CHAPTER 3 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 the information stored in the memory for processing the data. The storage capacity of a memory depends on the type of the memory package used.
3.1 Types of Memory
	Memory can be categorized into two types, namely volatile memory and non-volatile memory. Volatile memory loses the data as soon as the system is turned off. On the other hand, non-volatile memory does not lose the data even after the system is turned off. Apart from this classification, you can classify the memory into physical memory, virtual memory, and flash memory.
	3.1.1 Physical Memory
	Physical memory comprises of memory chips. Physical memory stores programs and data that the microprocessor requires. Therefore, it enables the microprocessor to access the required programs and data quickly. The different types of physical memory are:
	Random Access Memory (RAM) – Random access means any byte can be accessed in any order. The microprocessor can read and write programs and data to the RAM. It is a type of a volatile memory and therefore it is referred to as a temporary data storage area.
	Read Only Memory (ROM) – Read only means that the programs and data can only be read from it. Therefore, programs and data cannot be written to it. Most PCs contain small amount of ROM to stores programs that enables the computer to boot. It is a non-volatile memory and therefore, it is referred to as a permanent data storage area.
	HYBRID – It is a type of memory that reads and writes data like the RAM and maintains data even when the system is turned off like the ROM.
	The physical memory comprises of memory chips such RAM, ROM, and Hybrid.
	The different types of RAM are :
	_ Dynamic Random Access Memory (DRAM) - Retains data for a few milliseconds.
	The different types of DRAM are:
	_ Extended Data-Out DRAM ( EDO DRAM) – Faster than DRAM. _ Synchronous Dynamic Random Access Memory (SDRAM) – Twice as fast as the EDODRAM. DDR-SDRAM is a type of SDRAM, commonly known as Double Data Rate      Synchronous DRAM. _ Rambus Dynamic Random Access Memory (RDRAM) - Fastest type of DRAM. _ Static Random Access Memory (SRAM) – Faster than the DRAM because it is not refreshed. _ Video RAM (VRAM) – Stores the images that are to be displayed on the computer screen .
	The different types of ROM are :
	_ Programmable Read-Only Memory ( PROM) – Stores the programs permanently. Data can only be written once to the PROM. A PROM is a non-volatile memory. _ Erasable Programmable Read-Only Memory (EPROM) – Clears the program when exposed to the ultra-violet rays. Therefore, the EPROM can be reprogrammed after the program has been erased.
	The different types of ROM are :
	_ Programmable Read-Only Memory ( PROM) – Stores the programs permanently. Data can only be written once to the PROM. A PROM is a non-volatile memory. _ Erasable Programmable Read-Only Memory (EPROM) – Clears the program when exposed to the ultra-violet rays. Therefore, the EPROM can be reprogrammed after the program has been erased.
	The different types of hybrid memory are :
	_ Non-volatile RAM (NVRAM) – Type of SRAM that does not lose the data when the system is turned off _ Electrically Erasable Programmable Read-Only Memory (EEPROM) – Clears the program when exposed to the electrical voltage. Therefore, the EPROM can be reprogrammed after the program has been erased. _ Flash – Type of EEPROM that can be erased and reprogrammed in blocks.
	3.1.2 Virtual Memory
	Today, most PCs use 64MB or 128MB of RAM. However, this MB of RAM may not be sufficient enough to run all the programs that you want to run at a time. Virtual memory enables the computer to work as if it has more RAM than it actually has. Therefore, it helps to increase the speed of the PC. For example, if you simultaneously work with multiple applications such as the Windows operating system, a Windows Explorer, an e-mail program, a Web browser, a Word processor, 64MB of RAM may not be sufficient to run all these programs. In order to run another program, you would have to close some of the currently running programs. However, this problem can be solved using the virtual memory. Virtual memory copies the programs not used frequently to the hard disk. Therefore, it frees up the space in the RAM to run the new programs.Virtual Memory comprises of a set of memory addresses called virtual addresses. Programs can use these virtual addresses to store data and instructions. To copy the virtual memory into the RAM, the operating system divides the virtual memory into unitscalled pages. These pages are stored on the hard disk. When a program is executed, the pages are copied from the hard disk to the RAM. The process of copying the pages from the hard disk to the main memory is called paging or swapping. Windows operating systems from Windows 95 onwards supports virtual memory. The DOS operating system does not supports the concept of virtual memory.
	3.1.3 Flash Memory
	Flash memory is used in the digital camera, cellular phones, LAN switches, PC Cards for notebooks, and video games. This memory performs the action in a flash and as a result, it is termed as the flash memory. Flash memory is a non-volatile memory. It is a type of EEPROM consisting of blocks. EEPROM is erased and reprogrammed in blocks. Flash memory is faster than EEPROM since it is reprogrammed at the block level whereas EEPROM is reprogrammed at the byte level.
