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icon What is memory system? Explain the memory hierarchy?
  The system memory is the place where the computer holds current programs and data that are in use. There are various levels of computer memory (memory), including ROM, RAM, cache, page and graphics, each with specific objectives for system operation. This section focuses on the role of computer memory, and the technology behind it.

Although memory is used in many different forms around modern PC systems, it can be
divided into two essential types: RAM and ROM. ROM, or Read Only Memory, is
relatively small, but essential to how a computer works. ROM is always found on
motherboards, but is increasingly found on graphics cards and some other expansion
cards and peripherals. Generally speaking, ROM does not change. It forms the basic
instruction set for operating the hardware in the system, and the data within remains intact even when the computer is shut down. It is possible to update ROM, but it's only done rarely, and at need. If ROM is damaged, the computer system simply cannot function.

The hierarchical arrangement of storage in current computer architectures is called the memory hierarchy. It is designed to take advantage of memory locality in computer programs. Each level of the hierarchy has the properties of higher bandwidth, smaller size, and lower latency than lower levels.

Most modern CPUs are so fast that for most program workloads, the locality of reference of memory accesses and the efficiency of the caching and memory transfer between different levels of the hierarchy are the practical limitation on processing speed. As a result, the CPU spends much of its time idling, waiting for memory I/O to complete. This is sometimes called the space cost, as a larger memory object is more likely to overflow a small/fast level and require use of a larger/slower level.

The memory hierarchy in most computers is:
• Processor registers – fastest possible access (usually 1 CPU cycle), only hundreds
of bytes in size
• Level 1 (L1) cache – often accessed in just a few cycles, usually tens of kilobytes

• Level 2 (L2) cache – higher latency than L1 by 2× to 10×, often 512 KiB or more

• Main memory (DRAM) – may take hundreds of cycles, but can be multiple gigabytes. Access times may not be uniform, in the case of a NUMA machine.

• Disk storage – millions of cycles latency, but very large

• Tertiary storage – several seconds latency, can be huge The various major units in a typical memory system can be viewed as forming a hierarchy of memories (m1,m2,...,mn) in which each member mi is in a sense subordinate to the next highest member mi-1 of the hierarchy.

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