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Decimal to binary• Read from right to left
•1001 = 2^0+2^3 = 1 + 8 = 9
Unsigned and Signed Notation
1 0001101
• 1 0001101 in unsigned notation is 128+1+4+8 = 141
• 1 0001101 in signed notation is -1 + -4 + -8 = -13
• 1 means a negative number and 0 means a positive number.
Excess notation.10001101 in excess notation is the unsigned – 2^8 -1
Unsigned = 141 so;141 – 2^8-1 = 14 = excess value.
This is assuming it is 8bit. (if it was 16 bit it would be 2^16-1.
Two’s compliment with 1001100
• A quick method to find the solution is:• 1001100 (signed)• 0110100 (switch 0’s and 1’s after the first 1)
• -0110100 (the answer should be signed).
Ans: -01101002
Ans: -5210
Benefits and disadvantages
• Unsigned – cant be negative• Signed – two representations of 0• Signed – Difficult for arithmetic• Signed – Can represent negative values• Excess – Two numbers can be compared easily• Excess – Can represent negative numbers• Twos Compliment – Arithmetic easily done and
can represent negative numbers easily.
Floating point representation• 11000011100010100000000000000000• Signed bit, Biased Exponent, Normalised Mantissa
• Signed bit determines if its negative or positive• Biased Exponent can be used to workout the real exponent
which is 2^8-1(127) less than the 8 digit value shown.• Normalised Mantissa should be converted to the real
mantissa by adding a 1. at the beginning and continuing the number until you reach the last 1 in the sequence. So:
• Real mantissa = 1.000101• Final representation is –(1.000101)*2^8 = -100010100.• The decimal point is moved 8 places along because of multiplying by 2^8
Ans: 100010100 = -256+-16+-4 = -276.
Cache Memory• Enhances a computers performance using temporal
locality principle and spatial locality principle.• Temporal Locality Principle holds recently used data• Spatial Locality Principle collects nearby nodes to
recently used pieces/chunks of data.
CPU CacheMain
memory
Cache Mapping
• Associative mapped cache – Each memory block can be stored in any slot within the cache.
• Direct Mapped Cache – Each block in the main memory is assigned its own slot in the Cache memory.
• Set Associative Mapped Cache – Blocks in the main memory can be stored in a set of slots in the cache
Address Bus, Data Bus and Control Bus
Control unit
ALU
RegistersMAR
MBR
Address bus
Data bus
Control bus
• Address bus: this determines the location in memory that the processor will read data from or write data to.
• Data bus: this contains the contents that have been read from the memory location or are to be written into the memory location.
• Control bus: this manages the information flow between components indicating whether the operation is a read or a write and ensuring that the operation happens at the right time.
Address Bus, Data Bus and Control Bus
CPU fetch execute cycle
• Without pipelining
• A two-stage cycle.• Suppose we have 3 instruction I1, I2, I3.
– Without pipelining this will take 6 time units.
1 2 3 4 5 6Fetch I1 I2 I3
Execute I1 I2 I3
CPU fetch Execute cycle with pipelining
• With pipelining
• A two-stage cycle.• Suppose we have 3 instruction I1, I2, I3.
– With pipelining it will take only 4 time units.
1 2 3 4 5 6Fetch I1 I2 I3
Execute I1 I2 I3
Disadvantages of Pipelining
• Data hazards - Data hazards occur when data is modified. For example one instruction may still be running before another one ends.
(Hence why increasing the number of pipeline stages doesn’t necessarily increase performance. )
• Structural hazards – occurs when two instructions are trying to access the same memory resource. Causing a ‘queue’.
• Control hazards – occurs when the processor is told to branch. Can cause the processor to perform unwanted actions.
Memories: CPU register
• Program Counter (PC) – Contains the Address of the Next Instruction
• Instruction Register (IR) – Holds instructions just fetched from the main memory.
• Accumulator(AC) – A temporary register for when the instructions executed.
• Memory Address Register (MAR) – Stores the address of the cell the CPU is about to execute.
• Memory Buffer Register (MBR) – Contains Instruction or data just read from the memory.
• (think back to the audio array buffer in AV)
Memories: CPU register
A model of an operation System
Operation System
User command interface
Resource management
Process Manager
Memory manager
Device Manager
File manager
Network manager
Resource Management
Resource management
Process Manager:• Determines the next program to be
executed. • Time to be given to each program?
Memory manager:• Best use of the memory to run as many programs as possible
I/O Device (e.g.printer) Manager:• Determines which program should use a
particular I/O device.Network manager:
• Determines which computer should execute a particular program.
Starting up
OperatingSystem
Main memory
BootstrapProgram
Operating System
Disk storage ROM
RAM
Input/Output (I/O) Management
• There are four main I/O operations.–Control: tell the system to perform
some action (e.g. rewind tape).–Test: check the status of the device–Read: read data from the device–Write write data to the device.