	3.2 Types of RAM
	RAM is the main memory of the computer. It holds the operating system, programs, and data that are currently in use. Therefore, it enables the processor to quickly access the programs and data. The processor can quickly read from and write to the RAM as compared to the other storage devices such as the hard disk, floppy disk, and CD-ROM. RAM is small, in both the physical size as well as the amount of data it can hold. It is much smaller than the hard disk. It is available in the form of separate microchips as well as in the form of modules. The different types of RAM are Static RAM, Dynamic RAM, and Video RAM.
	3.2.1 Dynamic Random Access Memory (DRAM)
	Today, most PCs use DRAM as the temporary data storage area. DRAM stores data in the memory cells. Each memory cell contains a transistor and a capacitor. Capacitors lose their charge quickly and as a result, they need to be refreshed. Refreshing helps to retain the data in the memory. The life of data in DRAM is very short for about few milliseconds. In order to retain the data, the storage cells need to be refreshed after few milliseconds. Because of the constant refreshing of cells, this memory is referred to as Dynamic RAM.
	3.2.2 Static Random Access Memory (SRAM)
	This memory is referred to as Static because of the fact that it does not require refreshing like DRAM. SRAM stores the data as long as the power is supplied to the system. This is because SRAM is made up of transistors that do not require refreshing. SRAM is used as the computer's cache memory.
	Difference between SRAM and DRAM
	SRAM stores data till the power is switched off, whereas DRAM stores data only for a few milliseconds.
	SRAM uses a set of transistors for each memory cell, whereas DRAM uses a single transistor and capacitor for each memory cell
	SRAM does not refresh the memory cell after each reading of the transistors reading of the capacitor whereas DRAM needs to refresh the memory cell after each reading of the transistors reading of the capacitor
	In case of SRAM Data access is faster whereas in case of a DRAM data access is slower.
	SRAM consumes more power whereas in case of a DRAM power consumption is less
	SRAM is more expensive then DRAM
	3.2.3 Video Random Access Memory (VRAM)
	VRAM is used in video adapters or video systems. It stores the images that are to be displayed on the computer screen. Before displaying the image on the computer screen, the processor first reads the image from the memory and then writes it in VRAM. From VRAM, the digitally encoded images are converted into analog signals that can be displayed by a monitor. This conversion is done by Random Access Memory Digital-to-Analog Converter, which is a small chip built on the video adapters. Therefore, VRAM acts as a buffer between the processor and the monitor. It provides more bandwidth to work with systems requiring higher resolution and color depths than DRAM and EDO DRAM. VRAM reads and writes the data simultaneously. The processor starts writing the new image in VRAM while the current displayed image is still refreshing. Therefore, VRAM chips are called dual-ported chips.
	3.2.4 Extended Data-Out DRAM (EDO DRAM)
	EDO DRAM is faster than normal DRAM. This is because EDO DRAM starts fetching the next block of memory before the previous block of memory is sent to the processor. It is also known as hyper page mode DRAM.
	3.2.5 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. There are two modules of SDRAMs. They are: _ 2-clock SDRAM - In a 2-clock SDRAM, each clock signal controls two different DRAM chips. _ 4-clock SDRAM - In a 4-clock SDRAM, each clock signal controls four different DRAM chips. The recent computers use 4-clock SDRAM.
	3.2.6 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. 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.
	3.2.7 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. There is one big difference between SDRAM and DDR.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 and non-parity. The server uses the error correction code, normally called as ECC. DDR-SDRAM RDRAM Latest version of the SDRAM Type of the DRAM Connects the DIMM in parallel to the data bus Connects the RIMM in series to the data bus Data transmission is faster Data transmission is slower DIMM and RIMM are the memory packages used for holding the memory chips.
3.3 Working of RAM
	RAM stores the data until the processor is executing the current data. Once the processor finishes the current execution, RAM forwards the next data to the processor. The processor accesses the data from the RAM in a random order with the help of the memory cell address. The cell address contains the row number and the column number of the memory cell. When the processor receives the data, it actually receives the memory cell address of the data. The RAM controller accepts the memory cell address. The RAM controller reads the data from the cell address and then sends the data in its respective address lines. The address lines contain the transistors and the capacitors for reading the data. The control circuit reads the capacitor when the transistor opens the cell. When the capacitor is charged, the memory cell returns a bit value as 1. When the capacitor is empty, the memory cell returns the bit value as 0. In DRAM, each cell contains a pair of transistor and capacitor. Each cell represents a single bit of data in the binary format of 0 and 1. The capacitor stores the data in the memory cell. The transistor with the help of the memory control circuit reads these memory cells. A transistor acts as a switch. It turns on when the control circuit reads the capacitor. It turns off when the capacitor is empty. The DRAM controller refreshes the capacitor after each reading of the memory cell. DRAM refreshes automatically.
