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- #[[CT213 - Computer Systems & Organisation]]
- No previous topic.
- **Relevant Slides:** ![Lecture01.pdf](../assets/Lecture01_1662828507609_0.pdf)
-
- ## Traditional Classes of Computer Systems
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- What is a **Personal Computer (PC)**?
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- A **Personal Computer** is a computer designed for use by an individual, usually incorporating a graphics display, a keyboard, and a mouse.
- What is a **Server**? #card
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- A **server** is a computer used for running larger programs for multiple users, often simultaneously, and typically accessed only via a network.
- What is a **Supercomputer**?
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- A **supercomputer** is a member of a class of computers with the highest performance (and cost). They are configured as servers and typically cost tens to hundreds of millions of dollars.
- What is an **Embedded Computer**? #card
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- An **embedded computer** is a computer inside another device, used for running one predetermined application or collection of software.
- What are **Personal Mobile Devices**?
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- **Personal Mobile Devices** are small, wireless devices that connect to the internet.
- They rely on batteries for power, and software is installed by downloading apps.
- Conventional examples include smartphones and tablets.
- What is **Cloud Computing**? #card
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- **Cloud Computing** refers to large collections of servers that provide services over the internet.
- Some providers rent dynamically varying number of servers as a utility.
- What is **Software as a Service**? #card
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- **Software as a Service** delivers software & data as a service over the internet, usually via a thing program, such as a browser.
- Examples include web search & email.
-
- ## Computer Systems
- ![image.png](../assets/image_1662829382080_0.png){:height 410, :width 414}
- What is **Application Software**? #card
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- **Application Software** consists of user-installed applications & programs.
- Application Software provides services to the user that are commonly useful.
- What is the purpose of the **Operating System**? #card
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- The **Operating System** interfaces between a user's program and the hardware, provides a variety of services, and performs supervisory functions.
- What is the purpose of the **Hardware**?
- The **Hardware** performs the tasks.
-
- ## Seven Great Ideas in Computer Organisation
- ### 1. Use **Abstraction** to Simplify Design #card
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- A major productivity technique for hardware & software is to use **abstractions** to characterise the design at different levels of representation
- Lower-level details are hidden to offer a simpler model at higher levels.
- ### 2. Make the **Common Case** Fast #card
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- Making the **common case fast** will tend to enhance performance better than optimising the rare case.
- The common case is often simpler than the rare case, and hence is usually easier to enhance.
- ### 3. Performance via **Parallelism** #card
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- Involves speeding up performance by using designs that compute operations in **parallel**.
- ### 4. Performance via **Pipelining** #card
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- **Performance via Pipelining** is a particular pattern of **parallelism** that is so prevalent in computer architecture that it merits its own name.
- ### 5. Performance via **Prediction** #card
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- In some cases, it can be ^^faster on average to guess and start working^^ that to wait until you know for sure (assuming that the mechanism to recover from a misprediction is not too expensive, and your prediction is relatively accurate).
- ### 6. Hierarchy of Memories #card
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- Computer Architects have found that they can address conflicting demands with a **hierarchy of memories**.
- The ^^fastest, smallest, & most expensive memory per bit^^ is at the top of the hierarchy.
- The ^^slowest, largest, & cheapest per bit^^ is at the bottom of the hierarchy.
- ### 7. Dependability via **Redundancy** #card
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- Since any physical device can fail, we make systems **dependable** by including ^^redundant components^^ that can take over when a failure occurs *and* help detect failures.
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- ## Hardware Organisation
- What does basic computer organisation look like? #card
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- ![image.png](../assets/image_1662830400492_0.png)
- What is an **integrated circuit**?
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- An **integrated circuit**, also called a **chip**, is a device combining dozens to millions of transistors.
- ### The CPU
- What is a **CPU**? #card
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- The **Central Processing Unit (CPU)**, also called the **processor**, is the ^^active part of the computer^^, which contains the datapath & control, and which adds numbers, tests numbers, signals I/O devices to activate, and so on.
- The CPU is ^^responsible for executing programs.^^
- What are the steps that the CPU takes to process programs? #card
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- **1. Fetch:** Retrieve an instruction from ^^program memory.^^
- **2. Decode:** Break down the instruction into parts that have significance to specific sections of the CPU.
- **3. Execute:** Various portions of the CPU are connected to perform the desired operation.
