System RAM and Virtual Memory

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How Memory Works

CPU

• The CPU works by processing the instructions generated by software (processes) in a pipeline
• Instructions that are at the top of the pipeline are stored in the CPU’s registers and cache
• The CPU only has a small amount of cache, however
• Consequently, the operation of the CPU must be supported by additional storage technologies
  • Enter, RAM

System Memory (RAM)

• When a process is executed or a data file opened, the image is loaded from the fixed disk into system memory
• Instructions are fetched from system memory and into the CPU’s cache and registers as required
• This process is handled by a memory controller
• System memory is implemented as random-access memory (RAM) devices
  • RAM is faster than the flash memory used for SSDs and much faster than an HDD, but it is volatile
    • Volatile means the memory device can only store data when it is powered on
  • System memory is measured in gigabytes (GB)
  • amount of system RAM determines the PC’s ability to work with multiple applications at the same time and to process large files efficiently

Virtual RAM/Virtual Memory

• If there is not enough system RAM, the memory space can be extended by using disk storage
  • referred to as a pagefile or swap space
• total amount of addressable memory (system RAM plus swap space) is referred to as virtual memory or virtual RAM
  • With virtual memory, the OS assigns memory locations to processes in 4 kilobyte chunks called pages
  • memory controller moves inactive pages of memory to the swap space to free up physical RAM
  • retrieves pages from the swap space to physical RAM when required by process execution
  • An excessive amount of such paging activity will slow the computer down because disk transfer rates are slower than RAM transfer rates

Importance of Virtual Memory

• Virtual memory is not just used to supplement RAM with swap space
• It serves an important function in protecting the operation and integrity of the PC
• Multiple processes can share the RAM device resource as a virtual memory space that is mediated by the operating system
• This is more secure and reliable than allowing each process to use physical RAM devices

Address Space

The bus between the CPU, memory controller, and memory devices consists of a data pathway and an address pathway:

• The width of the data pathway determines how much information can be transferred per clock cycle
  • In a single channel memory controller configuration, the data bus is usually 64 bits wide
• The width of the address bus determines how many memory locations the CPU can keep track of and consequently limits to the maximum possible amount of physical and virtual memory
  • A 32-bit CPU with a 32-bit address bus can access a 4 GB address space
  • In theory, a 64-bit CPU could implement a 64-bit address space (16 exabytes),
  • but most 64-bit CPUs actually use a 48-bit address bus, allowing up to 256 terabytes of memory