3.4 Types of Memory Packages
	Memory package is a small circuit board that contains memory chips. SIMM, DIMM, SODIMM, and RIMM are some of the memory packages. These packages are the form factors of a RAM chip. The installation of a memory depends on the form factors of a RAM. A form factor is the size and shape of the memory packages.
	3.4.1 Single In-line Memory Module (SIMM)
	SIMM is a small circuit board designed to hold the memory chips. It contains pins for accepting data from the control circuit. These circuit boards or modules are known as packages. There are various SIMM packages available based on the number of pins it contains. For example, 30-pin package and 72-pin package.
	30-pin SIMM Package The capacities of a 30-pin SIMM package are 256KB, 1MB, 2MB, 4MB, 8MB, and 16MB RAM. It contains 2, 4, or 8 data chips per module. A 30-pin SIMM package has a data bus width of 8 bits with 9-bit parity. Parity checks the accuracy of the data transmission. An odd parity and an even parity are the two modes of parity checking. 72-pin SIMM Package A 72-pin SIMM package has 32-bit data width with 36-bit parity. PS/2 is another term used for the 72-pin SIMM package. The capacities of a 72-pin package are 1MB, 2MB, 4MB, 8MB, 16MB, 32MB,64MB, and 128MB RAM. It contains 2, 4, 8, or 16 data chips per module. FPM DRAM uses a 72-pin SIMM package. _ EDO DRAM may use either a 30-pin SIMM package or a 72-pin SIMM package. A SIMM package is packed either in Small Outline J-Lead (SOJ) package or in Thin Small Outline Package (TSOP Type II). The SOJ packages are surface mount packages. The pins of this package are shaped as letter ‘J' and are coming out of the chip. As a result, this package is named as SOJ. SOJ Package The TSOP packages are also surface mount packages. These packages are thinner than the SOJ packages. These packages are used in the notebook computers. TSOP Package SOJ and TSOP packages are the soldered DRAM chips. TSOP packages are 1mm in height whereas SOJ packages are 3mm in height.
	3.4.2 Dual In-Line Memory Module (DIMM)
	DIMM package is also a small circuit board that contains the memory chips. The difference between the SIMM and DIMM is that DIMM is a 168-pin package. The data widths of the DIMM packages are 64-bit, 72-bit, or 80-bit. A 168-pin DIMM package is available in the SDRAM, EDO, or FPM DRAM chips. A 168-pin DIMM package has 84 pins on each side of the package. This type of pin configuration helps in placing the DIMM package on the memory socket. The latest computers use a DIMM package. Its pin configuration does not support the motherboard of the old computers. A DIMM package is available in 8MB, 16MB, 32MB, and 64MB sizes. It supports 3.3V and 5.0V of electricity. Check the motherboard manual before installing the DIMM memory module in the system.
	3.4.3 Small Outline Dual In-line Memory Module (SODIMM)
	The laptops and notebook systems use this package. It is the smallest version of the DIMM. The SODIMM package has a notch at the bottom of the circuit board. This notch helps in inserting the SODIMM package in the memory socket. SODIMM packages are available with 144 and 200 pins. A 144-pin SODIMM package has 64-bit data path. The FPM DRAM and EDO DRAM use this package. The 72 pins on both the sides of the package divide a 144-pin package. A 200-pin SODIMM package has 64-bit data path. PC2100 memory and PC2700 memory use this package. The 100 pins on both the sides of the package divide a 200-pin package.
	3.4.4 Micro DIMM
	Micro DIMM stands for Micro Dual Inline Memory Module. This package is smaller than DIMM and SODIMM packages. The sub-notebook systems use these memory packages. The Micro DIMM package pins connect the memory module with the memory socket. These pins provide two communication lines for the module and the system. This package does not have the notch at the bottom. Micro DIMM packages are available with 144 and 172 pins. A 144-pin Micro DIMM package has 64-bit data path. PC100 SDRAM uses this package. The 144- pin Micro DIMM package has two sides. It contains 72 pins on each side of the package. The height of this chip is 1.545 inch long and 1 inch high. A 172-pin DDR Micro DIMM package has 64-bit data path. It is installed in 64-bit systems. The high performance network applications performed by the systems use this package.
