Memory geometry
Encyclopedia
In the design of modern personal computer
s, memory geometry describes the internal structure of random-access memory
. Memory geometry is of concern to consumers upgrading their computers, since older memory controllers may not be compatible with later products. Memory geometry terminology can be confusing because of the number of overlapping terms.
Memory geometry describes the logical configuration of a RAM module, but consumers will always find it easiest to grasp the physical configuration. Much of the confusion surrounding memory geometry occurs when the physical configuration obfuscates the logical configuration. The first defining feature of RAM is form factor. RAM modules can be in compact SO-DIMM
form for space constrained applications like laptops, printers, embedded computers, and small form factor
computers, and in DIMM format, which is used in most desktops.
The other physical characteristics, determined by physical examination, are the number of memory chips, and whether both sides of the memory "stick" are populated. Modules with the number of chips equal to some power of 2 do not support memory error detection or correction. If there are extra chips (between powers of 2), these are used for ECC. RAM modules are 'keyed' by indentations on the sides, and along the bottom of the module. This determines the technology, and classification of the modules, for instance whether it is DDR2, or DDR3, and whether it is suitable for desktops, or for servers. It is important to make sure that the keying of the module matches the key of the slot it is intended to occupy. Additional, non-memory chips on the module are an indication that it could be designed for high capacity memory systems for servers, and that the module may be incompatible with desktop systems.
As the next section of this article will cover the logical architecture, which covers the logical structure spanning every populated slot in a system, the physical features of the slots themselves becomes important. By consulting the documentation of your motherboard, or reading the labels on the board itself, you can determine the underlying logical structure of the slots. When there is more than one slot, they are numbered, and when there is more than one channel, the different slots are separated in that way as well - usually color-coded.
are used.
Channels are the highest level structure at the local memory controller level. Modern computers can have two, three or even more channels. It is usually important that, for each module in any one channel, there is a logically identical module in the same location on each of the other populated channels.
Module capacity is the aggregate space in a module measured in byte, or - more generally - in words. Module capacity is equal to the product of the rank density and the number of ranks, and where the rank density is the product of rank depth, and rank width. The standard format for expressing this specification is (rank density) Mbit
x (rank width)x(number of ranks).
Banks are one of the chief sources of physical/logical confusion. Banks are a logical distinction that corresponds to CSRows in low level addressing, for more details on the addressing of RAM in this POV, see dynamic random access memory
. As memory density has increased, the numbers of ranks in each chip has also increased. As an example of a simple configuration, say there is one module with 8 chips, or devices, on each side, and one rank in each of the 16 chips, and each bank was 8 bits wide, that module would have one bank for each side. One bank on one side, and one bank on the back. Today you could have a module composed of Micron Technology
MT47H128M16 chips with the organization 16Mb x 16 x 8 with a bank width of 16. With a module which has of 8 of those on each side, there would be 2 "x16" "banks" on each side. Memory controllers such as the Intel 945 Chipset
list the configurations they support: "Supports 256-Mb, 512-Mb, and 1-Gb DDR2 technologies for x8 and x16 devices." "Supports four banks for all DDR2 devices up to 512-Mbit density. Supports eight banks for 1-Gbit DDR2 devices." As an example, take an i945 memory controller with 4 Kingston
KHX6400D2/1Gs.
Kingston describes each module as having a geometry of 128Mx64, meaning that each one has 64 bits 128 million deep, equaling 8.192 billion bits, or 1.024 Gigabytes. Kingston describes each "device", or chip as having a geometry of 64Mx8, so each module has four banks. So from the MCH
point of view when there are 4 1GB modules, it sees 2 channels, each with 8 banks.
that make up each module, or module of RAM. The most important measurement of a chip is its density, measured in bits. Because memory bus width is usually larger than the number of chips, most chips are designed to have width, meaning that they are divided into equal parts internally, and when one address "depth" is called up, instead of returning just one value, more than one value is returned. In addition to the depth, a second addressing dimension has been added at the chip level, banks. Banks allow one bank to be available, while another bank is unavailable because it is refreshing
. An example of chip notation is 64Mb (depth) X 8 (width) X 8 Banks.
module could be composed of four 8-bit wide (x8) chips. As noted in the memory channel part, one physical module can be made up of one or more logical ranks. If that 32-bit SIMM were composed of eight 8-bit chips the SIMM would have two ranks. An example of Module notation is 128 Mb x 64-bit.
of memory controllers. For example, each socket of a two socket AMD K8
can have a two channel memory controller, giving the system a total of four memory channels.
