• Dynamic RAM (DRAM) – DRAM is a "classic" form of RAM. DRAM is slower than SDRAM and has since been replaced by the faster and less expensive SDRAM. DRAM works by storing data electronically in a storage cell, and every few milliseconds the storage cell is refreshed.
• Extended Data-Out RAM (EDO RAM) – EDO RAM is faster than DRAM. EDO RAM has also been replaced by SDRAM. EDO RAM was an improvement on DRAM because it has advanced timing features. EDO extends the amount of time data was stored and has a reduced refresh rate. This alleviated the CPU and RAM from timing constraints and improved performance.
• Synchronous DRAM (SDRAM) – SDRAM replaced DRAM, FPM, and EDO. SDRAM was an improvement because it synchronized data transfer between the CPU and memory. SDRAM allowed the CPU to process data while another process is being queued.
• Double Data Rate SDRAM (DDR SDRAM) – DDR SDRAM is a newer form of Synchronous Dynamic RAM (SDRAM) that can theoretically improve memory clock speed to at least 200 MHz.
• Single In-Line Memory Module (SIMM) – SIMM is a memory module with 72 or 30 pins. SIMMs are considered legacy components and can be found in older machines. SIMMs with 72 pins can support 32-bit transfer rates, and 32-pin SIMMs can support 16-bit transfer rates.
• Dual In-Line Memory Module (DIMM) – DIMM is a memory module with 168 pins. DIMMs are widely used today and support 64-bit transfer.
• Rambus Inline Memory Module (RIMM) – RIMM is a 184-pin memory module that uses only the Direct Rambus memory chips (RDRAM). Smaller ones called SO-RIMM have a 160-pin connector. Some systems require that RIMM modules be added in identical pairs, and others allow single RIMMs to be installed.

More information about specific memory types can be obtained from the manufacturer website.

Troubleshooting RAM Issues
Random Access Memory (RAM) failures come in either sudden or intermittent form. Overused or bad memory can cause the system to fail at any time. A good indication of the state of bad memory is the performance of the system. If the system is running smoothly and applications rarely stall, the RAM workload is well within the RAM specifications and it is functioning correctly. If the computer is multitasking and frequently freezes, the RAM is probably insufficient for the workload. The RAM might need to be upgraded.

Troubleshooting the RAM modules is rather straightforward. Today, RAM is inexpensive and easy to replace. Technicians can easily take out the bad memory and add a known good module. If the problem is resolved, the technician can infer that the RAM module is bad and discard the old module. If the memory problem still exists, consult the motherboard documentation. Some motherboards require memory modules to be installed in a particular slot order, or jumpers to be set. Figure shows the way to install SIMM and show the way to install DIMM.

Also, verify that the module has been installed correctly. Memory modules are notched and insert one way. If a bad install is suspect, remove the module and visually inspect the module socket. Check for any debris, dust, or dirt. Remove any debris and reset the memory module.

Today, computers run software applications that are very memory intensive. These programs continually put stress on the memory modules, potentially causing them to fail. There are several common symptoms for failed memory:

• HIMEM.SYS has problems loading. 
• Computer appears dead and does not boot. 
• Windows program is unstable or programs are freezing. 
• POST errors. 
• No video display.

RAM Compatibility Issues
SDRAM memory modules come in various speeds. The most common SDRAM speeds are PC-66, PC-100, and PC-133. The speed of SDRAM memory is measured in megahertz (MHz). SDRAM with a higher MHz rating indicates a higher performing memory stick. SDRAM memory has compatibility issues with the bus on the motherboard. The speed of the SDRAM module must match the speed of the bus. Common bus speeds are PC100 or PC133. When looking to purchase RAM modules, verify the bus speed and buy a compatible RAM module.

The speed of EDO and FPM memory modules are measured in nanoseconds (ns). The memory module with the lowest ns rating is the fastest. EDO and FPM also have compatibility issues with the system bus.

Faster DRAM can be installed on a slower system bus and it will not affect performance. The system will operate at the bus speed even if faster memory is installed. However, a slower or mixed DRAM module cannot be installed on a system with faster DRAM requirements or different clocked DRAM.

Legacy machines might require parity RAM. Parity RAM performs error-checking calculations for every eighth bit of data stored. Today, RAM is non-parity and does not perform parity calculations on data. Never mix parity and non-parity SIMMs. For older systems, the setup utility has an option for enabling or disabling RAM parity checking.

Also, Error Correction Code (ECC) and non-ECC RAM cannot be mixed. ECC has the ability to correct data errors and is typically found in file servers.

Scenario 
After a recent update, the computer does not boot. The memory is not being correctly recognized. These errors usually occur when there is a clocking issue with SDRAM. Computers require SDRAM to be either 2-clock or 4-clock. Legacy equipment usually requires 2-clock SDRAM. Newer computers usually require 4-clock SDRAM. Different clock rates are not compatible and cannot be mixed. Most likely the clock rating is not compatible with the motherboard specifications and cannot be used. Consult the motherboard documentation to find the compatible clock rating for SDRAM modules.