	3.4.5 Rambus In-line Memory Module (RIMM)
	RDRAM chip uses the RIMM memory package. This package is same as the DIMM package. It only differs in the pin configuration. The high bandwidth and the low latency applications use this memory package. The RIMM package has a data storage speed of 600 MHz, 711 MHz, 800 MHz, and 1066 MHz. It has 184 connecting pins. The distance between each pin in the RIMM package is 1mm. This package starts operating from 2.5 voltage supply. The RIMM packages are available in 16-bit data buses, 32-bit data buses, and 64-bit data buses. The memory bandwidth of the RIMM package is up to 9.6 GB per second.
3.5 Memory Specifications
	A memory is selected for the system depending on its motherboard configuration. The motherboard should support the different characteristics of the memory. The different memory characteristics are: _ Size of a Data Bus of a Memory Module _ Error Checking and Correcting Module _ Parity and Non-Parity chips
	3.5.1 Size of the Data Bus of a Memory Module
	The size of the data bus specifies the amount of information that is flowing in each clock cycle. The data bus size differs in the different memory modules. A single sided memory module will have a different data bus size from a double-sided memory module. A 30-pin single sided memory module will have an 8-bit data bus whereas a 72-pin single sided memory module will have a 32-bit data bus. A 72-pin double-sided memory module will have a 64-bit data bus. _ Before installing SIMM on the motherboard, check the type and the capacity of the DRAM chip.
	3.5.2 Error Checking and Correcting Module
	The error checking and correcting modules detect single-bit and multiple-bit errors. The memory bus uses this module for detecting and correcting errors of the memory block. A BIOS program is set in the ECC module. This program uses the parity-checking program for checking the accuracy of the data transmission from the memory to the processor. The memory chips group the parity bits into 7-bit block of memory for an ECC module. For example, to protect 32 bits of data, an ECC module uses 7 bits of memory block.
	3.5.3 Parity and Non-Parity
	When the data is transferred from the CPU to the memory, different types of error may occur. The parity-checking module detects these errors. This module only detects and notifies the user about the error. It does not log the error. The error is logged using the ECC module. A SIMM module of 3 bit, 6 bit, 9 bit, 12 bits, 18 bits, or 36 bits chips may either have the parity-checking or the nonparity checking modules with it. If the system requires parity-checking module, then count the integrated chips of the module. The recent Pentium processors do not have parity chips. This is because the memories of the recent processors are generally error free.
3.6 Installing the Memory
	RAM is the main memory of the system. The installation of RAM depends on the amount of memory required for the processor. It also depends on the configuration of the motherboard. Before installing RAM, we need to have some knowledge about the requirement of the system. The following points should be considered before installing the RAM: _ Capacity of the RAM required _ Form Factor of the RAM _ Type of RAM needed _ Tools required for mounting the RAM on the motherboard _ Warranty of the RAM Recently, 128MB, 256MB, and 512MB RAMs are available. While selecting the RAM according to its capacity, check the type of the processor used in the system. The Pentium processors support all the recently available RAMs. A form factor is the size and shape of any electronic device. The installation of the RAM also depends on the form factor of the RAM. The different form factors of a RAM are SIMM, DIMM, and SODIMM.
	Best Practice
	1. Read the manual before starting the installation. 2. Wear the anti-static wristband or any other static control device before touching any of the system components. 3. Check the number of memory sockets present on the motherboard for installing the RAM module.
	Precautions
	1. Hold the RAM module by its edges. Do not touch the chips mounted on the RAM module. 2. Ensure that the power supply to the system is switched off. 3. Unplug all devices before opening the system case.
	Hands on exercise
	To install the 72-pin SIMM package : 1. Remove the anti-static cover of the RAM module. 2. Check the notches of the RAM module. 3. Hold the RAM module at an angle of 45 degree. 4. Match the line of the RAM module notches with the notches of the memory socket. 5. Place the RAM module on the memory socket in the perpendicular position. The chips placed on the module should be facing towards us. 6. Align the metal connectors of the RAM module with the metal connectors of the memory socket. 7. Put a slight pressure while inserting the RAM module on the memory socket. 8. Check whether all the pins of the RAM module are in line with the memory socket. 9. Switch on the computer and follow the instructions given on the manual. The system may either recognize the new RAM on itself or display a message to update the configuration settings. 10. Follow the manual for configuring the new RAM for the system. _ The DIMM package should be inserted in perpendicular to the memory socket. The SODIMM package should be inserted at an angle of 30 degree.
