Q. Regarding SIMMS memory modules - What does 4 X 32 or 4 X 36 mean ?
A. Simply put, the 32 (bytes data path) refers to non-parity memory & 36 refers to parity( has an extra parity bit for error checking capability) memory. The first #,here 4, when multiplied by 4
( a constant for simms ) gives the amt. of memory on that module which in this example is 16Mb.
Also note: Memory capacity is always given in bytes. When information is displayed on the chip label, it is listed in bits. This number is always a multiple of 8 or 9. If it is a multiple of 8, it is non-parity. If it is a multiple of 9, it supports parity. ( For example a 16 x 32 means 32 is a multiple of 8 times 4 this, so this means it does not support parity. Each chip holds 16 x 32,000,000 bits = 512,000,000 bits. 512,000,000 bits divided by 8 bits = 64,000,000 bytes.)( 8 bits = 1 byte ) ( 1 MB = 1,000,000 bytes )
Q. Regarding DIMMS memory modules - What does 4 X 64 mean ?
A. The 64 (bytes data path) refers to non-parity memory. The 4 is multiplied by 8 ( a constant for dimms ) to give you 32 Mb of memory on that module. ( see above )
A DIMM OUTLOOK ON THE FUTURE
( Information Partially Provided by DataRam Web Site )
What is a 168 pin DIMM? A DIMM is an add-in memory board used in high performance PCs and workstations. The word DIMM is an acronym for Dual In-Line Memory Module. The "Dual In-Line" refers to the connector arrangement. The connector is the part of the board that makes electrical contact when the DIMM is installed in the system. All data, address, control signals and power pass from the system board to the DIMM through this connector. The connector consists of a dual row of contacts built-in along the bottom edge of the board. You may have heard some people refer to these contacts as fingers, but for consistencies sake we will refer to them as contacts. A DIMM has two sets of unique contacts with 84 gold contacts found on the front of the module and 84 gold contacts found on the back of the module. This is a total of 168 individual contacts in contrast to only 72 contacts on a SIMM. The "Memory" refers to the memory chips on the module.
The DIMM can be thought of as an evolutionary step up from the SIMM used in most of today PC's. The higher connector pin count allows the DIMM to have a wider data bus. This wider data bus translates to more memory on one board. In many cases, only one DIMM needs to be installed in a system whereas a SIMM required two or four boards to have equal capacity. DIMMs are available in "capacities" as small as 4 Mega Bytes up to 128 Mega Bytes. As DRAM technology grows so will the capacities and speeds of the DIMMs. DIMMs are available in organizations of non-parity, parity and ECC and a data width of 64, 72 or 80. The 168 pin DIMM should not be confused with SO-DIMM that are mainly used in notebook computer applications.
DIMMs are available in a variety of organizations, voltages, speeds, memory types , physical dimensionsa, buffered and unbuffered. All DIMMs regardless of type have certain characteristics in common. All DIMMs have connector keying that prevents the wrong type of DIMM from being installed in your system. All DIMMs - Presence Detect so the system can identify the module features when it is installed. All DIMMs are comprised from the same basic components: Printed Circuit Board ,Memory Chips, Buffers, Resistors and Capacitors.
The DIMM installation is simple and in most cases requires no special tools. A word of caution about static protection: DIMMs contain static sensitive devices. When installing DIMMs or any other electronic component in you computer it is vital that you ground yourself. Static discharge will damage the DIMM and any other component in your computer. Static may not cause an immediate failure but it can weaken a component to cause a failure at some future time.
DIMMS are presently available in two voltages: +5 VDC and +3.3 VDC
A 5 volt DIMM must not be installed in a 3.3 system. Fortunately DIMMs have KEYING that prevents the wrong type of DIMM from being installed. Refer to your systems manufacture guidelines on DIMM type required.
DIMMs are available in buffered and unbuffered configuration. This is another option where the user does not have a choice. Your system dictates what type of DIMM is used. Refer to your systems manufacturers guidelines for the DIMM type required.
Buffered DIMMs use a logic device called a "Buffer" to help reduce loading on the bus and improve signal quality at the DRAM. All address signals and most control signals are buffered. Row Address Strobe is not buffered to maintain RAS access time. Data is not buffered.
Unbuffered DIMMs have no buffering between the bus and DRAMs. The entire DRAM is connected directly to the bus. Systems that are capable of using unbuffered DIMMs have been carefully designed taking into account the loading effects an unbuffered DIMM has on the system.
Buffers
The buffer is an Integrated Circuit (IC) that is solder on the PCB. The buffer assists in reducing the loading on the system bus and improves signal quality to the DRAM. Not all DIMMs require buffers. The need for a buffer is dictated by the system for which the DIMMs are intended.
