Types of RAM
Types of Chips: SIMMs or DIMMs
SIMM Nomenclature or How Many Pins My Computer Needs
3.3-volt Components or 5-volt Components
There are different types of RAM used for different purposes. They include: DRAM, FPM DRAM, EDO RAM, SDRAM,
SRAM, EDRAM, VRAM, WRAM and RDRAM.
DRAM
Dynamic Random Access Memory, or DRAM is the most common type of memory chip. DRAM chips are the
Integrated Circuits, or ICs, positioned on a small Printed Circuit Board, or PCB. They come in many capacities (usually 1,
2, 4, 8, 16 or 32 M) on the same PCB, and install easily in special slots on the system board. (Older computers may use
socketed chips or may not be expandable at all.) It is inexpensive. but slow so if you can afford more expensive memory go
for EDO RAM.
EDO Memory
Extended Data Output, or EDO RAM, is one of a series of recent innovations in DRAM chip technology. On computer
systems designed to support it, EDO memory allows a CPU to access memory 10 to 15 percent faster than comparable
fast-page mode chips.
Synchronous DRAM
Synchronous DRAM, or SDRAM, is a new DRAM technology that uses a clock to synchronize signal input and output on
a memory chip. The clock is coordinated with the CPU clock so the timing of the memory chips and the timing of the CPU
are in 'synch'. Synchronous DRAM saves time in executing commands and transmitting data, thereby increasing the overall
performance of the computer.
The summarised overview of different DRAM chips is shown in the table below:
| Type | Where Used | Description and Facts |
| DRAM Dynamic Random Access Memory | Main System Memory and Video Memory | DRAM is the most common type of computer memory. DRAMs are used mostly in main memory systems. DRAM is volatile and slow, but inexpensive. DRAM is used for memory read and writes and the data must be refreshed after each transfer. |
| FPM DRAM Fast Page Mode DRAM | Main System Memory and Video Memory | FPM is also commonly used as the main system memory. As microprocessor speeds advanced, more memory throughput was required, resulting in the development of the Fast Page Mode DRAM. FPM DRAM is similar to regular DRAM but faster. |
| EDO RAM Extended Data Output RAM | Main System Memory and Video Memory | EDO is an improvement on the FPM design. Depending on the type of system and the software applications, EDO can provide approximately a 3% performance gain over FPM DRAM if a secondary (L2) cache is in the system, and approximately a 10% to 12% performance gain if the L2 cache is not present. |
| SDRAM Synchronous DRAM | Main System Memory and Video Memory | Synchronous DRAM is a new memory with improved performance, simpler design, and faster transfer rates than standard DRAM. |
| SRAM Static RAM | Cache Memory | SRAM is an extremely fast device that does not require periodic refreshing. SRAM is used primarily in cache memories. SRAM is faster, bigger, and generates more heat than DRAM. It is also more expensive than DRAM. |
| EDRAM Enhanced DRAM | Cache Memory | EDRAM is positioned as a high-performance specialty memory that combines DRAM and SRAM caches on one chip. |
| VRAM Video RAM | Video Memory | VRAM, specialized RAM for video, offers higher performance than DRAM. VRAM is dual ported, which allows simultaneous reads and writes of data. It requires a larger package than DRAM, and is more expensive than DRAM, too. |
| WRAM Windows RAM | Video Memory | WRAM technology evolved from its predecessor, VRAM. Named after its ability to offer full-motion video, WRAM comes in a smaller package than VRAM. It has added intelligence that makes it perform up to 50% faster than VRAM. |
| RDRAM RAMBUS DRAM | Video Memory | Future generation of DRAM with performance increases of up to 20 fold over standard DRAM. With costs just over 10% of standard DRAM, it is gaining acceptance in the industry. |
Memory on the motherboard is arranged in groups called memory banks. The number of memory banks and their specific configurations vary from one computer to another because they're determined by the computer's CPU and how it receives information. The needs of the CPU determine the number of memory sockets required in a bank.
Figure 1. The memory banks where memory chips will be installed.
Types of Chips: SIMMs or DIMMs
For main memory mainly two types of chips are used: SIMMs (Single In-Line Memory Modules) or DIMMs (Dual In-Line
Memory Modules).
SIMMs (Single In-Line Memory Modules)
SIMMs were developed to make memory upgrades easier. A typical SIMM consists of a number of DRAM chips on a PCB, which fits into a SIMM socket on a computer's motherboard. They usually come in two formats:
There are other sizes, but they're typically used for video RAM or on specialized boards.
