INSTRUCTION CYCLE
The fundamental sequence of steps that a CPU performs. Also known as the "fetch-execute cycle," it is the time in which a single instruction is fetched from memory, decoded and executed. The first half of the cycle transfers the instruction from memory to the instruction register and decodes it. The second half executes the instruction.
RAM TYPES
Today, most RAM implementations are synchronous dynamic RAM (SDRAM) and Rambus DRAM (RDRAM). SDRAM with 168-pin DIMMs are the most common modules. Before SDRAM and RDRAM, there was dynamic RAM (DRAM). Older Pentiums used fast page mode (FPM) and extended data-out (EDO) RAM. FPM and EDO RAM are 72-pin memory modules.
Dynamic RAM (DRAM) DRAM is a classic form of RAM and has since been replaced by the faster and less expensive SDRAM. DRAM stores data electrically in a storage cell and refreshes the storage cell every few milliseconds.

Extended Data-Out RAM (EDO RAM) EDO RAM is faster than DRAM. EDO RAM has also been replaced by SDRAM. EDO RAM is an improvement on DRAM because it has advanced timing features. EDO extends the amount of time data is stored and has a reduced refresh rate. This alleviates the CPU and RAM from timing constraints and improves performance.

Synchronous DRAM (SDRAM) SDRAM replaced DRAM, FPM, and EDO. SDRAM is an improvement because it synchronizes data transfer between the CPU and memory. SDRAM allows the CPU to process data while another process is being queued. Figure shows an SDRAM.

Double Data Rate SDRAM (DDR SDRAM) DDR SDRAM is a newer form of SDRAM that can theoretically improve memory clock speed to 200 megahertz (MHz) or more.
Chip specification DDR-200: DDR-SDRAM memory chips specified to run at 100 MHz DDR-266: DDR-SDRAM memory chips specified to run at 133 MHz DDR-333: DDR-SDRAM memory chips specified to run at 166 MHz DDR-400: DDR-SDRAM memory chips specified to run at 200 MHz
Stick/module specification PC-1600: DDR-SDRAM memory module specified to run at 100 MHz using DDR-200 chips, 1.600 GByte/s bandwidth per channel. PC-2100: DDR-SDRAM memory module specified to run at 133 MHz using DDR-266 chips, 2.133 GByte/s bandwidth per channel PC-2700: DDR-SDRAM memory module specified to run at 166 MHz using DDR-333 chips, 2.667 GByte/s bandwidth per channel PC-3200: DDR-SDRAM memory module specified to run at 200 MHz using DDR-400 chips, 3.200 GByte/s bandwidth per channel.
DDR SDRAM DIMMs have 184 pins (as opposed to 168 pins on SDR SDRAM, or, 240 pins on DDR-2), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDR SDRAM has two). DDR operates at a voltage of 2.5 V, compared to 3.3 V for SDR SDRAM.

* The different pin configuration for DDR RAM Modules: 1. 172-pin MicroDIMM, used for DDR SDRAM 2. 184-pin DIMM, used for DDR SDRAM 3. 200-pin SO-DIMM, used for DDR SDRAM and DDR2 SDRAM 4. 204-pin SO-DIMM, used for DDR3 SDRAM 5. 214-pin MicroDIMM, used for DDR2 SDRAM 6. 240-pin DIMM, used for DDR2 SDRAM, DDR3 SDRAM and FB-DIMM DRAM
Single Inline Memory Module (SIMM) SIMM is a memory module with 72 or 30 pins, as shown in Figures and . 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 Inline Memory Module (DIMM) DIMM is a memory module with 168 pins as shown in Figure . DIMMs are commonly used today and support 64-bit transfer.

Rambus Inline Memory Module (RIMM) RIMM is a 184-pin memory module that uses only the RDRAM, as illustrated in Figure . Smaller modules called SO-RIMM have a 160-pin connector. Some systems require that RIMM modules be added in identical pairs while others allow single RIMMs to be installed.
More information about specific memory types can be obtained from the manufacturer website.
Troubleshooting RAM Issues
RAM failures are either sudden or intermittent. Overused or defective memory can cause the system to fail at anytime. System performance is a good indication of the state of the memory. If the system is running smoothly and applications rarely stall, the RAM workload is well within the RAM specifications. If the computer is multitasking and frequently freezes, the RAM is probably insufficient for the workload.
Troubleshooting the RAM modules is straightforward. RAM is inexpensive and easy to replace. Technicians can easily remove the memory that is a suspected problem and add a valid module. If the problem is resolved, the RAM module is probably no longer operative. If the memory problem still exists, consult the motherboard documentation. Some motherboards require memory modules to be installed in a particular slot order, or require jumpers to be set. Figures , , and show the correct way to install SIMMs, DIMMs, and RIMMs.
Also, verify that the module has been correctly installed. Memory modules are notched and insert in one direction. If the user suspects an improper installation, remove the module and visually inspect the module socket. Remove any debris, dust, or dirt and reset the memory module.

Modern 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 inoperative and does not boot.
• Windows program is unstable or programs are freezing.
• POST errors exist.
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 module. 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 PC-100 or PC-133. 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. The following scenario helps to illustrate an issue with RAM.