|
In digital computers, it is helpful to store a state electrically so the machine doesn't need eyes or hands to check for the string, marble, or marzipan. Possible candidates for electrical state-saving systems include those that depend on whether an electrical charge is present or whether a current will flow. Both of these techniques are used in computer memories for primary storage systems.
The analog of electricity, magnetism, can also be readily manipulated by electrical circuits and computers. In fact, a form of magnetic memory called core was the chief form of primary storage for the first generation of mainframe computers. Some old-timers still call primary storage "core memory" because of this history. Today, however, magnetic storage is mostly reserved for mass storage because magnetism is one step removed from electricity. Storage devices have to convert electricity to magnetism to store bits and magnetic fields to electrical pulses to read them. The conversion process takes time, energy, and effort-all of which pay off for long-term storage, at which magnetism excels, but are unnecessary for the many uses inside the computer.
Using electrical circuits endows primary storage with the one thing it needs most-speed. Only part of its swiftness is attributable to electricity, however. More important is the way in which the bits of storage are arranged. Bits are plugged into memory cells that are arranged like the pigeon holes used for sorting mail-and for the same reason. Using this arrangement, any letter or bit of memory can be instantly retrieved when it is needed. The microprocessor does not have to read through a huge string of data to find what it needs. Instead it can zero in on any storage unit at random. Consequently, this kind of memory is termed Random Access Memory, more commonly known by its acronym, RAM.
Random Access Memory
The vast majority of memory used in PCs is based on storing electrical charges rather than magnetic fields. Because all the other signals inside a PC are normally electronic, the use of electronic memory is only natural. It can operate at electronic speed without the need to convert technologies. Chip makers can fabricate electronic memory components exactly as they do other circuits, even on the same assembly lines. Best of all, electronic memory is cheap, the most affordable of all direct-access technologies.
Dynamic Memory
The most common electronic memory inside today's personal computers brings RAM to life using minute electrical charges to remember memory states. Charges are stored in small capacitors. The archetypical capacitor comprises two metal plates separated by a small distance that's filled with an electrical insulator. A positive charge can be applied to one plate and, because opposite charges attract, it draws a negative charge to the other nearby plate. The insulator separating the plates prevents the charges from mingling and neutralizing each other.
The capacitor can function as memory because a computer can control whether the charge is applied to or removed from one of the capacitor plates. The charge on the plates can thus store a single state and a single bit of digital information.
In a perfect world, the charges on the two plates of a capacitor would forever hold themselves in place. One of the imperfections in the real world results in no insulator being perfect. There's always some possibility that a charge will sneak through any material, although better insulators lower the likelihood that they cannot eliminate it entirely. Think of a perfect capacitor as being like a glass of water, holding whatever you put inside it. A real-world capacitor inevitably has a tiny leak through which the water (or electrical charge) drains out. The leaky nature of capacitors themselves is made worse by the circuitry that charges and discharges the capacitor because it, too, allows some of the charge to leak off.
This system seems to violate the primary principal of memory-it won't reliably retain information for very long. Fortunately, this capacitor-based system can remember long enough to be useful-a few or few dozen milliseconds-before the disappearing charges make the memory unreliable. Those few milliseconds are sufficient that practical circuits can be designed to periodically recharge the capacitor and refresh the memory. For example, some Motorola 1MB SIMMs require memory refreshing every 8 milliseconds. Some 8MB SIMMs need a refresh only every 32 ms.
Refreshing memory is akin to pouring extra water into a glass from which it is leaking. Of course, you have to be quick to pour the water while there's a little left so you know which glass needs to be refilled and which is supposed to be empty.
To assure the integrity of its memory, PCs periodically refresh memory automatically. During the refresh period, the memory is not available for normal operation. Accessing memory also refreshes the memory cell. Depending on how a chip maker has designed its products, accessing a single cell also may refresh the entire row or column containing the accessed memory cell.
Because of the changing nature of this form of capacitor-based memory and its need to be actively maintained by refreshing, it is termed dynamic memory. Integrated circuits that provide this kind of memory are termed dynamic RAM or DRAM chips.