- **4. Write Back:** Simply "writes back" the results of the execute step ^^if necessary.^^
- ### CPU Organisation
- What does the **organisation** of the CPU look like? #card
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- Processors are made up of:
- A **Control Unit**
- **Execution Unit(s)**
- A **Register File**
- ![image.png](../assets/image_1662830846361_0.png){:height 339, :width 418}
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- #### Control Unit
- What does the **Control Unit** do? #card
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- The **Control Unit** ^^controls the execution^^ of the instructions stored in main memory.
- It ^^retrieves & executes^^ them.
- What is the architecture of the control unit? #card
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- The control unit contains a **fetch unit**, a **decode unit**, and an **execute unit**.
- It also contains two special registers:
- **Program Counter (PC):** keeps the address of the next instruction
- **Instruction Register (IR):** keeps the instruction being executed
- ![image.png](../assets/image_1662837864357_0.png){:height 266, :width 550}
-
- ### The Memory Subsystem
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- How is the memory divided into storage locations? #card
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- Memory is divided into a set of storage location which can hold data.
- Locations are numbered.
- Addresses are used to tell the memory which location the processor wants to access.
- What are the two hierarchies of memory? #card
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- **1. Nonvolatile / ROM (Read Only Memory):** Read only memory.
- Used to store the BIOS and / or a *bootstrap* or *bootloader* program.
- **2. Volatile / RAM (Random Access Memory):** Read / Write memory.
- Also called **Primary Memory**.
- Used to hold the programs, operating system, and data required by the computer.
- #### Primary Memory
- How is primary memory connected to the CPU? #card
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- **Primary Memory** is directly connected to the Central Processing Unit of the computer.
- It must be present for the CPU to function correctly.
- What are the three types of Primary Storage? #card
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- **1. Processor Register:**
- Contains information that the CPU needs to carry out the current instruction.
- **2. Cache Memory:**
- A special type of internal memory used by many CPUs to increase their **throughput**.
- **3. Main Memory:**
- Contains the programs that are currently being run and the data that the programs are operating on.
- What is the **address width**? #card
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- The **address width** is the number of bits used to represent an address in memory.
- The **width** limits the amount of memory that a computer can access.
- Most computers use a **64 bit address**, which means that the maximum number of locations is 2^{64} \approx 16 billion gigabytes.
- What operations does the memory subsystem support? #card
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- The memory subsystem supports two operations:
- **Load** (or read) + the address of the data location to be read.
- **Store** (or write) + the address of the location & the data to be written.
- How many bytes may the memory system read or write at a time? #card
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- Read & Write operations ^^operate at the width of the system's data bus^^, usually 32 bit or 64 bit.
- How is a section of memory addressed? #card
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- The address ^^contains only the address of the lowest byte^^, and a number of bytes to be read is specified, e.g., 4 bytes.
- #### Memory Alignment & Words of Data
- When the computer's **word size** is 4 bytes, the data to be read should be at a memory address which is ^^some multiple of four.^^
- When this is not the case, e.g., the data starts at address 14 instead of 16, then the computer has to read two or more 4 byte chunks and do some calculation before the requested data has been read, or it may generate ^^an alignment fault.^^
- Even though the previous data structure end is at, for example, address 13, the next data structure should start at address 16. Two **padding bytes** are inserted between the two data structures at addresses 14 & 15 to align the next data structure at address 16.
- ### The I/O Subsystem
- What are **input devices**? #card
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- Anything that feeds data into the computer.
- What are **output devices**? #card
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- Display / transmit information back to the user.
- What does the **I/O Subsystem** contain? #card
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- The **I/O Subsystem** contains the devices that the computer uses to communicate with the outside world and to store data.
- How do I/O devices communicate with the processor? #card
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- I/O devices usually communicate with the processor using the **I/O Bus**.
- PCs use the **PCI Express (Peripheral Component Interconnect Express)** bus for their I/O bus.
- The Operating System needs a **device driver** to access a given I/O device.
- What is a **device driver**? #card
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- A **device driver** is a program that allows the OS to control an I/O device.
- #### I/O Read / Write Operations
- The I/O read & write operations are similar to the memory read & write operations.
- How does the processor address I/O devices? #card
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- A processor may use:
- **Memory-Mapped I/O:** when the address of the I/O device is in the **direct memory space**, and the ^^sequences to read/write data in the device are the same as the memory read/write sequences.^^
- **Isolated I/O:** similar process to Memory-Mapped I/O, but the processor has a ^^second set of control signals to distinguish between a **memory access** and am **I/O access**.^^
- What is **IO/M**? #card
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- **IO/M** is a **status signal** in **Isolated I/O** that denotes whether the read/write operation pertains to the memory or to the I/O subsystem.
