Sim-Ex™ Tutorial for A+ Essentials

1.0 Personal Computer Components

  • 1.1 Identify the fundamental principles of using personal computers
    • Identify the names, purposes and characteristics of memory
      • Types of memory (e.g. DRAM, SRAM, SDRAM, DDR / DDR2, RAMBUS)
      • Operational characteristics
        • Memory chips (8, 16, 32)
        • Parity versus non-parity
        • ECC vs. non-ECC
        • Single-sided vs. double-sided

Types of memory (e.g. DRAM, SRAM, SDRAM, DDR / DDR2, RAMBUS)

PC memory stores data and programs currently being executed by the computer. It is important that the information is fetched by the CPU quickly to further processing. There are several memory types available. Important among there include the following:

  • Dynamic RAM (DRAM)
  • Synchronous RAM (SRAM)
  • Synchronous DRAM (SDRAM)
  • Rambus DRAM (RDRAM)
  • Video RAM (VRAM)

RAM stands for Random Access Memory.

Dynamic RAM (DRAM): In dynamic RAM, the RAM gets refreshed continually by the controller. DRAM has been introduced in the earlier stages, and RAM versions available today are much bigger and faster than the earlier simple DRAMs.

DRAMs store data in the form of capacitive charges. Since any capacitor tends to be leaky, a DRAM needs to be refreshed on a continual basis.

Synchronous RAM (SRAM): SRAM contains a clock built onto the memory module, enabling the SRAM to be in synchronization with the motherboard cloak. SDRAM doesn't require frequent recharge like DRAM. L-2 memory caches are usually made of SRAM and exhibit very fast read and write operations.

Synchronous DRAM (SDRAM): SDRAM works in sync with the motherboard, and hence works quite fast. SDRAMs have speeds of the order of 133MHz, 800MHz, etc.

Double Data Rate SDRAM (DDR SDRAM):DDR SDRAM is similar to SDRAM, but for the difference that DDR reads data on both the rising and falling edges of the clock. SDRAM reads only on the rising edge of a signal. This technique allows the DDR module to achieve speeds twice that of SDRAM. For example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz.

Rambus DRAM (RDRAM): RDRAM technology was developed originally by Rambus, Inc. Rambus memory is integrated onto Rambus Inline Memory Modules (RIMMs). RDRAM chips are synchronized to the processor's memory bus.

Comparison Chart:

Memory Type Synchronization
DRAM CPU synchronized to the motherboard. DRAM is neither synchronized to the motherboard nor CPU.
SRAM CPU and memory module are synchronized to the motherboard.
RDRAM Synchronizes to the memory bus clock. Memory bus clock is much faster than the motherboard clock. Hence faster data transfer between the CPU and the memory module occurs.

Video RAM (VRAM): VRAM is primarily used on video cards. It is dual ported, in the sense that while one device write to VRAM, another device can simultaneously do read operation. This is quite useful in animation and other speed sensitive video applications. VRAMs are more expensive than DRAMs, but provide better graphic display.

Operational characteristics

  • Memory chips (8, 16, 32)
  • Parity versus non-parity
  • ECC vs. non-ECC
  • Single-sided vs. double-sided

Memory chips (8, 16, 32)

Parity versus non-parity : Parity enables some basic error detection, and correction capabilities. This comes at the cost of increased overhead, and additional cost. Normally, parity is not used for memory chips in personal computers. If there is any specific requirement for added error detection and correction capabilities, one may go for parity enabled memory modules.

ECC vs. non-ECC :ECC memory, short for Error-Correcting Code memory, a type of memory that enables some basic error detection and correction capabilities, similar to parity. Here again, you need to pay additional cost for ECC memory modules, and regular memory modules do not support ECC or parity.

Single-sided vs. double-sided :Memory modules are available with single sided contacts (SIMM) and double sided contacts (DIMM). SIMM stands for Single In-line Memory Module, and DIMM stands for Dual In-line Memory Module. Even the memory chips may be soldered on either side of the PCB or on a single side. Double sided soldering of memory chips enables higher memory density.30-pin SIMMs are single-sided. 72-pin SIMMs are either single-sided or double-sided.

SIMMs have contacts on either side of the circuit board but they are electrically shorted. So a 72-pin (or a 30-pin) SIMM has pads on each side of the circuit board, but each pair of pads is connected together. DIMMs however have different connections on each side of the circuit board. So a 168-pin DIMM has 84 pads on each side and they are NOT shorted electrically. This results in smaller packaging volume for DIMM modules. DIMMs support 64-bit data transfer.

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