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
s, memory geometry describes the internal structure of random-access memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
. Memory geometry is of concern to consumers upgrading their computers, since older memory controllers may not be compatible with later products. Memory geometry terminology can be confusing because of the number of overlapping terms.
Physical features
SO-DIMM
A SO-DIMM, or small outline dual in-line memory module, is a type of computer memory built using integrated circuits.SO-DIMMs are a smaller alternative to a DIMM, being roughly half the size of regular DIMMs...
form for space constrained applications like laptops, printers, embedded computers, and small form factor
Small form factor
A small form factor is a computer form factor designed to minimize the volume of a desktop computer. For comparison purposes, the size of an SFF case is usually measured in litres. SFFs are available in a variety of sizes and shapes, including shoeboxes, cubes, and book-sized PCs...
computers, and in DIMM format, which is used in most desktops.
The other physical characteristics, determined by physical examination, are the number of memory chips, and whether both sides of the memory "stick" are populated. Modules with the number of chips equal to some power of 2 do not support memory error detection or correction. If there are extra chips (between powers of 2), these are used for ECC. RAM modules are 'keyed' by indentations on the sides, and along the bottom of the module. This determines the technology, and classification of the modules, for instance whether it is DDR2, or DDR3, and whether it is suitable for desktops, or for servers. It is important to make sure that the keying of the module matches the key of the slot it is intended to occupy. Additional, non-memory chips on the module are an indication that it could be designed for high capacity memory systems for servers, and that the module may be incompatible with desktop systems.
As the next section of this article will cover the logical architecture, which covers the logical structure spanning every populated slot in a system, the physical features of the slots themselves becomes important. By consulting the documentation of your motherboard, or reading the labels on the board itself, you can determine the underlying logical structure of the slots. When there is more than one slot, they are numbered, and when there is more than one channel, the different slots are separated in that way as well - usually color-coded.
Logical features
In the 90s specialized computers were released where two computers that each had their own memory controller could be networked at such a low level that the software run could use the memory, or CPU of either computer as if they were one unit. With AMD's release of the Opteron, and Intel's corresponding systems that share more than one memory controller in a single system have become common in applications that require the power of more than one common desktop. For these systems schemes like Non-Unified Memory ArchitectureNon-Uniform Memory Access
Non-Uniform Memory Access is a computer memory design used in Multiprocessing, where the memory access time depends on the memory location relative to a processor...
are used.
Channels are the highest level structure at the local memory controller level. Modern computers can have two, three or even more channels. It is usually important that, for each module in any one channel, there is a logically identical module in the same location on each of the other populated channels.
Module capacity is the aggregate space in a module measured in byte, or - more generally - in words. Module capacity is equal to the product of the rank density and the number of ranks, and where the rank density is the product of rank depth, and rank width. The standard format for expressing this specification is (rank density) Mbit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
x (rank width)x(number of ranks).
Banks are one of the chief sources of physical/logical confusion. Banks are a logical distinction that corresponds to CSRows in low level addressing, for more details on the addressing of RAM in this POV, see dynamic random access memory
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
. As memory density has increased, the numbers of ranks in each chip has also increased. As an example of a simple configuration, say there is one module with 8 chips, or devices, on each side, and one rank in each of the 16 chips, and each bank was 8 bits wide, that module would have one bank for each side. One bank on one side, and one bank on the back. Today you could have a module composed of Micron Technology
Micron Technology
Micron Technology, Inc. is an American multinational corporation based in Boise, Idaho, USA, best known for producing many forms of semiconductor devices. This includes DRAM, SDRAM, flash memory, SSD and CMOS image sensing chips. Consumers may be more familiar with its consumer brand Crucial...
MT47H128M16 chips with the organization 16Mb x 16 x 8 with a bank width of 16. With a module which has of 8 of those on each side, there would be 2 "x16" "banks" on each side. Memory controllers such as the Intel 945 Chipset
Chipset
A chipset, PC chipset, or chip set refers to a group of integrated circuits, or chips, that are designed to work together. They are usually marketed as a single product.- Computers :...
list the configurations they support: "Supports 256-Mb, 512-Mb, and 1-Gb DDR2 technologies for x8 and x16 devices." "Supports four banks for all DDR2 devices up to 512-Mbit density. Supports eight banks for 1-Gbit DDR2 devices." As an example, take an i945 memory controller with 4 Kingston
Kingston Technology
Kingston Technology Company, Inc. is an American privately held, multinational computer technology corporation that develops, manufactures, sells and supports flash memory products and other computer-related memory products. Headquartered in Fountain Valley, California, USA, Kingston Technology...
