Memory is categorized into Volatile and Non- volatile Memory
Volatile Memory – Stores data temporarily
Non -Volatile Memory – Stores data permanently even if system supply is turned off
Programmable Read Only Memory (PROM) – Stores programs permanently and is a non-volatile memory. Each binary 1 bit is treated as a fuse
Erasable Programmable Read Only Memory (EPROM) – Ultra violet (UV) rays can remove programs from this memory by causing a chemical reaction and can be easily recognized by clear quartz crystal window
Electrically Erasable Programmable Read Only Memory (EPROM)/ Flash ROM – Can be erased by an electric field. Flash memory can erase or reprogram blocks of bytes (512 bytes) where as EEPROM erases individual bytes
Defines a way to organize physical memory such as RAM and cache. This enables the Operating System to arrange memory into a logical manner such as assigning a logical address. Logical address is a memory location and it is accessed by an application program. System maps the logical address to real physical storage address
Computer's main memory is divided into different categories such as conventional, upper and extended memory based on the motherboard, CPU and Operating System.
Conventional Memory – Size is 640 KB, used by DOS. Also called DOS or base memory
Upper Memory – Lies between 640 KB to 1 MB area. 386 or higher processor can map empty locations to some real memory area
Extended Memory – Memory area lies above 1 MB. Available in 286 and later processor based computers. User can view memory details by typing mem command from the command prompt
Virtual memory is a part of the hard disk which is used as a memory. It has a set of memory addresses and stores the instructions or the data. When the processor executes the instructions, it converts the virtual memory addresses into real memory addresses. The main use of the virtual memory is to increase the address space
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. Flash memory is available as a PC card that you can plug into the PCMCIA slot.
PC BIOS chip in the system is the flash memory.
Cache memory is a small and fast memory which is placed between the CPU and RAM. 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.
Write Through – The write through type of cache memory makes the data go through the cache to the main memory. Once the data is written to the main memory then only the processor can continue with next operation. Processor has to wait till the cache memory updates the main memory
Write Back - When some data is written to this type of cache, the cache accepts the data and informs the processor that the data is written. Afterwards this data is written back to the main memory by the cache whenever it finds some free time.
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 – L2 cache is located outside the processor. It is commonly present on the motherboard. If L2 cache is located on the processor then the cache on the motherboard is referred as L3 cache. The most common size of this type of cache memory is 64 KB to 2 MB
L3 - Today, nearly all modern processors have L2 cache on them, thus the cache on the motherboard is referred to as L3 cache
Hybrid reads and writes data similar to the RAM but, maintains data similar to the ROM. It is a mixture of RAM and ROM. The NVRAM, FLASH, and EEPROM are the various hybrid memories
Static Random Access Memory (SRAM) – Stores the data as long as the power is supplied to the system
Dynamic Random Access Memory (DRAM) – Stores the data for few milliseconds
Fast Page Mode DRAM (FPMDRAM) - Standard DRAM is accessed through a technique called paging. Paging enables faster access to all the data within a given row of memory by keeping the row address the same and changing only the column. Memory that uses this technique is called Page Mode or Fast Page Mode memory.
Extended Data-Out DRAM (EDO DRAM) – Fetches the data from the next cell before the previous process completes. Also known as Hyper Page Mode DRAM
Synchronous Dynamic Random Access Memory (SDRAM) – Synchronizes the memory speed with the CPU clock speed. Speed depends on the speed of the CPU bus. It is faster and runs with an average speed of 133 MHz
Rambus Dynamic Random Access Memory (RDRAM) – It uses 2KB SRAM as cache. It uses a dedicated communication channel with the processor. And the packet size used by the RDRAM is 256 bytes.
DDR-SDRAM – Stands for Double Data Rate - Synchronous DRAM. 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 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, 333, 400, 533 and 667
Video Random Access Memory (VRAM) – Stores the images that are to be displayed on the computer screen. Video adapter or video system uses VRAM. This memory acts as a buffer between the CPU and the video card. It provides more bandwidth than DRAM and EDO DRAM. VRAM is costly as compared to SRAM.
VRAM reads and writes the data simultaneously in one process. It is a multitasking memory. As a result, VRAM chips are dual-ported
Synchronous Graphics RAM (SGRAM) – A less expensive approach to speed up memory access for graphics functions. It is not dual ported and therefore performance is somewhat lower. SGRAM is used in AGP Cards.
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
Processor accesses the data from the RAM in a random order with the help of the memory cell address. Cell address contains the row number and the column number of the memory cell. Each cell contains a pair of transistor and capacitor and represents a single bit of data either 0 or 1. The capacitor stores the data in the memory cell. 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 automatically
Single In-line Memory Module (SIMM) – Contains pins for accepting data from the control circuit. SIMM available in 30 pin and 72 pin packages. Has a data bus with parity. FPM DRAM and EDO DRAM use SIMM package
Dual In-line Memory Module (DIMM) – Similar to a SIMM. It is a 168-pin package and 84 pins on each side of the package. SDRAM, EDO or FPM DRAM use this package. Latest computers use this package. Supports 3.3V and 5V of electricity.
Small Outline Dual In-line Memory Module (SODIMM) – Has a notch at the bottom of the circuit board. Laptop and notebook systems use this package. It is available in 144 and 200 pins. FPM DRAM and EDO DRAM use this package. 72 pins on both sides of the package divide a 144 pin package
Micro DIMM – This is the smallest version of the DIMM. Sub-notebook systems use these memory packages. This package does not have the notch at the bottom. Micro DIMM packages are available with 144 and 172 pins. SDRAM and DDRRAM use this package. It contains 72 pins on each side of the 144 pin package. High performance network applications performed by the system use 172 pin Micro DIMM package.
Rambus in-line Memory Module (RIMM) – RDRAM chip uses this package. Similar to DIMM package. It only differs in the pin configuration. The high bandwidth and low latency applications use this memory package. It has 184 connecting pins. It operates in 2.5 voltage supply
Size of a Data Bus of a Memory Module - Specifies the amount of information that is flowing in each clock cycle. 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
Error Checking and Correcting Module - Error checking and correcting modules detects single-bit and multiple-bit errors. 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
Parity and Non-Parity chips - Parity-checking module detects these errors when the data is transferred from the CPU to the memory. This module only detects and notifies the user about the error. The error is corrected using the ECC module. To determine the parity checking module in 30 and 72 pin SIMMS, then count the number of DRAM chips in the module. An odd number indicates a parity chip and an even number indicates a non-parity chip
Memory Frequency – Front Side Bus (FSB) is speed of the Data Bus. The FSB and Memory must work at the same frequency. To increase the FSB frequency, the memory frequency has to be increased. However, it is possible to run FSB and Memory at different frequencies. In the Pentium 4 processor, a ratio of 5:4 is achievable
Memory Bank - Memory bank is a logical unit of memory in a computer. It consists of one or more memory modules. Processor uses a process called interleaving to switch between two or more memory banks. It needs one clock cycle to reset the bank. CPU can call other memory bank. For example, a 64 bit CPU needs two 72 pin SIMMs (32-bit) or a single 168 pin DIMM (64-bit) to complete a bank. All the memory modules within a bank must be identical
Increases the speed of the system
Change either the previous RAM or add one more RAM with the previous RAM to upgrade the memory
Depends on the number of the free available memory slots on the motherboard.
Recommended to use the memory of the same FSB as of the existing one.
Check the type of RAM before installing a new RAM
The new RAM should match with the configuration of the previous RAM and should be supported by motherboard and processor