Polling (Programmed) I/O
• The CPU controls the device directly via the module.
• The CPU sends a command to the module and waits until the operation is completed before sending another command.
• The performance is BAD as the CPU spends too much time waiting for the device.
Issue Read to I/O module
Check status
Read word from I/O module
Write word To memory
done
yes
NO
Next instruction
Ready
Interrupt Driven I/O Management.
• The CPU issues a command to the I/O module and then continues executing other instructions.
• The I/O module interrupts the CPU when it is ready with the data. Then the CPU takes the data.
Issue Read to I/O module
Check status
Read word from I/O module
Write word To memory
done
yes
NO
Ready
CPU goes to do Other things
When the statusIs ready the I/O module sendsAn interrupt-signal
Disadvantages of Interrupt Driven I/O
• CPU is responsible for managing I/O data transfer.
• Every transferred word must go through the CPU.
• Devices with large transfer, e.g. disk drive, the CPU wastes time dealing with data transfer.
• Solution: Direct-memory-access(DMA).
Direct-Memory Access (DMA)
• Directly accesses the memory without intervention from the CPU
• Special-purpose processor.• Handles data transfer.• DMA sends an interrupt to the CPU when the
transfer is completed.
System bus
CPUMain
memoryI/O module
I/Odevice
DMA
Memory management
• When a process is executed it has to be in main memory as the main memory can be accessed quicker.
• An efficient use of the main memory is an important task of the operation system.
• Different memory management techniques are used for this purpose.
Memory Partitions• How processes are arranged in the main memory
before been executed?• Fixed-sized partitions Variable-sized partitions
OS 8M
8M
8M
8M
8M
OS 8M
2M
4M
8M
18M
Memory Management TechniqueSwapping
• I/O operations are slow • If a running process requires an I/O
operation. The CPU will move to another process in the main memory.
• Suppose the main memory is full of processes waiting on I/O.
• CPU becomes idle• To solve this problem Swapping technique
is used.
Simple Paging (inefficient)
• Better than fixed and variable-sized partitions.• OS - loads all pages of a process in the main
memory.• However, not all pages of a process need to be
in the main memory in order to be executed.• OS - can still execute a process if only some of
the pages are loaded
Demand Paging (efficient)
• Loads a page only when it is required• No swapping in or out of unused
pages is needed.• Better use of memory.• CPU can access only a number of
pages of a process at one time then asks for more pages to be loaded.
Multithreading vs Single Threading
HTML, XML, XHTML, CSS
• Hypertext Mark-up Language (HTML)• eXtensible Mark-up Language (XML)• eXtensible Hypertext Mark-up language
(XHTML)• Cascading Stylesheets (CSS)
XHTML• ADVANTAGES AND DISADVANTAGES
XHTML has better security features.XHTML elements must be properly nestedXHTML elements must always be closedXHTML elements must be in lowercaseXHTML documents must have one root elementXHTML Does not have error tolerance
HTML
• Easy to use• Loose Syntax (error tolerance)• Widely used• Compatible with nearly every browser.• Security features offered by HTML are limited• Deprecated tags because of updates
• ADVANTAGES AND DISADVANTAGES
XML
• You can define your own tags• Reflects the structure and semantics of
documents and better navigation.• Tagging is separate from display.• Allows a single document to be used many
ways.
Why XHTML?
• XHTML is a combination of HTML and XML (EXtensible Markup Language).
• XHTML consists of all the elements in HTML 4.01, combined with the strict syntax of XML.
(The browser doesn’t have to fix it for you)
HTML to XHTML• Change tags to lowercase• Quote attribute values and expand attributes.• Closing open elements• Nesting inline elements inside block elements• Removing presentational mark-up• Changing name to id and encoding
ampersands• Changing the document type
Advantages of CSS
• Allows you to format a whole website in one single file by creating variables to reference to from your XHTML or HTML files.
• Ability to edit multiple pages just by changing a single variable in the CSS file.
• Consistency of look across pages.
ID and Class Selector
• The id selector is used to specify a style for a single, unique element.
• #para1{text-align:center;color:red;}
• The Class Selector is used to specify a style for a group of elements.
• .center {text-align:center;}• p.center {text-align:center;}
Computer Networks
• TCP/IP model • Layer 4: Application• Layer 3: Transport• Layer 2: Internet• Layer 1: Network access
Figuring out the Subnet AddressWhat is the subnetwork address if the destination address is 200.45.34.56 and the subnet mask is 255.255.240.0?
Convert the address to binary
11001000 00101101 00100010 00111000
200 . 45 . 34 . 56
Convert the subnet mask to binary
11111111 . 11111111 . 11110000 . 00000000
255 . 255 . 240 . 0
Apply the AND operation to the subnet (address AND subnet)
11001000 . 00101101 . 00100000 . 00000000 (are both a 1? Else 0).
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