CompTIA 220-601 – A+ PC components

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This post is outdated. For an updated guide see Jed Reisner’s A+ 220-801 and 220-802 guide.

These questions are based on: 220-601 – A+ Essentials CompTIA Self-Test Software Practice Test.


Objective: Personal computer components
SubObjective: Identify the fundamental principles of using personal computers


Single Answer, Multiple Choice


What is the minimum number of DIMMs that can be installed in a typical system?



  1. 1
  2. 2
  3. 4
  4. 8


A. 1


DIMMs are 64 bits wide. This width matches the memory bus of modern computer systems, which means DIMMs can be installed one at a time.


The DIMM modules are eight bytes wide and transfer eight bytes (64 bits) of data at a time. With DIMMs made of older SDRAM (synchronous DRAM) memory chips, the data is transferred once per clock cycle. But with DDR SDRAM and DDR2 SDRAM, the data is transferred two times per clock cycle — once on the leading edge of the clock signal and again on the falling edge. This allows a tremendous amount of data to be transferred per clock cycle. For example, in a Pentium 4 system with a front-side bus operating at 400 MHz, up to 6,400 MB (megabytes) of data is transferred to and from the DDR2-DIMMs per second.


The total speed is affected by the clock and bus speed supported by the module. For example, DDR 1600 uses a clock speed of 100 MHz with a bus speed of 200, resulting in a total transfer rate of 1600 Mbps.


SIMMs come in two flavors: 30-pin and 72-pin. The 30-pin SIMMs are used in x486 systems, which have a 32-bit memory bus. Each 30-pin SIMM is only eight bits wide, which is why four 30-pin SIMMs are needed to form a single bank of memory. The larger 72-pin SIMMs expand the chip bus to 32-bits wide. They were developed for later x486 systems and the original Pentium systems, which have a 64-bit memory bus. This means that only one 72-pin SIMM can be installed in an x486 system, but two are required for Pentium systems because of their larger bus. SIMMs were phased out in favor of DIMMs because of performance limitations. Their performance is dramatically inferior to DIMM and RIMMs because of the slower speeds at which they operate. For example, the maximum data transfer of an EDO SIMMs operating on a 33-MHz clock is about 266 MBps.


RIMMs come in 64-, 32- and 16-bit packages. The 64-bit modules have an eight-byte bus, and the 32-bit and 16-bit modules have four- and two-byte buses, respectively. The 64-bit RIMMs can operate at very high speeds and can transfer an amazing amount of data per second. For example, RIMM modules operating on a clock speed of 600 MHz can transfer eight bytes of data twice per clock cycle, for a total transfer rate of up to 9,600 MBps.





  1. Upgrading and Repairing PCs, 17th edition, Scott Mueller, pp. 492-495.
  2. Upgrading and Repairing PCs, 17th edition, Scott Mueller, p. 487.
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