COMPUTER MEMORY

Introduction :

Memory is one of the functions of the brain that enables to store and remember the past events. Similarly, in computers the term memory refers to a chip that stores data. It also enables us to retrieve the stored data. The processor retrieves information stored in the memory for processing data. The storage capacity of a memory depends on the type of the memory package used.

1. Memory Specifications

Memory is selected for the system depending on its motherboard configuration. The motherboard should support different characteristics of the memory. The different memory characteristics are:

  • Number of Pins
  • Memory Frequency (FSB)
  • Single channel or dual channel memory

Number of Pins

Each RAM has certain number of pins or connectors using which RAM communicates with the processor through RAM slots.

Memory Frequency

Front Side Bus (FSB) is used to describe the Data Bus. CPU frequency is set as a multiple of FSB. For example, a CPU of 3.2GHz may have a FSB of 1066MHz. This means that the CPU is set to run 3 times the FSB.

The Processor FSB and Memory must work at the same frequency.

Single channel and dual channel memory

Generally RAM speed is lower than that of CPU. So CPU has to wait for the RAM memory to deliver data. In a perfect PC, RAM memory would be as fast as the CPU. RAM increases its data rate by using a technique known as dual channel.

Single channel memory

Dual Channel memory

we enable a dual channel technique for the above single channel memory, memory controller expands their width of data bus 64 to 128 bits. It doubles the data transfer rate of memory Clock rate, capacity and timing will be same as in single channel.

Since each memory module can accept only 64 bits per cycle, two memory modules are used in dual channel to accommodate 128 bit data buses. For dual channel memory we need even number of memory module on system dual channel works by accessing two memory modules in parallel. Figure 2.2 shows working of dual channel memory.

For example, DDR2-800 memory module has a maximum theoretical transfer rate (MTTR) of 6400 MB/s (Megabytes/Second). If we enable dual channel technique with DDR2-800 memory modules MTTR is doubled. I.e. MTTR will be 12, 800 MB/s.

2. Types of Memory

Memory can be divided into two types, volatile memory and non-volatile memory. Volatile memory temporarily stores the data. It loses data as soon as the system supply is turned off. Non-volatile memory stores data permanently. It does not lose the data even if the system supply is turned off.

Further, memory is classified into physical, flash and cache memory.

1. Physical Memory

Physical memory is the total amount of memory installed in the computer. For example, if the computer has two 1GB memory modules installed, it has a total of 2GB of physical memory.

The different types of physical memory are:

  • RAM – Stands for Random Access Memory. It is a semiconductor-based memory where the CPU or the other hardware devices can read and write data. It temporarily stores the data and it is a volatile memory, Once the system turns off, it loses the data. As a result, RAM is used as a temporary data storage area.
  • ROM-Stands for Read Only Memory, It stores the data permanently and it is a non volatile memory. It does not the data even after the system turns off. As a result ROM is a permanent data storage area.
  • The different types of ROM are:
  • PROM-Stands for the Programmable Read-Only Memory. It stores programs permanently and is a non-volatile memory Programming the ROM is sometimes referred to as burning and it requires a special machine called a device programmer or ROM burner. Each binary 1 bit can be thought of as a fuse; when a PROM is programmed, a higher voltage blows or burns the fuses at the desired locations thus turning any given 1 into a 0. Due to this, PROM chips are often referred to as One Time Programmable (OTP) chips.
  • EPROM – Stands for the Erasable Programmable Read-Only Memory. Ultra-violet (UV) rays can remove the programs from this memory. It can be easily recognized by the clear quartz crystal window set on the top of chip. An EPROM eraser is a device that contains a UV light source that erases the chip by causing a chemical reaction, which essentially melts the fuses back together.
  • EEPROM/Flash ROM-Stands for the Electrically Erasable Programmable Read Only Memory. Electrical signal removes the programs from this memory. EEPROM can be erased by an electric field rather than exposed to UV. Also, the data can be erased bit by bit allowing only selected portions of the code to be replaced. But in case of Flash ROM, data is rewritten in blocks of 512 bytes instead of bit by bit, so rewriting is faster. Flash memory can erase or reprogram blocks of bytes where as EEPROM erases individual bytes. This is also called hybrid memory as it reads and writes data similar to the RAM but, maintains data similar to the ROM. It is a mixture of RAM and ROM.

2. Flash Memory

Flash memory is the high-density device. It is a non-volatile memory. It is fast in reading and writing data. This is because it writes data in chunks or blocks. Flash memory is an electrically re-programmable device. The contents from the flash memory are erased in blocks and not in bytes. The block size can range from 256 bytes to 16 KB. Flash memory can replace hard disk in portable computers.

