Chips vs Modules

Memory modules are often referred to as chips, and vice versa. To be clear, a memory module looks like the one below, and contains a number of memory chips (the small black square ones) soldered to one or both sides of the module.

Chip Characteristics

• DRAM density: This is the size of the chips on the module in megabits. Example: 256 Mbit - equivalent to a 32 MB (Megabyte) chip.
• DRAM organization: You will commonly see this notation on our website, usually written in a format like "64M x4", where 64M is a number of storage units (64 million), x4 (pronounced "by 4") — by the number of bits per chip, which equals the number of bits per storage unit. So, a configuration stated "16-chip 64M x8", means that the module has 16 chips, and each chip has a capacity of 512Mbit (64 Million x8), or 64 MB.
  
  There are x4, x8, and x16 DDR, DDR2 and DDR3 chips. (DDR3 chips are more commonly found in x8 and x16 forms). The x4 chips allow the use of advanced error correction features like Chipkill, and are commonly found in memory used in servers or high end workstation motherboards.

Module Characteristics

• Capacity: Also commonly referred to as size. e.g. 512MB, 1GB, 2GB.
• Number of DRAM Devices: The number of chips (devices) is a multiple of 8 for non-ECC modules and a multiple of 9 for ECC modules. Chips can occupy one side (Single Sided) or both sides (Dual Sided) of the module. The maximum amount of chips per DDR module is 36 (9x4).
• Number of DRAM ranks:
(also known as rows or sides). Any given module can have 1, 2 or 4 ranks, but only 1 rank of a module can be active at any moment of time. When a module has two or more ranks, the memory controller must periodically switch between them by performing close and open operations. Do not confuse rows in this context with rows used to describe internal chip architecture (that is why the term rank is to be preferred). The term sides is also confusing because it incorrectly suggests that this is tied to the physical placement of chips on the module. Read a more detailed explanation of Ranks.
• Timings: CAS Latency (CL), Clock Cycle Time (tCK), Row Cycle Time (tRC), Refresh Row Cycle Time (tRFC), Row Active Time (tRAS).
• Buffering: Typically Registered vs. Unbuffered, but may also be "Buffered" (such as Apple Mac Pro memory), or Registered with Parity.
• Packaging: Typically DIMM or SO-DIMM.
• Voltage: Usually 2.5V for DDR, 1.8V for DDR2, and 1.5V for DDR3.
• Power Consumption: This increases with the clock rate, but is less on lower voltage modules. As a result, newer DDR2 and DDR3 modules consume less power, even though they run at a higher clock rate.

Module and chip characteristics are inherently linked.

Total module capacity is a product of one chip's capacity by the number of chips. ECC modules multiply it by 8/9 because they use one bit per byte for error correction. A module of any particular size can therefore be assembled either from 32 small chips (36 for ECC memory), or 16(18) or 8(9) larger chips.

DDR memory bus width per channel is 64 bits (72 for ECC memory). Total module bit width is a product of bits per chip by number of chips. It also equals number of ranks (rows) multiplied by DDR memory bus width. Consequently a module with greater amount of chips or using x8 chips instead of x4 will have more ranks.

Example: Variations of 1 GB PC2100 Registered DDR SDRAM module with ECC

This example compares different real-world server memory modules with a common size of 1 GB. One should definitely be careful buying 1 GB memory modules, because all these variations can be sold under one price position without stating whether they are x4 or x8, single or dual ranked. Using the RamCity website and finding your specific system or motherboard will ensure you are buying the right upgrade.

There is a common belief that number of module rows or ranks equals number of sides. As the above data shows, this is not true. One can find (2-side, 1-rank) or (2-side, 4-rank) modules. One can even think of 1-side, 2-rank memory module having 16(18) chips on single side x8 each, but it's unlikely such a module was ever produced.

Some content on this page is reproduced from the Wikipedia article DDR SDRAM. Our thanks go to the Authors involved for their contribution.

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