Most SIMMs are not buffered. Therefore, the system must be designed so that each signal can "drive" all of the devices on that SIMM. If there are an unusually high number of devices on the SIMM, a buffer may be used to reduce the loading of the devices. Another case where a buffered SIMM might be used is in a system with a large number of SIMMs.
If the width of the SIMM is X32, then the module is a four byte, non-parity module. In this case there are four control lines (/CASx) that allow the system to access each of the four bytes separately. For example, the CPU could write to bits 0 through 7 (BYTE 0) without changing the contents of the other twenty-four bits. This scheme allows the system to write to any or all of the four bytes. A X36 parity SIMM has the same organization as the non-parity, but contains an extra bit for each byte. This bit is called the parity bit. Systems that are designed for non-parity SIMMs will work with parity SIMMs, the extra bits are simply ignored. The opposite is not true however, unless there is some means of disabling the parity checking.
If the width of the SIMM is X40, then the module is an ECC module. In this case, individual bytes can not be controlled separately. Since an ECC memory system writes the full thirty-two bits of data along with the eight ECC check bits at the same time, these modules do not use individual DRAM chips for the parity. Instead, a X4 or X8 DRAM chip will be used. This reduces the total chip count. For example, a parity 4MX36 SIMM might use eight 4MX4 DRAMs (32 bits) and four 4MX1 DRAMs (one bit per byte) for a total of twelve chips. A 4MX36 ECC SIMM might use nine 4MX4 DRAMs (36 bits) instead. JEDEC only defines X40, 72 pin SIMMs for 5 volt, not 3.3 volt
SIMMs that are X36 might be parity or they might be ECC modules. Typically, a system is designed to use either parity or ECC and will not work with the other organization. To add to the confusion, systems can be designed to use parity memory as ECC memory. The reason this works, is that even though the parity module has the four controls to write individual bytes, the system could manipulate the four controls as one.
To add to the variety, there are also X33 SIMMs. Systems that use this type of memory, calculate parity for the entire thirty-two bit word instead of for each byte.
These explanations are a little simplistic. In reality, the 72 pin SIMM has two other control lines (/RASx) that controls half of the thirty-two bit data word. However on most systems, these signals are tied together.
ECC-ON-SIMM
For systems that require high availability, but do not have ECC logic on the motherboards, a hybrid type product is available. By adding the ECC logic to the SIMM board itself, the module can correct any single bit errors. Because of the redundancy required for ECC, four bits per byte are required instead of one bit per byte for parity. Because of the additional memory and ECC logic on these boards, they are significantly more expensive than parity modules.
Although most SIMMs are +5 volts, the 72 pin and the proprietary SIMMs may also be 3.3 volts. Since most computer devices have been operating at 5 volts for the last several decades, why change to 3.3 volts? There are two primary reasons why a system would be designed for 3.3 volts modules. The first is power. Since 3.3 volt DRAMs use less power than +5 volt DRAMs, the SIMM module uses less power. The second reason is that the larger capacity, 64Mb DRAMs will only operate at 3.3 volts.
In order to prevent installing a 3.3 volt module in a 5 volt socket, the "key" along the edge of the board is different from a 5 volt module.
DRAM refresh is a topic often misunderstood due to the many ways refresh can be accomplished. This article addresses the most often asked question about the difference between 2K and 4K DRAMs.
JEDEC-Approved Versions
The Joint Electronics Design Engineering Council (JEDEC) has two approved refresh types for 4MEG by x DRAMs. For example, one of these JEDEC versions for a 4M x 4 DRAM requires 12 row-address bits and 10 column-address bits for 4,096 (4K) cycle refresh in 64ms. The other, for the same capacity DRAM, requires 11 row-address bits and 11 column-address bits for 2,048 (2K) cycle refresh in 32ms. Except for this addressing difference, the performance of the DRAMs is the same.
Why Two Types of Refresh?
The main reason behind the addition of the 4K refresh version is decreased power consumption. A DRAM device with 4K refresh draws less current than the same capacity DRAM with 2K refresh. The current is decreased by increasing the number of rows and decreasing the number of columns in the DRAM array. The number of columns defines the "depth" of a page. A 2K device has a page depth of 2,048 -- whereas a 4K device has a page depth of 1,048.
The DRAM controller in your workstation/server determines the type of refresh it can support. Some controllers only have 11 address drivers, so they are limited to 2K refresh. Many newer DRAM controllers have been designed to support both refresh standards. And still others support only 4K refresh.
