 remainder 1
0 remainder
610 = 01102
OCTAL NUMBER – Octal numbers are in base 8
Conversion from Decimal to Octal
4910 to base 8
8 49
8
... Read more »
NETWORKS
Definitions: An Interconnected collection of autonomous processors. A Network is a group of
two or more computers connected to each other by a cable, over telephone lines or through
wireless communication. When you are connected to a network, you can share resources on your
computer such as documents, programs, printers, modems and use other resources from other
computer. The key idea of a Network is that users need equal access to resources such as data,
but they do not necessarily have to have equal computing power.
LANs, WANs and ‘Client Server” concepts are perhaps the most important trend in modern
computing.
There are two types of Networks
LOCAL AREA NETWORK (LAN)
WIDE AREA NETWORK (WAN)
LOCAL AREA NETWORK (LAN)
Definition: A network of computers located in a single building or an a single site.
A LAN may also include a Bridge which is a connection to another similar LAN and a Gateway
which is a connection to a different types of network outside
... Read more »
INTRODUCTION TO PROGRAMMING & FLOWCHARTS
PRINCIPLE OF GOOD PROGRAMING
a. The program requirements must be specified in full and in writing. These specifications will
be prepared by a systems analyst. A programmer has the task of converting these
specifications into a written program.
b. In developing a program, a programmer should keep working papers. He can refer back to
these papers later to check what he has done in case:
(i) there is an error in the program for correction;
(ii) the user of the program asks for a change in the program – e.g. for an extra bit of
processing on input data, perhaps to produce an additional report.
c. The working papers might include a decision table or flowchart (or both).
d. When writing a program, the programmer should try to keep it as short as possible, since this
will make more efficient use of storage capacity in the CPU. The program should therefore
be logically well- structured
e. Program should be tested when they have
... Read more »
Dear Friend have you ever think of a better place to be after we have completed our mission on earth. Well, I will like to inform you that no matter your achievments in this world surely an end will come to every thing you possessed. Life is full of challenges and Ad-Hoc; troubles , confusion, comotions and the like. Sometimes Christians are panic of some unsual circumstances that happens around them. Yes, Jesus Christ has already declared in his words that "In the World ye shall face tribulation but be of good cheer I have overcome the world. My words for you today dear friend is that we should always be looking unto Jesus Christ who is the author and the finisher of our faith. More so, we should be sober, watchful, vigilant lest the coming of the son of man(Jesus Christ) catch us unawares. He may come like a thief in the night, in the morning or afternoon no one knows, not even the Angel in heaven, Only the Heavenly Father knows the exact time. God bless you for
... Read more »