Difference Between Serial And Random Access Memory Images
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Random-access memory (RAM; /ræm/) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code.[1][2] A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory, in contrast with other direct-access data storage media (such as hard disks, CD-RWs, DVD-RWs and the older magnetic tapes and drum memory), where the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.
Non-volatile RAM has also been developed[3] and other types of non-volatile memories allow random access for read operations, but either do not allow write operations or have other kinds of limitations on them. These include most types of ROM and a type of flash memory called NOR-Flash.
The first practical form of random-access memory was the Williams tube starting in 1947. It stored data as electrically charged spots on the face of a cathode-ray tube. Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. The capacity of the Williams tube was a few hundred to around a thousand bits, but it was much smaller, faster, and more power-efficient than using individual vacuum tube latches. Developed at the University of Manchester in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June 1948.[6] In fact, rather than the Williams tube memory being designed for the Baby, the Baby was a testbed to demonstrate the reliability of the memory.[7][8]
Prior to the development of integrated read-only memory (ROM) circuits, permanent (or read-only) random-access memory was often constructed using diode matrices driven by address decoders, or specially wound core rope memory planes.[citation needed]
An integrated bipolar static random-access memory (SRAM) was invented by Robert H. Norman at Fairchild Semiconductor in 1963.[14] It was followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964.[9] SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each bit of data.[15] Commercial use of SRAM began in 1965, when IBM introduced the SP95 memory chip for the System/360 Model 95.[10]
Dynamic random-access memory (DRAM) allowed replacement of a 4 or 6-transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility. Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every few milliseconds before the charge could leak away. Toshiba's Toscal BC-1411 electronic calculator, which was introduced in 1965,[16][17][18] used a form of capacitive bipolar DRAM, storing 180-bit data on discrete memory cells, consisting of germanium bipolar transistors and capacitors.[17][18] While it offered improved performance over magnetic-core memory, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory.[19]
Synchronous dynamic random-access memory (SDRAM) was developed by Samsung Electronics. The first commercial SDRAM chip was the Samsung KM48SL2000, which had a capacity of 16 Mbit.[23] It was introduced by Samsung in 1992,[24] and mass-produced in 1993.[23] The first commercial DDR SDRAM (double data rate SDRAM) memory chip was Samsung's 64 Mbit DDR SDRAM chip, released in June 1998.[25] GDDR (graphics DDR) is a form of DDR SGRAM (synchronous graphics RAM), which was first released by Samsung as a 16 Mbit memory chip in 1998.[26]
One can read and over-write data in RAM. Many computer systems have a memory hierarchy consisting of processor registers, on-die SRAM caches, external caches, DRAM, paging systems and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as \"RAM\" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that access time to rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the highest possible average access performance while minimizing the total cost of the entire memory system (generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom).
Since 2006, \"solid-state drives\" (based on flash memory) with capacities exceeding 256 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random-access memory and \"disks\", dramatically reducing the difference in performance.
First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat... Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.
A different concept is the processor-memory performance gap, which can be addressed by 3D integrated circuits that reduce the distance between the logic and memory aspects that are further apart in a 2D chip.[35] Memory subsystem design requires a focus on the gap, which is widening over time.[36] The main method of bridging the gap is the use of caches; small amounts of high-speed memory that houses recent operations and instructions nearby the processor, speeding up the execution of those operations or instructions in cases where they are called upon frequently. Multiple levels of caching have been developed to deal with the widening gap, and the performance of high-speed modern computers relies on evolving caching techniques.[37] There can be up to a 53% difference between the growth in speed of processor and the lagging speed of main memory access.[38]
So, why is it valuable to understand the difference between storage and memory The answer comes down to performance. If your computer is sluggish or performing poorly, the root cause might lie in insufficient storage or memory. By understanding how both components make your computer run, you can make better decisions about what computer to buy (or whether it makes sense to consider upgrades).
The term random access as applied to RAM comes from the fact that any storage location, also known as any memory address, can be accessed directly. Originally, the term Random Access Memory was used to distinguish regular core memory from offline memory.
One significant difference between RAM and flash memory is that data must be erased from NAND flash memory in entire blocks. This makes it slower than RAM, where data can be erased in individual bits.
Embedded system designers must take into account many considerations when selecting a Flash memory: which type of Flash architecture to use, whether to select a serial interface or a parallel interface, does it need error correction code (ECC), and so on. If the processor or controller supports only one type of interface, this limits the options so the memory may be easy to select. However, this is often not the case. For example, some FPGAs support serial NOR Flash, parallel NOR Flash, and NAND Flash memory to store configuration data. This same memory can be used to store user data, which makes selecting the right memory to use more difficult. In this article series, the different aspects of Flash memories will be discussed, beginning with the differences between NOR Flash and NAND Flash.
The NOR Flash architecture provides enough address lines to map the entire memory range. This gives the advantage of random access and short read times, which makes it ideal for code execution. Another advantage is 100% known good bits for the life of the part. Disadvantages include larger cell size resulting in a higher cost per bit and slower write and erase speeds. For more details on how NOR Flash can be used in embedded systems, see An Overview of Parallel NOR Flash Memory.
As mentioned earlier, NOR Flash memory has enough address and data lines to map the entire memory region, similar to how SRAM operates. For example, a 2-Gbit (256MB) NOR Flash with a 16-bit data bus will have 27 address lines, enabling random read access to any memory location. In NAND Flash, memory is accessed using a multiplexed address and data bus. Typical NAND Flash memories use an 8-bit or 16-bit multiplexed address/data bus with additional signals such as Chip Enable, Write Enable, Read Enable, Address Latch Enable, Command Latch Enable, and Ready/Busy. The NAND Flash needs to provide a command (read, write or erase), followed by the address and the data. These additional operations makes the random read for NAND Flash much slower. For example, the S34ML04G2 NAND Flash requires 30µS compared to 120ns for S70GL02GT NOR Flash. Thus the NAND is 250 times slower.
To overcome or to reduce the limitations of slower read speeds, memory is often read as pages in NAND Flash, with each page being a smaller sub-division of erase blocks. The contents of one page is read sequentially with address and command cycles only at the beginning of each read cycle. The sequential access duration for NAND Flash is normally lower than the random access duration in NOR Flash devices. With the random access architecture of NOR Flash, address lines need to be toggled for each read cycle, thereby accumulating the random access for sequential read. As the size of block of data to read increases, the accumulated delay in NOR Flash becomes greater than NAND Flash. Thus, NAND Flash can be faster for sequential reads. However, due to the much higher initial read access duration for NAND Flash, the performance difference is evident only while transferring large data blocks, often for sizes above 1 KB. 153554b96e
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