KHX6400D2/1Gs.
Kingston describes each module as having a geometry of 128Mx64, meaning that each one has 64 bits 128 million deep, equaling 8.192 billion bits, or 1.024 Gigabytes. Kingston describes each "device", or chip as having a geometry of 64Mx8, so each module has four banks. So from the MCH
Northbridge (computing)
The northbridge has historically been one of the two chips in the core logic chipset on a PC motherboard, the other being the southbridge. Increasingly these functions have migrated to the CPU chip itself, beginning with memory and graphics controllers. For Intel Sandy Bridge and AMD Fusion...
point of view when there are 4 1GB modules, it sees 2 channels, each with 8 banks.
Memory chip
The lowest form of organization covered by memory geometry, sometimes called "memory device". These are the component ICsIntegrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
that make up each module, or module of RAM. The most important measurement of a chip is its density, measured in bits. Because memory bus width is usually larger than the number of chips, most chips are designed to have width, meaning that they are divided into equal parts internally, and when one address "depth" is called up, instead of returning just one value, more than one value is returned. In addition to the depth, a second addressing dimension has been added at the chip level, banks. Banks allow one bank to be available, while another bank is unavailable because it is refreshing
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...
. An example of chip notation is 64Mb (depth) X 8 (width) X 8 Banks.
Memory module
Some measurements of modules are size, width, speed, and latency. A memory module consists of a multiple of the memory chips to equal the desired module width. So a 32-bit SIMMSIMM
A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module , the most predominant form of memory module today, in that the contacts on a SIMM are redundant...
module could be composed of four 8-bit wide (x8) chips. As noted in the memory channel part, one physical module can be made up of one or more logical ranks. If that 32-bit SIMM were composed of eight 8-bit chips the SIMM would have two ranks. An example of Module notation is 128 Mb x 64-bit.
Memory channel
A memory channel is made up of ranks. Physically a memory channel with just one memory module might present itself as having one or more logical ranks.Controller organization
This is the highest level. In a typical computer there will only be a single memory controller with only one or two channels. The logical features section described NUMA configurations, which can take the form of a networkComputer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
of memory controllers. For example, each socket of a two socket AMD K8
Opteron
Opteron is AMD's x86 server and workstation processor line, and was the first processor which supported the AMD64 instruction set architecture . It was released on April 22, 2003 with the SledgeHammer core and was intended to compete in the server and workstation markets, particularly in the same...
can have a two channel memory controller, giving the system a total of four memory channels.
See also
- DIMMDIMMA DIMM or dual in-line memory module, comprises a series of dynamic random-access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers, workstations and servers...
- List of device bandwidths
- Dynamic random access memoryDynamic random access memoryDynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
- Random-access memoryRandom-access memoryRandom access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
- Memory organisationMemory organisationThere are several ways to organise memories with respect to the way they are connected to the cache:# one-word-wide memory organisation# wide memory organisation# interleaved memory organisation# independent memory organisation-One-Word-Wide:...
- Memory addressMemory addressA digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
- Memory bankMemory BankA memory bank is a logical unit of storage in electronics, which is hardware dependent. In computer the memory bank may be determined by the memory access controller and the CPU along with physical organization of the hardware memory slots....
- Bank switchingBank switchingBank switching is a technique to increase the amount of usable memory beyond the amount directly addressable by the processor. It can be used to configure a system differently at different times; for example, a ROM required to start a system from diskette could be switched out when no longer...
- Double-sided RAMDouble-sided RAMDouble-sided RAM is a type of random-access memory which has its chips divided into two sides , only one of which can be seen at a time by the computer. Initially, these were created by essentially attaching two single-sided SIMM cards to the same PCB, but more modern chips use different wiring...
- Dual-channel architectureDual-channel architectureMulti-channel architecture is a technology that increases the transfer speed of data between the RAM and the memory controller by adding more channels of communication between them. Theoretically this multiplies the data rate by exactly the number of channels present. Dual-channel memory employs...
- Page address registerPage address registerA page address register contains the physical addresses of pages currently held in the main memory of a computer system. PARs are used in order to avoid excessive use of an address table in some operating systems. A PAR may check a page's number against all entries in the PAR simultaneously,...