3.7 Upgrading the Memory
	The memory of the system must be upgraded to increase the speed of the system. The memory is upgraded either by changing the previous RAM or by adding one more RAM with the previous RAM. The installation of a new RAM depends on the number of the free available memory sockets on the motherboard.
	Best Practice
	1. Read the manual before starting the installation. 2. Wear the anti-static wristband or any other static control device before touching any of the system components. 3. Check the number of memory sockets present on the motherboard for installing the RAM. 4. Check the type of RAM for mounting on the motherboard. The new RAM should match with the configuration of the previous RAM. 5. Check the difference between the total RAM required and the currently installed RAM, to get the amount of additional RAM required for upgrading.
	Precautions
	1. Hold the RAM module by its edges. Do not touch the chips mounted on the RAM module. 2. Ensure that the power supply to the system is switched off. 3. Unplug all devices before opening the system case.
	Hands on exercise
	To upgrade the memory of the system: 1. Open the system case. 2. Place the RAM module on the memory socket according to the type of the RAM. If the additional RAM module is a SIMM, hold the RAM module in 45 degrees while placing on the memory socket. If the additional RAM module is a DIMM, hold the RAM module perpendicular to the memory socket. 3. Check the notches of the RAM module before placing it on the memory socket. 4. Put a slight pressure while inserting the RAM module on the memory socket. 5. Close the system case. 6. Connect all the peripheral devices to the computer. 7. Switch on the computer and check the current amount of the RAM.
3.8 Troubleshooting
	Troubleshooting the memory using various methods solves the memory problems. The beep codes and the memory diagnostic software are the ways for troubleshooting the memory. The following troubleshooting methods are: _ BIOS Beep Codes _ Award BIOS Beep Codes _ PC Memory Diagnostic Software _ AMI BIOS Beep Codes
	3.8.1 Using the BIOS Beep Codes
	The BIOS program does the POST testing as the system turns on. The POST is Power on Self Test. This testing is done to ensure that the system is working properly. If the computer fails to start, a long beep occurs. This beep indicates that there is a requirement of the hardware engineer to check the BIOS of the motherboard.
	3.8.2 Using the Award BIOS Beep Codes
	Award is a BIOS developer. The Award developer uses beep codes for recognizing the beeps when the computer boots up. If the computer boots up with one short beep, the computer is working fine. If more than one beep occurs, then there is a requirement of the hardware engineer. The following beeps that should be taken care are: _ 1 Long Beep – Shows the memory problem _ 1 Long Beep and 2 Short Beeps – Shows the failure with the DRAM parity _ 1 Long Beep and 3 Short Beeps – Shows the video error _ Continuous Beep – Shows the failure in memory or video memory
	3.8.3 Using the PC Memory Diagnostic Software
	The processor uses the PC Memory Diagnostic software to solve the memory problems. This software is downloaded from the Internet for solving the memory problems. For example, if the computer boots up properly but hangs up after working for an hour, troubleshoot the memory using this software. To troubleshoot the memory: 1. Download the software from the Internet. 2. Follow the setup instructions. 3. Run the diagnostic test on the memory. If the memory shows an error such as an incorrect memory sizing, there must be a problem with the software setting in the CPU. The error number for this error is 164. The computer uses the PC Memory Diagnostic Software to fix this error. If the memory diagnostic test does not show any error, check the motherboard, CPU, and other peripheral devices. The hardware such as hard disks, chips, and memory packages also performs diagnostic tests. They generally show problems due to its aging. The hardware errors can also occur due to the bad soldering performed while mounting the components on the motherboard.
	3.8.4 Using the AMI BIOS Beep Codes
	AMI BIOS is a BIOS used by the motherboard manufacturers. The BIOS indicates the beeps those occur during the POST. Some of the faulty beeps are: _ 1 Beep Tone – Indicates the failure in refreshing a DRAM _ 2 Beep Tone – Indicates the failure in DRAM parity _ 3 Beep Tone – Indicates the failure in Base 64K RAM _ 4 Beep Tone – Indicates an error in the system time _ 5 Beep Tone – Indicates the failure in CPU _ 6 Beep Tone – Indicates an error in the keyboard controller _ 7 Beep Tone – Indicates an error in the virtual mode _ 8 Beep Tone – Indicates an error in the read and write procedure of the display memory _ 9 Beep Tone – Indicates an error in the checksum of the ROM BIOS _ 10 Beep Tone – Indicates an error in the read and write procedure of the CMOS register _ 11 Beep Tone – Indicates an error in the cache memory _ Continuous Beep Tone – Indicates the failure in memory or video memory