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RAM And ROM

The document provides an overview of computer memory, distinguishing between primary and secondary memory. Primary memory includes RAM and ROM, with RAM being volatile and faster for active processes, while ROM is non-volatile and used for permanent data storage. Various types of RAM, including DRAM and SRAM, as well as advancements like SDRAM, DDR, and their successors, are discussed in detail.

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By Gihan C. Ranasinghe
 Memory is required in computers to store data and instructions.  Memory is physically organized as a large number of cells that are capable of storing one bit each.  Logically they are organized as groups of bits that are assigned an address.  Data and instructions are accessed through these memory address.  The speed with which these memory addresses can be accessed determines the cost of the memory.  Faster the memory speed, higher the price.
 Computer memory can be said to be organized in a hierarchical way where memory with the fastest access speeds and highest costs lies at the top whereas those with lowest speeds and hence lowest costs lie at the bottom.  Based on this criteria memory is of main two types.  Main two types are; Primary Memory Secondary Memory Although Cache consider as a Memory, it is an intermediate memory. So it doesn't group as a Main type of Memory.
Primary Memory  Primary memory is computer memory that a processor or computer accesses first or directly.  It allows a processor to access running execution applications and services that are temporarily stored in a specific memory location.  Primary memory is also known as primary storage or main memory.  Primary memory is considered faster than secondary memory.
 As soon as a computer starts, primary memory loads all running applications, including the base operating system (OS), user interface and any user-installed and running software utility.  A program/application that is opened in primary memory interacts with the system processor to perform all application specific tasks.  There are two types of Primary Memory,  RAM  ROM
RAM  Alternatively referred to as main memory, primary memory, or system memory, Random Access Memory (RAM) 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, the computer can access the data much faster.
Characteristics of RAM  These are semiconductor memories.  It is known as the main memory.  Volatile memory.  Data is lost in case power is switched off.  It is the working memory of the computer.  Faster than secondary memories.  A computer cannot run without the primary memory.
 As the computer boots, parts of the operating system and drivers are loaded into memory, which allows the CPU to process the instructions faster and speeds up the boot process.  After the operating system has loaded, each program you open, such as the browser you're using to view this page, is loaded into memory while it is running.  If too many programs are open the computer will swap the data in the memory between the RAM and the hard disk drive.  RAM as we know it today, as solid state memory, was first invented in 1968 by Robert Dennard.  Known specifically as dynamic random access memory, or DRAM, transistors were used to store bits of data.
 There are two main categories of RAM;  DRAM  SRAM Dynamic random-access memory (DRAM)  DRAM is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit.  The capacitor can be either charged or discharged. These two states are taken to represent the two values of a bit, conventionally called 0 and 1.
Static random-access memory (static RAM or SRAM)  SRAM is a type of semiconductor memory that uses bistable latching circuitry (flip-flop) to store each bit.  SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.  The term static differentiates SRAM from DRAM which must be periodically refreshed.  SRAM is faster and more expensive than DRAM; it is typically used for CPU cache while DRAM is used for a computer's main memory.
SDRAM  SDRAM, which is short for Single Data RAM or 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 also supports up to 133 MHz system bus cycling.  Since 1993, this has been the prevalent type of memory used in computers around the world.  In the picture below is an example of a SDRAM.
 It was one of the first memory architectures to support Synchronous Memory architectures and was only known as SDRAM at its time.  Single Data Rate means that it can transfer one machine word (16 bits for the x86 architecture) of data during one clock cycle.  Common SDR memory standards included PC-100 and PC-133 which ran on clock speeds of 100MHz and 133MHz respectively.
DDR  Short for Double Data Rate, DDR is memory that was first introduced in 1996 and has since been replaced by DDR2.  DDR utilizes both the rising and falling edge of the system clock, potentially doubling the speed of the memory.  Today, DDR technology is found on high-end video cards and computer memory  DDR improved upon the SDR design by providing double the data during one clock cycle.  This provided a significant increase in performance over the traditional architecture.
 For marketing purposes, DDR memory clocks have always been promoted at speeds twice their original value.  For example, common memory standards for DDR included DDR-200, DDR-266, DDR-333 and DDR-400 which actually had respective clock speeds of 100MHz, 133MHz, 166MHz and 200MHz.
RDRAM  Short for Rambus Dynamic Random Access Memory, RDRAM was developed by Rambus and first became available to computers in 1999.  RDRAM is a memory technology that offers a 1.6 GB/s of theoretical memory bandwidth and offers improved memory access at a slightly higher cost when compared to SDRAM.  After Intel's contract with Rambus expired in 2001, Intel shifted its focus primarily to DDR SDRAM and today, RDRAM is not often found in computers.
DDR2  Short for double data rate two, DDR2 is the second generation of DDR memory that was released in September of 2003.  DDR2 is capable of operating at greater speeds than DDR, offers a greater bandwidth potential, operates on less power, and generates less heat.  Due to architectural differences, DDR2 memory modules are incompatible with DDR slots.
DDR3  Short for double data rate three, DDR3 is a type of dynamic random-access memory (DRAM) released in June of 2007 as the successor to DDR2.  DDR3 chips have bus clock speed of 400 MHz up to 1066 MHz, range in size from 1 to 24 GB, and consume nearly 30% less power than their predecessors.  DDR3 RAM sticks for a desktop computer have 240 pins. For a laptop computer, DDR3 RAM sticks have 204 pins.
DDR4  Short for double data rate four, DDR4 is a type of system memory known as SDRAM and was released in September of 2014 as the successor to DDR3.  DDR4 has bus clock speeds that range from 800 to 1600 MHz and range in storage capacity from 4 to 128 GB per DIMM (Dual In-line Memory Module).  DDR4 is also more efficient at 1.2 V when compared to DDR3's 1.5 to 1.65 V range.
Difference between DDR1, DDR2 & DDR3
Difference between SDRAM versions….
Difference between DDR3 & DDR4
ROM  ROM stands for Read Only Memory.  This is a non-volatile type of memory which is found in computers and other electronics devices.  It is used for storing data permanently.  This type of memory cannot be modified by the computer but it can be read and therefore it is known as the read only memory.  It keeps the bootstrap program which runs on it own when the system is turned on or reset every time.  Some of the common types of ROM include PROM, EPROM and EEPROM.
PROM  It stands for Programmable Read Only Memory. It was first developed in 70s by Texas Instruments.  It is made as a blank memory.  A PROM programmer or PROM burner is required in order to write data onto a PROM chip.  The data stored in it cannot be modified and therefore it is also known as one time programmable device.
EPROM  It stands for Erasable Programmable ROM.  It is different from PROM as unlike PROM the program can be written on it more than once.  This comes as the solution to the problem faced by PROM. The bits of memory come back to 1, when ultra violet rays of some specific wavelength falls into its chip’s glass panel.  The fuses are reconstituted and thus new things can be written on the memory.
 It stands for Electrically Erasable Read Only Memory.  These are also erasable like EPROM, but the same work of erasing is performed with electric current.  Thus, it provides the ease of erasing it even if the memory is positioned in the computer. It stores computer system’s BIOS.  Unlike EPROM, the entire chip does not have to be erased for changing some portion of it.  Thus, it even gets rid of some biggest challenges faced by using EPROMs. EEPROM