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Memory: Types and Forms

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This chapter is from the book

Terms you'll need to understand:

  • Volatile and nonvolatile
  • Memory register and memory cell
  • BIOS and CMOS
  • Synchronous (synchronized), and asynchronous (not synchronized)
  • Cache hierachy, cache, buffer
  • Fast Page Mode (FPM), Extended Data Out (EDO) memory
  • RAM, DRAM, SRAM, SDRAM, RDRAM, DDR-SDRAM, VRAM
  • DIP, SIMM, DIMM, RIMM
  • Parity, ECC

Concepts you'll need to master:

  • Read-only memory (ROM), Programmable ROM (PROM), and Random Access Memory (RAM)
  • Memory address, grids, and matrix
  • Memory refresh and wait states
  • Clock multipliers
  • Secondary memory caching, internal and external memory
  • Memory pages
  • Chip packaging and modular manufacturing
  • Odd and even parity, error detection

Perhaps the most familiar parts of a computer are the central processing unit and the main memory. The main memory is the same as the "computer's memory," also commonly referred to as system memory, or RAM. In this chapter we examine memory chips and how they're packaged. In Chapter 5, "Processors and Chipsets," we then look at different types of processors.

Memory is only a temporary place to store information until a device can get to it. Essentially, the CPU uses memory to move program instructions and data in and out of that temporary storage area. A typical instruction might be a request to store a data bit somewhere. Another instruction might be to retrieve that bit from a particular place—an address. Data might be a number, a letter, or any other bit of information. Remember that a data bit is also a small charge of electricity.

Data becomes information when it takes on context (surrounding circumstances). 76 means nothing on its own, other than the fact that it's a number. Surround that number with context: "Tomorrow, the temperature is expected to reach 76," and it becomes information. RAM is like a holding tank for data on its way to becoming information.

Storage is any location where information can be placed and retained for some amount of time. Computer memory is temporary storage, in that it generally requires the presence of electrical current. Volatile memory can hold information only when a normal electrical current is present. Nonvolatile memory can hold information in the absence of an electrical current. System memory is, for the most part, volatile.

Compact memory cards, smart memory cards, and memory sticks used in digital photography are examples of nonvolatile memory. Although some amount of current is necessary to change information, that information then remains stored even when there is no further current. Remember that volatile memory requires a continuing supply of current.

NOTE

Volatile, from the Latin "to fly," means that information "flies away" when there's no electricity to keep it in place. Television reporters often refer to an explosive situation as a volatile situation, meaning that it could change at any second. Volatile memory will lose all of its data when electricity is removed. Nonvolatile memory will maintain its data even without electricity.

Floppy disks, fixed disks, optical disks, and card media are all nonvolatile. However, although disks retain information without electrical current, we refer to them as permanent storage, not "memory." The terms volatile and nonvolatile are generally assigned to memory chips.

Permanence is a relative word. A burst of static electricity can completely wipe out any information on a magnetic storage device or in a memory chip. Optical disks store data in structural changes to the media, and so even ESD or close proximity to a magnet (electromagnetic interference) rarely affect that information. Chapter 6, "Basic Electronics," examines both ESD and EMI.

Conceptual Overview

Computer memory is fairly easy to remember when you've grasped the basic concepts. In a nutshell, a CPU uses transistors to handle bits of data. These transistors are grouped together into registers, making for small storage places inside the processor housing (the chip die). At some point, either the registers fill up or the instructions are completed. The CPU then works together with a memory controller to move data bits out to memory cells. Memory cells are typically capacitors that form small storage places on a memory chip. Both processor registers and memory cells have addresses. Every time a bit of data goes somewhere, it crosses a bus of some kind. That's it; now go pass the exam!

All right, so it's a bit—so to speak—more complicated than that. Most memory began as dynamic random access memory (DRAM). The main engineering changes that have taken place have all been attempts to find ways of either speeding up the memory to match CPU speeds, or to speed up the CPU to match memory speeds. The rest of memory technology relates to moving bits of information across buses more quickly.

TIP

When we refer to speeding up memory, we usually mean increasing the speed of the memory chips, increasing the clock speed of associated buses, or handling larger pieces of data.

To understand memory addresses, you should first understand a grid or matrix. We're therefore going to use Table 3.1 to a slightly different fashion, making it into a sort of "mind map." If you can see the way the overall types of memory break down on a grid, then perhaps they'll be easier to remember.

NOTE

A matrix is nothing more than an arrangement of columns and rows, like a spreadsheet or an Etch-a-Sketch. Columns go up and down across the page, and rows go left and right across the page. Remember the word "page." Column addresses are at the top; row addresses are along the side.

Cells going left to right (horizontally) in a row have an X coordinate. Cells going up and down (vertically) in a column have a Y coordinate. The direction of rows is called the X axis, and columns are called the Y axis. Combining X and Y coordinates gives us an address in the grid, like a cell address in a spreadsheet. A memory page is a range (group) of cell addresses within a row.

Table 3.1 Mind Map of Basic Memory Concepts

Permanent Storage/Nonvolatile Memory

Magnetic disks Optical disks Printed paper

Permanent

 

Swap files

Temporary

 

ROM BIOS Programmable ROM (Flash BIOS)

Nonvolatile

 

Memory cards

Nonvolatile

 

Volatile Memory

Acronym

Packaging

Random Access Memory

RAM

 

Dynamic RAM—main memory

DRAM

DIP

Static RAM Cache Memory (L-1 and L-2) CMOS—(uses trickle charge battery)

SRAM

SIMM or chip

Synchronous DRAM—main memory

SDRAM

DIMM

Rambus DRAM—main memory

RDRAM

RIMM

Double Data Rate SDRAM—main memory

DDR SDRAM

DIMM


NOTE

We discuss several additional types of memory in this chapter, but the A+ exam is focused primarily on the types listed in Table 3.1.

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