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Memory Technology

CMOS
Complimentary Metal Oxide Semi Conductor (CMOS) devices are low power devices frequently used in ISA systems to store the system setup.Data stored in CMOS are volatile and are lost after the power is removed. A number of packages are available that not only store system setup but also include a system clock and battery. The battery is used to maintain the stored configuration as well as the clock.

DRAM
Dynamic Random Access Memory (DRAM) is most commonly used in main memory subsystems. Memory technology is very slow compared to processor requirements. Consequently use of static RAM caches on computers is now common. Previously speeds were in the 60ns to 80ns range.
SRAM
Static Random Access Memory (SRAM) is an extremely fast, volatile memory device. It is nearly exclusively used in cache memories, takes up a large amount of space, and generates significant heat.

DRAM and SRAM devices

DRAM and SRAM devices from the two major categories of memory chip technology.

DRAM and SRAM similarities

DRAM chips are similar to SRAM chips in several areas:

• Both are random access devices. The term random access implies that when a RAM chip is addressed, the data in that address is found immediately. It is not necessary to search sequentially through other addresses before finding the information.
• Both DRAM and SRAM are volatile memory devices. The data they store remain intact only as long as power is applied to the memory chips.
• Both the ram s are read and write devices, unlike ROM memory devices.
• Both DRAM and SRAM are organized internally into rows and columns.At the intersection of a row and column is found one data bit.

DRAM and SRAM differences

Although DRAM and SRAM serve similar functions, there are some significant functional and physical differences between the two memory types:

• Information Storage - In a DRAM chip, the data bit (a zero or a one) that is stored at the intersection of a row and column is stored as a charge on a capacitor. In SRAM, a data bit is stored as the state of a transistor latch (a latch can be made of up to 4 transistors)

The fact that SRAM relies on up to four transistors per data cell is the fundamental reason why SRAM chips are so much hotter than DRAM chips and why they consume so much power. This is also the reason for the higher cost of SRAM chips.

• Performance - Because data is stored in a DRAM chip as a charge on a capacitor, and because all capacitors lose their charge in time , DRAM chips most periodically be refreshed This process re-energizes the bits before the charge is completely gone. SRAM chip does not suffer this limitation.. When a DRAM chip is undergoing refresh, it is unavailable to the CPU or any other devices in the computer that might need to use it.

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