Computer Science: Main Memory

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Questions and Answers

What is the primary storage accessed directly by the CPU in modern computer systems?

  • Optical storage
  • Main memory (correct)
  • Disk storage
  • Cloud storage

Machine instructions can take disk addresses as arguments.

False (B)

What is the first step in the instruction-execution cycle of the Von-Neumann architecture?

Fetching an instruction from memory

Memory consists of a large array of bytes, each with its own ________.

<p>address</p> Signup and view all the answers

Match the following components of the memory management process with their functions:

<p>Contiguous Memory Allocation = Allocating memory blocks in a single contiguous block Paging = Dividing memory into fixed-size units for easier management Swapping = Temporarily moving processes in and out of main memory Page Table = Tracking the mapping of virtual addresses to physical memory</p> Signup and view all the answers

What is the primary function of cache memory?

<p>To speed up access to main memory (C)</p> Signup and view all the answers

The operating system is responsible for controlling access to cache memory.

<p>False (B)</p> Signup and view all the answers

What is the Von Neumann bottleneck?

<p>The relative speed difference between the CPU and main memory that can cause delays.</p> Signup and view all the answers

During a memory stall, a multithreaded core can switch from the stalled hardware thread to another __________ thread.

<p>hardware</p> Signup and view all the answers

Match the following statements with their correct meanings:

<p>Memory stall = Delay caused by slow main memory access Shared memory = Memory accessed by multiple processes Process protection = Ensuring processes cannot access each other's address space Cache memory = Fast access memory located between CPU and main memory</p> Signup and view all the answers

What type of memory is primarily used to improve access speed between the CPU and main memory?

<p>Cache memory (D)</p> Signup and view all the answers

User processes do not require any form of protection from one another.

<p>False (B)</p> Signup and view all the answers

What type of address is generated by the CPU?

<p>Logical address (D)</p> Signup and view all the answers

In execution-time address-binding, logical addresses and physical addresses are identical.

<p>False (B)</p> Signup and view all the answers

What is the function of the memory-management unit (MMU)?

<p>To map virtual addresses to physical addresses.</p> Signup and view all the answers

The simplest implementation of a memory management method is the __________ scheme.

<p>base-register</p> Signup and view all the answers

Match the following terms with their definitions:

<p>Logical address = Memory address generated by the CPU Physical address = Memory address seen by the memory unit Relocation register = Base register in base-register scheme Virtual address = Another term for logical address in some contexts</p> Signup and view all the answers

What purpose do the base and limit registers serve in operating systems?

<p>They define the logical address space of a process. (D)</p> Signup and view all the answers

In compile-time and load-time address-binding schemes, how do logical and physical addresses relate?

<p>They are identical. (B)</p> Signup and view all the answers

Separate per-process memory space protects processes from each other.

<p>True (A)</p> Signup and view all the answers

The user program has access to both logical and physical addresses directly.

<p>False (B)</p> Signup and view all the answers

What does the limit register specify?

<p>Size of the memory range that a process can access.</p> Signup and view all the answers

What happens when the base register's value is added to a generated logical address?

<p>It maps the logical address to a physical address.</p> Signup and view all the answers

An example of an implementation of address binding is the __________ scheme.

<p>base-register</p> Signup and view all the answers

The CPU must check every memory address generated in user mode to ensure it is between ______ and ______.

<p>base, limit</p> Signup and view all the answers

At what point does address binding occur if the memory location is known at compile time?

<p>Compile time (B)</p> Signup and view all the answers

Instructions to load the base and limit registers are not privileged.

<p>False (B)</p> Signup and view all the answers

Address binding is a mapping from one address space to another during the process of ______.

<p>execution</p> Signup and view all the answers

Match the following stages of address binding with their descriptions:

<p>Compile time = Absolute code is generated if starting memory location is known. Load time = Relocatable code is generated if starting location is not known. Execution time = Binding is delayed until runtime, allowing movement between memory segments.</p> Signup and view all the answers

What does the term 'symbolic address' refer to in the context of address binding?

<p>Addresses in the source code that are identified with variable names.</p> Signup and view all the answers

Flashcards

Role of Memory in Program Execution

Memory is essential for computer operation because it's the only storage the CPU can access directly. This means instructions and data need to be loaded into memory before the CPU can work with them.

Memory Address

Machine instructions use memory addresses or registers as arguments. They can't directly access data from disk.

Direct Access Storage

Instructions and data are moved into memory from other storage devices (like disk) so the CPU can work with them.

Instruction-Execution Cycle

The CPU uses a cycle to execute instructions. First, it fetches the instruction from memory, then decodes it, fetches any needed data, and finally executes the instruction on the data, frequently storing the result back in memory.

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Von Neumann Architecture

The Von Neumann architecture, common in most computers, allows instructions and data to share the same memory space. This means the same instructions can operate on both instructions and data.

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Memory Unit's View

The CPU only sees a series of memory addresses and doesn't have any information about how they are generated.

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Memory Stall

The speed difference between the CPU and main memory, causing the CPU to wait for data, known as the Von Neumann bottleneck.

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Cache

A fast type of memory placed between the CPU and main memory to reduce memory stalls.

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Shared Memory

Multiple processes sharing the same memory to improve performance.

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Protection in Shared Memory

Protecting the OS and user processes from unauthorized access.

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Address Space Protection

Restricting a process's access to only its own memory addresses, usually enforced by hardware.

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Hardware Protection

Hardware-based mechanisms are preferred over software solutions for protection in shared memory because they are faster and more efficient.

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Memory Protection

It's a crucial concept in operating systems that prevents one process from interfering with the memory space of another. This protection guarantees that processes stay within their allocated memory boundaries. The term 'memory protection' refers to the mechanisms used to achieve this.