In personal computer memories, special semiconductor circuits that act like capacitors are used instead of actual capacitors with metal plates. A large number of these circuits are combined to make a dynamic memory integrated circuit chip. As with true capacitors, however, dynamic memory of this type must be periodically refreshed.
Static Memory
While dynamic memory tries to trap evanescent electricity and hold it in place, static memory allows the current flow to continue on its way. Instead, it alters the path taken by the power, using one of two possible courses of travel to mark the state being remembered. Static memory operates as a switch that potentially allows or halts the flow of electricity.
A simple mechanical switch will, in fact, suffice as a form of static memory. It, alas, has the handicap that it must be manually toggled from one position to another by a human or robotic hand.
A switch that can be itself controlled by electricity is called a relay, and this technology was one of the first used for computer memories. The typical relay circuit provided a latch. Applying a voltage to the relay energizes it, causing it to snap between not permitting electricity to flow to allowing it. Part of the electrical flow could be used to keep the relay itself energized, which would, in turn, maintain the electrical flow. Like a door latch, this kind of relay circuit stays locked until some force or signal causes it to change, opening the door or the circuit.
Transistors, which can behave as switches, can also be wired to act as latches. In electronics, a circuit that acts as a latch is sometimes called a flip-flop because its state (which stores a bit of data) switches like a political candidate who flip-flops between the supporting and opposing views on sensitive topics. A large number of these transistor flip-flop circuits, when miniaturized and properly arranged, together make a static memory chip. Static RAM is often shortened to SRAM by computer professionals. Note that the principal operational difference between static and dynamic memory is that static RAM does not need to be periodically refreshed.
Read-Only Memory
Note that both the relay and the transistor latch must have a constant source of electricity to maintain their latched state. If the current supplying them falters, the latch will relax and the circuit forgets. Even static memory requires a constant source of electricity to keep it operating. Similarly, if dynamic memory is not constantly refreshed, it too forgets. When the electricity is removed from either type of memory circuit, the information that it held simply evaporates, leaving nothing behind. Consequently, these electrically-dependent memory systems are called volatile. A constant supply of electricity is necessary for them to maintain their integrity. Lose the electricity, and the memory loses its contents.
Not all memory must be endowed with the ability to be changed. Just as there are many memories that you would like to retain-your first love, the names of all the constellations in the Zodiac, the answers to the chemistry exam-a computer is better off when it can remember some particularly important things without regard to the vagaries of the power line. Perhaps the most important of these more permanent rememberings is the program code that tells a microprocessor that it's actually part of a computer and how it should carry out its duties.
In the old-fashioned world of relays, you could permanently set memory in one position or another by carefully applying a hammer. With enough assurance and impact, you could guarantee that the system would never forget. In the world of solid-state, the principal is the same but the programming instrument is somewhat different. All that you need is switches that don't switch-or, more accurately, that switch once and jam. This permanent kind of memory is so valuable in computers that a whole family of devices called Read-Only Memory or ROM chips has been developed to implement it. These devices are called read-only because the computer that they are installed in cannot store new code in them. Only what is already there can be read from the memory.
In contrast, the other kind of memory, to which the microprocessor can write as well as read, is logically termed Read-Write Memory. That term is, however, rarely used. Instead, read-write memory goes by the name RAM even though ROM also allows random access to its contents.
Mask ROM
If ROM chips cannot be written by the computer, the information inside must come from somewhere. In one kind of chip, the mask ROM, the information is built into the memory chip at the time it is fabricated. The mask is a master pattern that's used to draw the various circuit elements on the chip during fabrication. When the circuit elements of the chip are grown on the silicon substrate, the pattern includes the information that will be read in the final device. Nothing, other than a hammer blow or its equivalent in destruction, can alter what is contained in this sort of memory.
Mask ROMs are not common in personal computers because they require their programming to be carried out when the chips are manufactured; changes are not easy to make and the quantities that must be made to make things affordable are daunting.