It is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products. Some of the latest memory cards include Multimedia card (MMC), Secure Digital (SD), CompactFlash, Memory Stick, xD Picture Card and SmartMedia. Figure 2.5 shows a flash ROM.

3. Cache Memory

Cache memory is a small and fast memory which is placed between the CPU and main memory. It is accessed at a very high speed than the system memory. As a result, the programs which access the same data or instructions over and over run faster. The CPU does not have to traverse to the main memory to get the data. It will first access the cache to find the data. Figure 2.6 shows the structure of cache memory.

CPU does a check on the cache to find the data, if the data is not found then it proceeds to the system memory. This results in faster access to the data in the cache. But, it will be lagging if the data is not in the cache and on the system memory. This can be countered with modern hardware and better cache design. The time taken to traverse the cache is very less as compared to the whole RAM as it is small and fast memory.

Cache Levels

Cache levels refer to the electronic pathways and connecting circuits via which cache memory is connected to the CPU. They also indicate the physical closeness of cache memory to CPU. Most of the modern computers use two or three cache levels so that processor does not have to wait for longer time for information from the memory.

Different types of cache levels are:

  • L1 – L1 cache is referred to the cache which is built in the processor. This is the fastest cache in the computer. This cache is also known as primary cache or internal cache. The most common size of this type of cache memory is 8 KB to 64 KB.
  • L2 – New CPUs like from Pentium pro onwards usually incorporate the L2 cache directly on the processor chip. Earlier, L2 cache was located outside the processor i.e. commonly present on the motherboard. The most common size of this type of cache memory is 64 KB to 8 MB. L2 cache consists of two components:
  • Data Store: This is the actual area where the cache data is stored.
  • Tag RAM: This is a small area used to keep a map of the addresses of the system memory which are cached in the data store.
  • L3 – Today, nearly all modem processors have L2 cache on them, thus the cache on the motherboard is referred to as L3 cache. Often only high end workstations and servers need L3 cache. Currently, only the Pentium 4 Extreme Edition features L3 cache. L3 has been part of the CPU or externally mounted near the CPU on the motherboard. It comes in many sizes and speeds. Figure 2.7 shows different cache levels.

3. Types of RAM

RAM is the main memory of the computer. It holds the data until the system is turned off. Once the system is switched off, the data is lost. As a result, it is known as the temporary data storage area. There are two types of RAM, Static RAM and Dynamic RAM.

Static Random Access Memory (SRAM)

SRAM is a type of semiconductor memory. It stores the data as long as the power is supplied to the system. Once the power is turned off or is lost temporarily, data stored in SRAM is lost. SRAM uses six transistors for each memory cell. Due to more number of transistors present in the cell, the cells do not refresh frequently. Hence, the data is stored for longer period. Refreshing a cell means re-writing data in a cell.

SRAM is faster in accessing data. The data accessing speed of SRAM makes it behave like a cache memory. SRAM is expensive as compared to DRAM. Example of SRAM is all types of cache memory.

Dynamic Random Access Memory (DRAM)

The lifetime of the data in DRAM is very short. It is approximately for four milliseconds.

The data in DRAM are stored in memory cells. Each memory cell contains a pair of a transistor and a capacitor. Each memory cell is referred as a bit of data, the smallest amount of information that the system can work with. The memory cells of DRAM are refreshed by the DRAM controller after every few milliseconds retain the data in the memory.

The cells in DRAM are arranged in rows and columns. Each cell has a row and a column reference number. DRAM accesses the data using the cell reference number. DRAM is less expensive than SRAM.

Types of DRAM

Different types of DRAMS which are used in a desktop computer are:

  • SDRAM RD RAM
  • DDR1 RAM
  • DDR2 RAM
  • DDR3 RAM

Synchronous Dynamic Random Access Memory (SDRAM)

SDRAM synchronizes the memory speed with the CPU clock speed. The speed of the SDRAM depends on the speed of the CPU bus. It is faster than SRAM, DRAM, EDO DRAM, and VRAM memories. The data transfer speed of SDRAM is measured in nanoseconds and megahertz units. It runs with an average speed of 133 MHz.

Rambus Dynamic Random Access Memory (RDRAM)

RDRAM is the fastest computer memory. It sends data very frequently on the data bus and reads data on every rise and fall of the clock cycle. As a result, the RDRAM memory gets the data transfer speed of 800 MHz. The video memory on the graphic accelerator cards and the cache memories use this chip. The high-performance workstations and the servers use the RDRAM chip. The high bandwidth and the low latency applications also use this memory chip. Figure 2.9 shows a RDRAM.