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Base and Limit Registers

Two special registers, base and limit, work together to define the memory address range a process can access. The base register holds the starting address, and the limit register signifies the maximum allowed space.

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CPU Memory Address Check

Every memory address generated by a process in user mode needs to be checked by the CPU to see if it falls within the allowed range between the base and limit registers for that user.

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Privileged Instructions

Instructions for modifying base and limit registers can only be executed by the operating system, not by user programs. This ensures that processes can't change their allotted memory space.

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Address Binding

It's the process of mapping one address space to another. It's crucial for running programs because it involves converting addresses represented in different ways (symbolic, relocatable, absolute) as a program transitions from source code to execution.

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Address Binding Time

This refers to the time when address binding is performed. It can happen at compile time, load time, or execution time, with each stage offering different advantages and disadvantages.

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Absolute Code

This type of code assumes the program will always start at the same memory location. The compiler can produce absolute code, which eliminates the need for any address modification during execution.

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Relocatable Code

This code is more flexible because it doesn't assume a specific starting memory location. It allows the program to be loaded at any available spot in memory.

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Execution Time Binding

This type of binding is performed during execution, allowing memory segments of a running program to be moved around dynamically. It requires hardware support for address mapping, like base and limit registers.

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Logical Address

The addresses used by the CPU to access memory, generated by the program. Think of it as the program's own view of memory.

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Physical Address

The actual addresses used by the physical memory hardware.

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Compile-Time/Load-Time Address Binding

The memory management technique where logical and physical addresses are the same at compile or load time. This means addresses are fixed before the program runs.

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Execution-Time Address Binding

The memory management technique where logical and physical addresses are different during program execution.

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Virtual Address

A logical address that can be changed to a physical address during execution. It allows programs to move around in memory without conflict.

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Memory Management Unit (MMU)

A hardware component that translates virtual addresses to physical addresses. This is crucial for memory protection and flexible memory management.

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Base-Register Scheme

A simple MMU technique where a base register holds a fixed offset added to each logical address to get the physical address.

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Relocation Register

The base register in the base-register scheme: It holds the starting address for a process.

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Study Notes

Main Memory

  • Main memory is central to the operation of modern computer systems.
  • Machine instructions operate on memory addresses or registers, not disk addresses.
  • Memory consists of a large array of bytes with unique addresses.
  • Instructions and data must be moved into memory before the CPU can use them.

Memory Management

  • Topics include background memory allocation, contiguous memory allocation, paging, page table structure, and swapping.

Role of Memory in Program Execution

  • Memory is central to the operation of a computer system.
  • CPU accesses memory and registers directly.
  • Machine instructions use memory addresses or registers as arguments.
  • Memory contains a large array of bytes, each with a unique address.
  • Instructions and data must be moved into memory before the CPU can use them.
  • A CPU fetches an instruction, decodes it, fetches operands if needed, executes the instruction on the operands, and stores the result back in memory.

Relative Speed Issue with Memory

  • Register access is very fast (one CPU clock cycle or less).
  • Main memory access takes many CPU clock cycles, causing a memory stall.
  • The speed of memory is slower than the CPU.
  • This difference is called the Von Neumann bottleneck.
  • A remedy is to add cache memory between the CPU and main memory.
  • Cache is typically built on the CPU chip for fast access.

Protection Issue with Shared Memory

  • Modern computers run multiple processes sharing memory to improve performance.
  • Operating System (OS) must protect processes from accessing each other's memory.
  • Basic protection is typically done by hardware, not OS.
  • OS intervention would significantly impact performance.

Protection by Hardware

  • Each process has its own separate memory address space.
  • This isolates processes from each other and helps protect the operating system.
  • Protection relies on a pair of base and limit registers that define the logical address space of a process.

Hardware Address Protection

  • Each process has a separate memory address space.
  • This isolation protects different processes from each other.
  • The CPU must check each memory address to ensure it's within the valid range for the current process.

Address Binding

  • Address binding maps one address space to another.
  • Programs usually reside on a hard disk as binary executables.
  • To run, the executable file must be loaded into memory within the context of a process.

Multi-Steps Processing of a Program

  • A user program undergoes several steps before execution.
  • These steps include compilation, assembly, linking, and loading into memory.

Address Binding Time

  • Address binding can occur at compile time, load time, or execution time.
  • Compile-time binding uses absolute addresses and is only useful if the starting memory location is known in advance.
  • Load-time binding requires relocatable code if the starting location is unknown until load time.
  • Execution-time binding provides flexibility to move a program during execution.

Logical vs. Physical Address Space

  • Logical addresses are generated by the CPU.
  • Physical addresses are seen by the memory unit.
  • In some schemes, logical and physical addresses are the same, while in others, they differ.

Memory-Management Unit (MMU)

  • The MMU is a hardware device that maps logical addresses to physical addresses at run time.

Base-Register Scheme of MMU

  • The relocation register (base register) is used to translate logical addresses to physical addresses.

Logical vs Physical Address

  • Logical addresses are the addresses seen by the CPU.
  • Physical addresses are the addresses seen by memory.

Dynamic Loading

  • Dynamic loading is a technique to load a routine only when called.
  • This enhances memory space utilization by avoiding unnecessary loading of routines.

Benefits of Dynamic Loading

  • Improved memory space utilization.
  • Enables handling infrequently occurring cases, such as error routines.
  • No special OS support needed.

Dynamic Linking

  • Dynamic linking postpones linking of system libraries until run time.
  • This allows libraries to be shared among multiple processes, thus reducing memory usage (shared libraries).

Memory Protection - Hardware Means

  • The CPU checks each generated memory address against values in base and limit registers to ensure it falls within the valid range for the current process.
  • Invalid addresses lead to a trap (error) to the OS.

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