Primary memory

• • a computer memory that a processor can access directly.

  • a computer memory that the CPU can write data to and retrieve data from while executing programs.

  • Registers, Cache memory and RAM are all part of primary memory.

Memory vs Storage

• • There is an important distinction between computer memory and storage:

Memory

• • • • • Stores program operations and data while a program is being executed.

        • There are several types of memory, including registers, cache, RAM and virtual memory.

Storage

• • • • • Stores programs and files long term, even when they are not in use.

        • Devices such as hard drives, USB memory sticks or SD cards are used to store files such as photos, music and software applications long term.

        • The data stored is referred to as persistent data.

Random access memory (RAM)

• • • “random access” means that the data in the memory can be directly accessed in any order.

    • Volatile memory, when  a computer is switched off, all data stored in its RAM is lost.

    • Data can be written to and read from memory very quickly

    • Acts as temporary storage and working space for the operating systems and its applications.

    • Stores data and instructions currently in use

What is stored in RAM

• • Active part of the operating system;

  • Applications;

  • The graphical user interface (GUI).

Read only memory (ROM)

• • It is a storage for data that can be read but not altered.

  • It cannot be changed by either a program or a user.

  • e.g. ROM stores the instructions for the computer to start up when it is turned on again, Basic Input and Output System (BIOS).

  • There are numerous ROM chips located on the motherboard and a few on expansion boards. The chips are essential for the basic input/out system (BIOS), boot up, reading and writing to peripheral devices, basic data management and the software for basic processes for certain utilities.

  • Stores permanent instructions

  • Is mainly used for firmware.

    • • Firmware are software programs or sets of instructions that are embedded into a hardware device.

      • It supplies the needed instructions on how a device communicates with various hardware components.

      • It could be used to start up a device or control the operation of a peripheral.

Memory caching

• • Is considered effective because in most programs the same data or instructions are accessed over and over.

  • By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM.

  • Cache is graded as Level 1 (L1), Level 2 (L2) and Level 3 (L3).

    • L1.

      • • Is usually part of the processor chip itself and is both the smallest and the fastest to access.

        • Its size is often restricted to between 8 KB and 64 KB.

    • L2 and L3.

      • • Are bigger than L1.

        • Are extra caches built between the CPU and the RAM.

        • Although sometimes L2 is built into the CPU with L1.

        • Takes slightly longer to access than L1.

        • The more L2 and L3 memory available, the faster a computer can run.

    • In a multi-core processor, each CPU core has its own L1 cache, but may share L2 and L3 caches.

Disk caching

• • A disk cache is a cache memory that is used to speed up the process of storing and accessing data from the host hard disk.

  • Enables faster processing of reading/writing, commands and other input and output processes between the hard disk, the memory and computing components.

  • Typically stores the most recent and frequently used programs and data.

    • • When a user or program requests data, the operating system first reviews the disk cache.

      • If the data is found, the OS and memory quickly deliver the data to the program.

  • Works under the same principle as memory caching, but instead of using high-speed SRAM, disk caching or a disk cache uses conventional main memory.

  • The most recently accessed data from the disk and its adjacent sectors are stored in a memory buffer.

  • When a program needs to access data from the disk, it first checks the disk controller cache.

  • The performance can be dramatically improved using disk caching because accessing a data in RAM can be thousands of times faster than accessing a hard disk.

SRAM vs DRAM

• • Cache memory is made out of high-speed static RAM (SRAM), rather than the slower and cheaper dynamic RAM (DRAM) which is used for main memory.

  • Static Random Access Memory (SRAM) is a type of RAM that holds data in a static form as long as the memory has power.

    • • Unlike DRAM, it does not need to be refreshed.

      • SRAM stores each bit of data on four transistors using two cross-coupled inverters.

      • The two stable states characterize 0 and 1.

  • Dynamic RAM has one capacitor and one transistor per bit.

  • SRAM does not have a capacitor to store the data, hence SRAM works without refreshing.

  • SRAM is faster and more reliable than DRAM.

  • While DRAM supports access times (access time is the time required to read or write data to/from memory) of about 60 nanoseconds, SRAM can give access times as low as 10 nanoseconds.

  • SRAM is significantly more expensive to produce than DRAM.

    • • Since SRAM is faster, it is used in registers and cache memory. Since it has a high cost, it is often used only as a memory cache.