by Memory is the faculty of the brain by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, it would be impossible for language, relationships, or personal identity to develop. Memory loss is usually described as forgetfulness or amnesia.
Memory is often understood as an informational processing system with explicit and implicit functioning that is made up of a sensory processor, short-term (or working) memory, and long-term memory. This can be related to the neuron. The sensory processor allows information from the outside world to be sensed in the form of chemical and physical stimuli and attended to various levels of focus and intent. Working memory serves as an encoding and retrieval processor. Information in the form of stimuli is encoded in accordance with explicit or implicit functions by the working memory processor. The working memory also retrieves information from previously stored material. Finally, the function of long-term memory is to store data through various categorical models or systems.
Declarative, or explicit, memory is the conscious storage and recollection of data. Under declarative memory resides semantic and episodic memory. Semantic memory refers to memory that is encoded with specific meaning, while episodic memory refers to information that is encoded along a spatial and temporal plane. Declarative memory is usually the primary process thought of when referencing memory. Non-declarative, or implicit, memory is the unconscious storage and recollection of information. An example of a non-declarative process would be the unconscious learning or retrieval of information by way of procedural memory, or a priming phenomenon. Priming is the process of subliminally arousing specific responses from memory and shows that not all memory is consciously activated, whereas procedural memory is the slow and gradual learning of skills that often occurs without conscious attention to learning.
Memory is not a perfect processor, and is affected by many factors. The ways by which information is encoded, stored, and retrieved can all be corrupted. The amount of attention given new stimuli can diminish the amount of information that becomes encoded for storage. Also, the storage process can become corrupted by physical damage to areas of the brain that are associated with memory storage, such as the hippocampus. Finally, the retrieval of information from long-term memory can be disrupted because of decay within long-term memory. Normal functioning, decay over time, and brain damage all affect the accuracy and capacity of the memory.
RAM
RAM (random-access memory) is a hardware device that allows information to be stored and retrieved on a computer. RAM is usually associated with DRAM, which is a type of memory module. Because information is accessed randomly instead of sequentially like it is on a CD or hard drive, access times are much faster. However, unlike ROM, RAM is a volatile memory and requires power to keep the data accessible. If the computer is turned off, all data contained in RAM is lost.
Main Types of RAM
There are two main types of RAM:
- DRAM (Dynamic Random Access Memory)
- SRAM (Static Random Access Memory)
DRAM (Dynamic Random Access Memory) – The term dynamic indicates that the memory must be constantly refreshed or it will lose its contents. DRAM is typically used for the main memory in computing devices. If a PC or smartphone is advertised as having 4-GB RAM or 16-GB RAM, those numbers refer to the DRAM, or main memory, in the device.
More specifically, most of the DRAM used in modern systems is synchronous DRAM, or SDRAM. Manufacturers also sometimes use the acronym DDR (or DDR2, DDR3, DDR4, etc.) to describe the type of SDRAM used by a PC or server. DDR stands for double data rate, and it refers to how much data the memory can transfer in one clock cycle.
In general, the more RAM a device has, the faster it will perform.
SRAM (Static Random Access Memory) – While DRAM is typically used for main memory, today SRAM is more often used for system cache. SRAM is said to be static because it doesn’t need to be refreshed, unlike dynamic RAM, which needs to be refreshed thousands of times per second. As a result, SRAM is faster than DRAM. However, both types of RAM are volatile, meaning that they lose their contents when the power is turned off.
ROM
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing software that is rarely changed during the life of the system, sometimes known as firmware. Software applications for programmable devices can be distributed as plug-in cartridges containing read-only memory.
Strictly, read-only memory refers to memory that is hard-wired, such as diode matrix or a mask ROM integrated circuit, which cannot be electronically changed after manufacture. Although discrete circuits can be altered in principle, integrated circuits (ICs) cannot. Correction of errors, or updates to the software, require new devices to be manufactured and to replace the installed device.
Erasable programmable read-only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM) can be erased and re-programmed, but usually this can only be done at relatively slow speeds, may require special equipment to achieve, and is typically only possible a certain number of times.
The term “ROM” is sometimes used to mean a ROM device containing specific software, or a file with software to be stored in EEPROM or Flash Memory. For example, users modifying or replacing the Android operating system describe files containing a modified or replacement operating system as “custom ROMs” after the type of storage the file used to be written to.
EDO RAM
Extended data out random access memory (EDO RAM/DRAM) is an early type of dynamic random access memory (DRAM) chip which was designed to improve the performance of fast page mode DRAM (FPM DRAM) that was used in the 1990s. Its main feature was that it eliminated wait times by allowing a new cycle to start while retaining the data output buffer from the previous cycle active, which allows a degree of pipelining (overlap in operation) that improved performance.
Extended data out dynamic random access memory was introduced in 1994 and began to replace fast page mode DRAM by 1995 when Intel first introduced the 430FX chipset that supports EDO DRAM. Before that, EDO DRAM could replace FPM DRAM, but if the memory controller was not specifically designed for the EDO, then the performance remained the same as FPM.
Single-cycle EDO DRAM is able to carry out an entire memory transaction in a single clock cycle, otherwise, it can do it in two cycles instead of three, once the page has been selected. The EDO’s capability allowed it to replace the slow L2 cache of PCs at that time and reduced the huge performance loss associated with the L2 cache, while making PCs cheaper to build overall. So a system using EDO with L2 cache was much faster compared to the FPM and L2 cache combination, while also being cheaper to build.
EDO was rated for 40 MHz maximum clock rate, 64 bits of bus bandwidth, 320 MBps peak bandwidth and ran at 5 volts. It was tangibly faster than the older FPM DRAM that had only 25 MHz max clock rate and 200 MBps peak bandwidth. However, it was superseded by the faster SDRAM starting in 1996, after only two years of major use.
SD RAM
SDRAM, which is short for Synchronous DRAM, is a type of memory that synchronizes itself with the computer’s system clock. Being synchronized allows the memory to run at higher speeds than previous memory types and asynchronous DRAM and also supports up to 133 MHz system bus cycling. Since 1993, this is the prevalent type of memory used in computers around the world. In the picture below is an example of a SDRAM DIMM. The original type, named SDRAM, up to the current type, DDR3, are all derivatives of the SDRAM memory type.
Synchronous dynamic random-access memory (SDRAM) is any dynamic random-access memory (DRAM) where the operation of its external pin interface is coordinated by an externally supplied clock signal.
The first commercial SDRAM was the Samsung KM48SL2000 chip, introduced in 1992. SDRAM is widely used in computers. Beyond the original SDRAM, further generations of double data rate RAM have entered the mass market – DDR (also known as DDR1), DDR2, DDR3, and DDR4, with the latest generation (DDR5) scheduled to be released commercially within the future 2020-2021 timeframe.
