Random Access Memory (RAM)

Dynamic RAM (DRAM) requires a constant supply of power to refresh and retain its contents. Recent advances in DRAM chips have produced three types of DRAM chips:

• Synchronous DRAM (SDRAM) is faster than conventional RAM chips and more expensive. SDRAM chips streamline operations by coordinating or synchronizing the movement of data and instructions between the chip and other components in the system unit.
• Double data rate SDRAM (DDR SDRAM), also known as SDRAM II, is faster, more reliable, and more expensive than SDRAM. DDR SDRAM chips are able to transfer twice (double) as much data in the same amount of times as SDRAM.
• Direct Rambus© DRAM (Direct RDRAM) is the fastest and most expensive.

Almost all of today’s microcomputers use a combination of DRAM chips.

Static RAM (SRAM), like DRAM, requires a constant supply of power. Compared to DRAM, SRAM does not require as much power, is faster, and is more reliable. SRAM is also more expensive and typically used for specialized applications. One of these applications is for cache memory or RAM cache.

Cache (pronounced “cash”) memory improves processing by acting as a temporary high-speed holding area between the memory and the CPU. In a computer with a cache (not all machines have one), the computer detects which information in RAM is most frequently used. It then copies that information into the cache. When needed, the CPU can quickly access the information from the cache.

There are three different types or levels of cache:

• Level 1 (L1), also known as primary cache and internal cache, is built into the microprocessor chip.
• Level 2 (L2), also known as external cache, is slower than L1 but has a greater capacity. In older computers with older microprocessors, the Level 2 cache is located on a chip that is plugged into the system board. New computers with newer microprocessors have the L2 caches built into the microprocessor. This arrangement, sometimes referred to as advanced transfer cache, provides faster response than cache located on the system board.
• Level 3 (L3), the newest type of cache, works with special microprocessor L2 caches. L3 caches are not built into the microprocessor. Rather, L3 caches use SDRAM chips located on the system board.

Most of today’s microcomputers have two or three types of cache. The most powerful have all three types.

Flash RAM or flash memory chips can retain data even if power is disrupted. This type of RAM is the most expensive and used primarily for special applications such as for digital cell telephones, digital video cameras, and portable computers.