The RIMM module carries the RDRAM chip. RIMM stands for Rambus Inline Memory Module. The number of RDRAM placed on the module depends on the bus width of the RAM. RDRAM (RAM Bus DRAM) of 160 or 184 Pins operates at 300-400 MHz

DDR-SDRAM

DDR-SDRAM stands for Double Data Rate-Synchronous-DRAM. It is the latest version of SDRAM. DDR is synchronous with the system clock. The difference between SDRAM and DDR is that SDRAM transfers data on the rising edge of the clock signal. However, DDR transfers data on both the edges of the clock signal that is on the rising and falling edges of the clock signal. As a result, the data transfer rate of DDR is faster than SDRAM. It is almost twice the speed of the SDRAM.

This memory chip consumes less power. DDR memory supports Error Correction Code (ECC) and non-parity.

Some of the variations of the DDR RAM available in the market are:

  • DDR 266- This is one of the cheapest DDR memories available in the market. It is also known as PC 2100.
  • DDR 333 – It is required for those users who use powerful applications such as video processing software, latest games etc. It is also known as PC 2700
  • DDR 400-DDR 400 memory, known as PC 3200, is recommended for some of the motherboards with at least P4 2.8 GHz or above or Athlon 64 Processor and should be operated in dual channel mode for better performance.
  • DDR2 533 – DDR-SDRAM memory chips specified to run at 133 MHz, I/O clock at 266 MHz. It is also known as PC2-4200.
  • DDR2 667/DDR2 800) These speeds are possible because of additional features such as a larger prefetch and more improvised registers, PC name for DDR2 667.

DDR3-DDR3 is the third generation of Double Data Rate (DDR) SDRAM memory. Similar to DDR2, it is a continuing evolution of DDR memory technology that delivers higher speeds up to 1866 MHz, lower power consumption and heat dissipation. It is an ideal memory solution where the bandwidth is the major concern for the systems equipped with dual and quad core processors and the lower power consumption is a perfect match for both server and mobile platforms.

The DDR3 memory modules physically look similar to the DDR2 modules. Both the modules, DDR2 and DDR3 have total 240 pins. The only noticeable difference between DDR2 and DDR3 is the location of the notch on the module. Figure 2.11 shows notch location of DDR, DDR 2 and DDR 3 memory modules.

DDR3 has the clock speeds between 1066MHz and 1.6GHz, which is double the speed of DDR2 DDR3 may reduce the power usage by dropping the voltage levels to 1.5V It consumes 30% less power compared to DDR2.

  • Technically, DDR3 memory support is available with the latest Intel chipsets e.g. Intel G41.
  • Improved thermal design (cooler) as compared to DDR2 memory.
  • Hynix, Samsung and Micron are the manufacturers of DDR3 memory.

Summary:

  • Memory can be categorized into a volatile memory or a non-volatile memory.
  • Volatile memory stores the data temporarily whereas the non-volatile memory stores the data permanently.
  • Physical memory is the total amount of memory installed in the computer.
  • RAM is a semiconductor-based memory where the CPU or the other hardware devices can write the data and read the same from it.
  • ROM is the permanent data storage area.
  • PROM stores the programs permanently.
  • EPROM uses the ultra-violet rays to remove the programs from the memory.
  • EEPROM uses the electrical charges to remove the programs from the memory.
  • Flash memories are the high-density devices that write data in blocks or chunks.
  • RAM is the main memory. The Static RAM and the Dynamic RAM are its two types.
  • Transistor is a semiconductor device that opens or closes a circuit for reading the data.
  • SRAM uses around four to six transistors for each memory cell.
  • DRAM uses a pair of a transistor and a capacitor for each memory cell.
  • EDO DRAM starts fetching the data from the next cell before the previous process completes.
  • VRAM stores the images that are to be displayed on the computer screen.
  • SDRAM synchronizes memory speed with the CPU clock speed.
  • RDRAM transfers data at the maximum speed of 800 MHz.
  • DDR-SDRAM transfers data on both the edges of the clock signal that is on the rising and falling edges of the clock signal.
  • RAM increases its data rate by using a technique known as dual channel.
  • Installation of RAM depends on the amount of the memory required for the processor and the configuration of the motherboard.
  • Memory of the system is upgraded either by changing the previous RAM or by adding one more RAM with the previous RAM.
  • Troubleshooting methods are: Using BIOS beep codes and Using PC Memon Diagnostic Software.

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As a computer engineer, live in Nagpur city, state Maharashtra, country India. country code - +91.
Posts created 5

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