SIMMs (Single Inline Memory Modules) are printed circuit boards (PCB), built with memory chips and having connection points (fingers) along one side. They are inserted into special SIMM sockets on the computers motherboard, or possibly memory carrier card. There are two standard types of SIMMs, a 30 pin SIMM and a 72 pin SIMM, but there are also custom SIMMs with other pin counts. Physically they are different, so that a 30 pin SIMM can not be inserted into a 72 pin socket. They do share some common features however. All are keyed to prevent them from being easily inserted backwards or into the wrong voltage system. Most SIMM sockets use a swing and latch insertion scheme.
A fundamental feature of a SIMM is that although there are connections on the front and the back of the PCB, these connections are not unique. The signal at pin 1 on the front of the PCB is connected to pin 1 on the back of the PCB. In other words, even though there might be 72 contacts on the front and 72 contacts on the back of the PCB, there are only 72 electrical contacts total that can be used to access the devices on the PCB.
When referring to a SIMM, there are a number of characteristics that need to be qualified to insure that the SIMM will match the requirement. The first feature usually discussed is the SIMMs organization (e.g. 16MX36). The data capacity, error checking and number of pins can all be determined from the organization of a standard SIMM. Other important functional characteristics are the speed, operating voltage, type of memory cycles (EDO or Fast Page) and buffering. In some cases, the type of refresh (e.g., 1K, 2K, 4K..) must also match for the SIMM to operate correctly.
The physical size is usually more of an issue with workstations than with PCs. The two primary measurements are the height and thickness of the module. The SIMMs height is determined by the PCB, while the thickness is determined by the package type (SOJ or TSOP) of the DRAMs that are soldered onto the PCB. One final characteristic is the type of plating on the connectors. This can be either gold or tin.
A relatively new and different kind of RAM, Synchronous DRAM or SDRAM differs from earlier types in that it does not run asynchronously to the system clock the way other types of memory do. SDRAM is tied to the system clock and is designed to be able to read or write from memory in burst mode (after the initial read or write latency) at 1 clock cycle per access (zero wait states) at memory bus speeds up to 100 MHz or even higher. SDRAM supports 5-1-1-1 system timing when used with a supporting chipset.
SDRAM is considered by some the "heir apparent" to EDO as the next PC memory standard. The reason is that its synchronized design permits support for the much higher bus speeds that will be coming in the next couple of years. Right now, SDRAM remains far less popular than EDO, mainly because it is more expensive. SDRAM doesn't offer that much "real world" additional performance over EDO due to the system cache masking much of that differential in speed, and the fact that most systems are running on relatively slow 66 MHz or lower system bus speeds.
There are several important characteristics and concerns regarding SDRAMs that are relatively unique to the technology:
Speed and Speed Matching:
SDRAM modules are generally rated in two
different ways: First, they have a "nanosecond" rating like conventional
asynchronous DRAMs, so SDRAMs are sometimes refered to as being "12
nanosecond" or "10 nanosecond". Second, they have a "MHz" rating, so they
are called "83 MHz" or "100 MHz" SDRAMs. Because SDRAMs are, well,
synchronous, they must be fast enough for the system in which they are being
used.
2-Clock and 4-Clock Circuitry:
There are two slight variations in the
composition of SDRAM modules; these are commonly called 2-clock and 4-clock
SDRAMs. They are almost exactly the same, and they use the same DRAM
chips, but they differ in how they are laid out and accessed. A 2-clock SDRAM
is structured so that each clock signal controls 2 different DRAM chips on the
module, while a 4-clock SDRAM has clock signals that can control 4 different
chips each. You need to make sure that you get the right kind for your
motherboard. The current trend appears to be toward 4-clock SDRAMs.
Serial Presence Detect:
Some motherboards are now being created that require
the use of special SDRAM modules that include something called a Serial
Presence Detect (SPD) chip. This is an EEPROM that contains speed and design
information about the module. The motherboard queries the chip for information
about the module and makes adjustments to system operation based on what it
finds. A great idea in theory, but you won't think it's great if you buy an
SDRAM module without the chip on it when your board requires SPD...
Packaging Concerns:
To make matters even more confusing, SDRAM usually
comes in DIMM packaging, which itself comes in several different formats
(buffered and unbuffered, 3.3 volts and 5 volts).
Abbreviation of Synchronous Graphic Random Access Memory, a type of DRAM used increasingly on video adapters and graphics accelerators. Like SDRAM, SGRAM can synchronize itself with the CPU bus clock up to speeds of 100 MHz. In addition, SGRAM uses several other techniques, such as masked writes and block writes, to increase bandwidth for graphics-intensive functions. Unlike VRAM and WRAM, SGRAM is single-ported. However, it can open two memory pages at once, which simulates the dual-port nature of other video RAM technologies.
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