- When the **signal is low** (IO/M = 0), i.e., IO/M is `true`, it denotes **memory-related operations**.
- When the **signal is high**, (IO/M = 1), i.e., IO/M is `false`, it denotes an **I/O operation**.
-
-
-
- ## Programs
- What are programs? #card
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- Programs are ^^sequences of instructions^^ that tell the computer what to do.
- To the computer, a program is made out of a ^^sequence of numbers that represent individual operations.^^
- These operations are known as **machine instructions** or just **instructions**.
- A set of instructions that a processor can execute is known as an **instruction set**.
- ### Program Development Tools
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- What is a **high-level programming language**? #card
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- A **high-level programming language** is a ^^portable language^^ such as C that is ^^composed of words & algebraic notation^^ that can be translated by a compiler into **assembly language**.
- What is a **compiler**? #card
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- A **compiler** is a program that translates statements in a given high-level language into assembly language statements.
- What is an **assembler**? #card
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- An **assembler** is a program that translates symbolic, assembly language versions of instructions into the ^^binary version.^^
- What is **Assembly Language**? #card
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- **Assembly Language** is a ^^symbolic representation^^ of **machine instructions**.
- What is **Machine Language**? #card
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- **Machine Language** is a ^^binary representation^^ of **machine instructions**.
- What is an **instruction**? #card
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- An **instruction** is a command that the computer hardware understands & obeys.
-
- ## Operating Systems
- What is an **Operating System**? #card
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- Possible definition: a program that runs on the computer that ^^knows about all the hardware^^ and usually ^^runs in privileged mode^^, having ^^access to physical resources that user programs can't control^^, and has the ^^ability to start & stop user programs.^^
- The OS is responsible for managing the physical resources of complex systems, such as PCs, workstations, mainframe computers, etc.
- It is also responsible for ^^loading & executing programs^^ and ^^interfacing with the users.^^
- Usually, there is no operating system for **small embedded systems**.
- Computers designed for one specific task.
-
- ### Multiprogramming
- What is **Multiprogramming**? #card
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- **Multiprogramming** is a technique that allows the system to ^^present the illusion that multiple programs are running on the computer simultaneously.^^
- Many multiprogrammed computers are **multiuser**.
- They allow multiple users to be logged in at a time.
- How is multiprogramming achieved? #card
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- Multiprogramming is achieved by ^^switching rapidly between programs.^^
- How does the processor decide which process to execute next? #card
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- **FCFS - First Come, First Served:** processes are moved to the CPU in the order in which they arrive.
- **SJN - Shortest Job Next:** looks at all processes in the **ready state** and dispatches the one with the smallest service time.
- **Round Robin:** distributes the processing time equitably among all ready processes.
- ### Context Switching
- What is a **Context Switch**? #card
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- When a program timeslice ends, the OS stops it, removes it, and gives another program control over the processor.
- This is a **context switch**.
- How does the OS go about a Context Switch? #card
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card-last-score:: 3
- copies the current program register file into memory
- restores the contents of the next program's register file into the processor
- starts executing the next program
- From the program point of view, ^^no program can tell that a context switch has been performed.^^
- ### Protection
- Three rules of Protection in multiprogrammed computers: #card
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card-last-reviewed:: 2022-11-17T19:35:55.000Z
card-last-score:: 3
- 1. The result of any program running on the multiprogram computer ^^must be the same as if the program was the only program running on the computer.^^
- 2. Programs ^^must not be able to access other programs' data^^ and must be confident that their data will not be modified by other programs (for security and privacy).
- 3. Programs ^^must not interfere with other programs' use of I/O devices.^^
- How is protection achieved? #card
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card-next-schedule:: 2022-11-29T21:01:17.902Z
card-last-reviewed:: 2022-11-14T20:01:17.902Z
card-last-score:: 3
- Protection is achieved by the ^^operating system having full control over the resources of the system (processor, memory, and I/O devices)^^ through:
- **Privileged Mode:** the operating system is the only one that can control the physical resources it executes in privileged mode.
- User programs execute in **user mode**.
- **Virtual Memory:** each program operates as if it were the only program on the computer, occupying a full set of the address space in its virtual space.
- The OS is *translating* memory addresses that the program references into physical addresses used by the memory system.
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- **Next Topic:** [[Programming Models]]
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