Welcome back once again, in today's article I will tech you most of reason computer memory fall.
Memory is a cornerstone of the modern PC. Memory that holds the program code and data that is processed by the CPU—and it is this intimate relationship between memory and the CPU that forms the basis of computer performance. With larger and faster CPUs constantly being introduced, and more complex software is developed to take advantage of the processing power. In turn, the more complex software demands larger amounts of faster memory. With the explosive growth of Windows (and more recently, Windows 95) the demands made on memory performance are more acute than ever.

 These demands have resulted in a proliferation of memory types that go far beyond the simple, traditional DRAM. Cache (SRAM), fast page-mode (FPM) memory, extended data output (EDO) memory, video memory (VRAM), synchronous DRAM (SDRAM), flash BIOS, and other exotic memory types (such as RAMBUS) now compete for the attention of PC technicians. These new forms of memory also present some new problems. This chapter will provide you an understanding of memory types, configurations, installation concerns, and troubleshooting options. Memory Package Styles and Structures Ultimately, the memory die is mounted in a package just like any other IC. The completed memory packages can then be soldered to your motherboard or attached to plug-in structures, such as SIMMs, DIMMs, and memory cards. Only four package styles are normally used for memory devices:

 
- DIP (Dual In-line Package) This classic IC package is used for through-hole mounting (prior to surface-mount technology). The advantage of DIP ICs is their compatibility with IC sockets, which allows ICs to be inserted or removed as required. Unfortunately, the long metal pins can bend and break if the IC is inserted or removed incorrectly. Also, the overall size of the package demands extra space.

 
DIP ICs were used in older PCs (286 and earlier systems) and older VGA/SVGA video boards. DIPs are still sometimes used on motherboards to provide cache

RAM.
- SIP (Single In-line Package) This type of IC package is rarely used today—there are simply not enough pins. However, they did make a short appearance with memory devices in late-model 286 and early 386 systems, which flirted with proprietary memory expansions. I remember NEC using such devices in a 2MB add-on for their 386SX/20— and you needed to add that module before you added even more memory in the form of proprietary SIMMs. SIPs can be troublesome because they are difficult to find replacements for, so expect replacement memory modules using them to cost a premium.

 
- SOJ (Small-Outline “J” Lead) This is the contemporary package style for surface mount circuits. The leads protrude from the package like a DIP, but are bent around just under the package in the form of a “j”. Sockets for SOJ packages are often used for replaceable memory ICs, such as the BIOS ROM, but most RAM devices are soldered directly to the motherboard as system memory (or a video board as video RAM). SIMMs often use SOJ memory components.

 
- TSOP (Thin, Small-Outline Package) Like the SOJ, a TSOP is also a surface-mount package style. However, its small, thin body makes TSOP memory ideal for narrow spaces. Expect to find such devices serving as memory in notebook/sub-notebook systems or PCMCIA cards (a.k.a., PC Cards).

Add-On Memory Devices

Memory has always pushed the envelope of IC design. This trend has given us tremendous amounts of memory in very small packages, but it also has kept memory relatively expensive. Manufacturers responded by providing a minimum amount of memory with the system, then selling more memory as an add-on option—this keeps the cost of a basic machine down and increases profit through add-on sales. As a technician, you should understand the three basic types of add-on memory.

SIMMs and DIMMs:

 By the time 386 systems took hold in the PC industry, proprietary memory modules had been largely abandoned in favor of the “Memory Module” (Fig. 23-3). A SIMM (Single In-line Memory Module) is light, small, and contains a relatively large block of memory, but perhaps the greatest advantage of a SIMM is standardization. Using a standard pin layout, a SIMM from one PC can be installed in any other PC. The 30-pin SIMM (Table 23-1) provides 8 data bits, and generally holds up to 4MB of RAM.


Proprietary Add-On Modules:
Once the Intel i286 opened the door for more than 1MB of memory, PC makers scrambled to fill the void. However, the rush to more memory resulted in a proliferation of non-standard (and incompatible) memory modules. Each new motherboard came with a new add-on memory scheme—this invariably led to a great deal of confusion among PC users and makers alike. You will likely find proprietary memory modules in 286 and early 386 systems.

Memory Considerations

Memory has become far more important than just a place to store bits for the microprocessor.
It has proliferated and specialized to the point where it is difficult to keep track of all the memory options and architectures that are available. This part of the chapter reviews established memory types, and explains some of the current memory architectures.

Memory Organization

The memory in your computer represents the result of evolution over several computer generations. Memory operation and handling is taken care of by your system’s microprocessor. As CPUs improved, memory-handling capabilities have improved as well. Today’s microprocessors, such as the Intel Pentium or Pentium Pro, are capable of addressing more than 4GB of system memory—well beyond the levels of contemporary software applications. Unfortunately, the early PCs were not nearly so powerful. Older PCs could only address 1MB of memory because of the limitations of the 8088 microprocessor. Because backward compatibility is so important to computer users, the drawbacks and limitations of older systems had to be carried forward into newer computers, instead of being eliminated. Newer systems overcome their inherent limitations by adding different “types” of memory, along with the hardware and software to access the memory. This part of the chapter describes the typical classifications of computer memory: conventional, extended, and expanded memory. This chapter also describes high memory concepts. Notice that these memory types have nothing to do with the actual ICs in your system, but the way in which software uses the memory.

Memory Speed and Wait States
The PC industry is constantly struggling with the balance between price and performance. Higher prices usually bring higher performance, but low cost makes the PC appealing to more people. In terms of memory, cost-cutting typically involves using cheaper (slower) memory devices. Unfortunately, slow memory cannot deliver data to the CPU quickly enough, so the CPU must be made to wait until memory can catch up. All memory is rated in terms of speed—specifically, access time.

 Access time is the delay between the time data in memory is successfully addressed, to the point at which the data has been successfully delivered to the data bus. For PC memory, access time is measured in nanoseconds (ns), and current memory offers access times of 50 to 60 ns. 70-ns memory is extremely common. The question often arises: “Can I use faster memory than the manufacturer recommends?” The answer to this question is almost always “Yes,” but rarely does performance benefit. As you will see in the following sections, memory and architectures are typically tailored for specific performance. Using memory that is faster should not hurt the memory or impair system performance, but it costs more and will not produce a noticeable performance improvement. The only time such a tactic would be advised is when your current system is almost obsolete, and you would want the new memory to be useable on a new, faster motherboard if you choose to upgrade the motherboard later on. A wait state orders the CPU to pause for one clock cycle to give memory additional time to operate. Typical PCs use one wait state, although very old systems might require two or A wait state orders the CPU to pause for one clock cycle to give memory additional time to operate.

 Typical PCs use one wait state, although very old systems might require two or three. The latest PC designs with high-end memory or aggressive caching might be able to operate with no (zero) wait states. As you might imagine, a wait state is basically a waste of time, so more wait states result in lower system performance. Zero wait states allow optimum system performance. Table 23-9 illustrates the general relationship between CPUs, wait states, and memory speed. It is interesting to note that some of the fastest systems allow the most wait states. This flexibility lets the system support old, slow memory,