Memory: RAM and ROM
RAM (Random Access Memory) is used as the temporary memory space, or workspace of the computer. It is where the operating system, application programs and documents currently in use are stored. RAM is semiconductor memory: it consists of networks of millions of transistors, capacitors and resistors manufactured in integrated circuits or chips. RAM needs continuous power to maintain its information: all data is erased when the power is shut off. (Computers use hard disks, floppy disks, CD-ROMS and other media for permanent storage.) RAM cards or modules are installed in slots on the mainboard, connected to the microprocessor by the processor bus.
Bits, Bytes and Megabytes
Because computers use binary numbers for their internal operations, RAM memory is measured in bytes.
Types of RAM
DRAM - Dynamic RAM uses one transistor and one tiny capacitor to store each bit as a voltage. The capacitor can only store the voltage for several microseconds so it must be recharged or refreshed every 15 microseconds. To refresh the RAM the microprocessor must read all the contents of RAM and rewrite it. This can take several clock cycles, but with the high clock speeds of modern microprocessors, it has become a relatively minor task.
SRAM - Static RAM uses a circuit of six transistors to store each bit of data. It is much faster, because it does not have to be refreshed, but it is also more expensive. It is used for cache memory operating at the same speed as the processor. Because less than 1MB of high speed SRAM connected directly to the microprocessor can dramatically increase the performance of the system, the higher cost is justified.
The data transfer
rate is a measure of the data transferred in one second. The transfer
rate is the product of frequency and bus width.
Access time is the length of time that it takes for the system to find and retrieve data in RAM or on disk. RAM access times typically range from 60 nanoseconds (DRAM) to 2.5 nanoseconds (DDR-SDRAM). Hard disk access times are measured in milliseconds. Access time is inversely related to the clock speed at which RAM can operate.
Until a few years ago, RAM speed was reported in terms of access time. However with the introduction of SDRAM the speed began to be reported in terms of clock speed. For example, 7.5ns SDRAM is known as PC133.
Many factors can cause errors in the transmission of data from RAM chips, including power glitches, radio frequency interference, timing errors and cosmic radiation. Non-parity RAM has no built-in error correction capability. Parity RAM includes an extra bit for every bite stored; the state of the extra bit is determined by the sum of the other 8 bits. The memory controller must have parity-checking capability in order to use Parity RAM. When an error is detected, the controller sends a message to the BIOS or the operating system. ECC RAM uses a system called Error Correcting Code to detect and repair memory errors. Although it is more expensive it provides highly reliable performance in servers and other critical applications.
Types of DRAM
Several improvements have been made to increase the speed of DRAM.
The earliest microcomputers had RAM mounted directly on the mainboard or on expansion cards. To make RAM easier to install or replace, it is now mounted on small printed circuit boards and installed in dedicated slots on the motherboard.
When first introduced, DRAM was packaged in 30-pin Single In-Line Memory Modules (SIMMs). Although the chips may be mounted on both sides of the circuit board, the contacts which make the electrical connections to the motherboard are the same on both sides. 30-pin SIMM capacities ranged from 256kB to 16 MB. Only one byte of memory could be accessed at one time; the other pins are used for power, addressing and control signals. 72-pin SIMMs have 32 pins for data (plus 4 pins for parity). Capacities range from 1 MB to 128 MB, stored in 8 or 9 chips soldered onto the card. 72-pin SIMMs were used in Pentium I systems; since the Pentium has a 64-bit external data bus, memory modules must be installed in pairs in Pentium I systems to make up 64 bits.
SDRAM is packaged in 168-pin DIMMs (Dual In-line Memory Modules) which permits data to be transferred 64 bits at a time. Unlike SIMMs, DIMMs have different contacts on each side of the module, so more than double the number of pins can fit onto a slightly larger area. DDR SDRAM is mounted on 184 pin DIMMs. Although the two types of DIMMs are almost identical in size, the notches on the bottom are different, to prevent the module from being plugged into the wrong slot. Up to 1GB of RAM can be packed on a single DIMM module.
Rambus memory is available in 184-pin and 232 packages, which transfer data 16 or 32 bits at a time.
Read-Only Memory (ROM) is used to store permanent data which will not be lost when the power is turned off. The BIOS information (Basic Input-Output System) is stored on ROM chips on the motherboard so that the computer will always have access to the instructions needed to boot up the computer. ROM memory is also Random Access Memory, but it cannot be used by the system for storing programs like the main memory. Some ROM chips are designed to be rewritten using a special procedure, which allows BIOS manufacturers to provide upgrades. Once it has been rewritten, the ROM will retain the new data when the power is off.
Types of ROM