80386 and later CPUs support 32 data bits. A common configuration for systems that use 30-pin SIMMs would be 30-pin
SIMM sockets, each of which supports 8 data bits. That means four 30-pin SIMMs should be installed to supply 32 bits.
The memory configuration on such a system is typically divided into two memory banks -- bank 0 and bank1. Each memory bank consists of four 30-pin SIMM sockets. The CPU addresses RAM one bank at a time. Memory banks must always be either completely full of SIMMs or completely empty. Chips in the same bank must be of the same capacity. Mixing different-capacity SIMMs within the same bank prevents the computer from accurately detecting the amount of available memory. This causes one of two problems to occur:
As was mentioned earlier, one 72-pin SIMM supports 32 data bits, which is four times the number of data bits supported by a single 30-pin SIMM. For a 32-bit CPU -- such as Intel 486 -- you need only one 72-pin SIMM per bank to provide the CPU with 32 data bits. That same CPU would require four 30-pin SIMMs per bank to get its 32 data bits.
In modern systems SIMMs are installed vertically in a slot.
|
|
| a). SIMMs taking up half the memory banks | b). SIMMs taking up all the memory banks |
Figure 2. SIMMs installed in the memory banks
SIMM connectors can be either tin plated or gold. If the these metal types are mixed, for example, a tin-plated SIMM is
placed into a gold-plated memory socket, it can cause accelerated corrosion which in turn can cause bad connections and
system failure as the worst outcome.
DIMMs (Dual In-Line Memory Modules)
One DIMM is basically equivalent to a pair of SIMMs but uses less space. DIMMs (with parity) are 72 bits wide, and
SIMMs are 36 bits wide (with parity). Currently the largest available SIMM is 64 MB, while the largest available DIMM is
128 MB.
Like SIMMs, most DIMMs install vertically into expansion sockets. The principal difference between the two is that on a
SIMM, opposing pins on either side of the board are 'tied together' to form one electrical contact; on a DIMM, opposing pins
remain electrically isolated to form two separate contacts.
DIMMs are often used in computer configurations that support a 64-bit or wider memory bus (usually based on 64-bit CPUs
like Intel's Pentium or IBM's PowerPC).
Many PCs require SIMMs that are capable of parity checking. These parity SIMMs have an extra chip or chipson the PCB.
Parity requires that an extra bit be sent at the end of each byte of data to hold the parity value. This parity bit is used to ensure
that your information is being sent properly.
Most of the modern computer systems (like Pentiums) and all Macintosh computers use non-parity memory.
Check the manual of your computer to see what type of memory chips are installed on your motherboard. If your manual
doesn't let you know if you have parity memory, an easy way to determine it is to count the number of chips on the 1 or 4MB
SIMMs already in your system. If they have two or eight chips on them, you don't need parity. If they have three, nine, or
twelve, you do.
If your PC does not require parity-checking SIMMs, do not get them. They might work in non-parity systems, but generally
are a little more expensive.
SIMM Nomenclature or How Many Pins My Computer Needs
SIMM chips are usually identified by two numbers, e.g. 4M x 8, 4M x 9. The first number is the density (complexity) of the
DRAM chip and the second is its width (how many bits can be sent to it simultaneously). The following tables provides a
summary of popular 30- and 72-pin SIMMs.
| SIMM Type | SIMM Format | SIMM Capacity |
30-pin | 256K x 8 1M x 8 4M x 8 256K x 9 (Parity) 1M x 9 (Parity) 4M x 9 (Parity) | 256K 1M 4M 256K 1M 4M |
72-pin | 256K x 32 1M x 32 2M x 32 4M x 32 8M x 32 256K x 36 (Parity) 1M x 36 (Parity) 2M x 36 (Parity) 4M x 36 (Parity) 8M x 36 (Parity) | 1M 4M 8M 16M 32M 1M 4M 8M 16M 32M |
All DRAM chips must be of the same speed which is represented by the last digit of the part number on a chip. This last digit
should be a 6, 7, or 8 which corresponds to 60,70, or 80 nanoseconds (ns). That is the speed of your SIMM. The lower the
number, the faster the DRAM.
A helpful generic (very generic) rule
3.3-volt Components or 5-volt Components
Computer memory components operate at either 3.3 volts or 5 volts. Until recently, 5 volts was the industry standard. Making integrated circuits, or ICs, faster requires a reduced cell geometry, that is, a reduction in the size of the basic 'building blocks'. As components become smaller and smaller, the cell size and memory circuitry also become smaller and more sensitive. As a result, these components cannot withstand the stress of operating at 5 volts. 3.3-volt components can operate faster and use less power. They also produce less heat.
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