Every program that you open, while working on your computer, fills the memory with data. This data in the main memory, which is temporarily stored in the RAM modules, is processed by the computer’s processor – also known as the CPU (Central Processing Unit). A small part of the RAM blocks is used as the cache memory. It is connected directly to the processor and allows particularly fast access to data. RAM is extremely cheap and easy to upgrade. It is a common practice to breathe new life into an older PC with more RAM. The two types of RAM that are in use today are DDR3-SDRAM and DDR4-SDRAM. They are standardized by JEDEC. SDRAM stands for Synchronous Dynamic Random Access Memory, i.e. DRAM with a clock rate specified by an external memory controller. With DDR-SDRAM (Double Data Rate), data is transmitted on the rising and falling edge of the clock signal which speeds up communication between the memory chip and the memory controller present in the processor. The clock is doubled in the DDR3-SDRAM. With DDR4-SDRAM the clock frequency of the memory chips is higher compared to the DDR3-SDRAM. DDR3-SDRAM is mostly used today in lower-tier computers. The memory types GDDR4 and GDDR5 used in current graphics cards are both modifications of DDR3-SDRAM. In desktop PCs, the main memory is usually mounted in the form of DIMMs (Dual Inline Memory Modules), while more compact SO-DIMMs (Small Outline Dual Inline Memory Modules) are used in notebooks. In some devices, the main memory is also soldered onto the mainboard and cannot be replaced or upgraded.