Primary storage
Primary storage (main memory / internal memory), often referred to simply as memory
It is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.
Early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were replaced by magnetic core memory. Undoubtedly, a revolution was started with the invention of a transistor, that soon enabled, then-unbelievable miniaturization of electronic memory via solid-state silicon chip technology.
In present days, primary memory identified as follows.
1. Random-access memory (RAM)
It is small-sized, light, but quite expensive at the same time. (The particular types of RAM used for primary storage are also volatile. So they lose the information when not powered).
As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM.As shown in above diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:
• Processor registers are located inside the processor. Each register typically holds data (often 32 or 64 bits). CPU instructions instruct the arithmetic and logic unit to perform various calculations or other operations on this data. Registers are technically among the fastest of all forms of computer data storage.
• Processor cache is an intermediate stage between ultra-fast registers and much slower main memory. It is introduced solely to increase performance of the computer. Most actively used information in the main memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. On the other hand it is much slower, but much larger than processor registers. Multi-level hierarchical cache setup is also commonly used. As,
1. Primary cache :smallest, fastest and located inside the processor.
2. Secondary cache : being somewhat larger and slower than primary cache.
Main memory is directly or indirectly connected to the central processing unit via a memory bus. It is actually two buses as address bus and a data bus. The CPU firstly sends a number through an address bus, a number called memory address, that indicates the desired location of data. Then it reads or writes the data itself using the data bus. Additionally, a memory management unit (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks.
2. Read Only Memory (ROM)
As the RAM types used for primary storage are volatile, a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program (BIOS) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for read-only memory (the terminology may be somewhat confusing as most ROM types are also capable of random access).Many types of "ROM" are not literally read only, as updates are possible; however it is slow and memory must be erased in large portions before it can be re-written.
Types of ROM
Types of ROM
• PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.
• EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.
• EEPROM (electrically erasable programmable read-only memory): An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge
Activity :
Listout five important points to discribe each of following topics
- Random Access Memory(RAM)
- Read Only Memory (ROM)
- Register
- Cache memory