PROM
One alternative is the Programmable Read-Only Memory chip or PROM. This style of circuit consists of an array of elements that work like fuses. Too much current flowing through a fuse causes the fuse element to overheat, melt, and interrupt the current flow, protecting equipment and wiring from overloads. The PROM uses fuses as memory elements. Normally, the fuses in a PROM conduct electricity just like the fuses that protect your home from electrical disaster. Like ordinary fuses, the fuses in a PROM can be blown to stop the electrical flow. All it takes is a strong enough electrical current, supplied by a special machine called a PROM programmer or PROM burner.
PROM chips are manufactured and delivered with all of their fuses intact. The PROM is then customized for its given application using a PROM programmer to blow the fuses one-by-one according to the needs of the software to be coded inside the chip. This process is usually termed "burning" the PROM.
As with most conflagrations, the effects of burning a PROM are permanent. The chip cannot be changed to update or revise the program inside. PROMs are definitely not something for people who can't make up their minds-or for a fast changing industry.
EPROM
Happily, technology has brought an alternative, the Erasable Programmable Read-Only Memory chip or EPROM. Sort of self-healing semiconductors, the data inside an EPROM can be erased and the chip re-used for other data or programs.
EPROM chips are easy to spot because they have a clear window in the center of the top of their packages. Invariably, this window is covered with a label of some kind, and with good reason. The chip is erased by shining high intensity ultraviolet light through the window. If stray light should leak through the window, the chip could inadvertently be erased. (Normal room light won't erase the chip because it contains very little ultraviolet light. Bright sunshine does, however, and can erase EPROMs.) Because of their versatility, permanent memory, and easy reprogrammability, EPROMs are ubiquitous inside personal computers.
EEPROM
A related chip is called Electrically Erasable Programmable Read-Only Memory or EEPROM (usually pronounced double-E PROM). Instead of requiring a strong source of ultraviolet light, EEPROMs need only a higher than normal voltage (and current) to erase their contents. This electrical erasability brings an important benefit-EEPROMs can be erased and reprogrammed without popping them out of their sockets. EEPROM gives electrical devices such as computers and their peripherals a means of storing data without the need for a constant supply of electricity. Note that while EPROM must be erased all at once, each byte in EEPROM is independently erasable and writable. You can change an individual byte if you want. Consequently, EEPROM has won favor for storing setup parameters for printers and other peripherals. You can easily change individual settings, yet still be assured the values you set will survive switching the power off.
EEPROM has one chief shortcoming-it can be erased only a finite number of times. Although most EEPROM chips will withstand tens or hundreds of thousands of erase and reprogram cycles, that's not good enough for general storage in a PC that might be changed thousands of times each second you use your machine. This problem is exacerbated by the manner in which EEPROM chips are erased-unlike ordinary RAM chips in which you can alter any bit whenever you like, erasing an EEPROM means eliminating its entire contents and reprogramming every bit all over again. Change any one bit in a EEPROM, and the life of every bit of storage is shortened.
Flash Memory
A new twist to EEPROM is Flash ROM, sometimes called Flash RAM (as in the previous edition of this book-we've altered our designation to better fit usage and continuity in our discussion of ROM technology) or just Flash memory. Instead of requiring special, higher voltages to be erased, Flash ROM can be erased and reprogrammed using the normal voltages inside a PC. Normal read and write operations use the standard five-volt power that is used by most PC logic circuits. (Three-volt Flash ROM is not yet available.) An erase operation requires a super-voltage, a voltage in excess of the normal operating supply for computer circuitry, typically 12 volts.
For system designers, the electrical re-programmability of Flash ROM makes it easy to use. Unfortunately, Flash ROM is handicapped by the same limitation as EEPROM-its life is finite (although longer than ordinary EEPROM)]md]and it must be erased and reprogrammed as one or more blocks instead of individual bytes.
The first generation of Flash ROM made the entire memory chip a single block, so the entire chip had to be erased to reprogram it. Newer Flash ROMs have multiple, independently erasable blocks that may range in size from 4K to 128K bytes. The old, all-at-once style of Flash ROM is now termed bulk erase flash memory because of the need to erase it entirely at once.
New multiple-block Flash ROM is offered in two styles. Sectored-erase Flash ROM is simply divided into multiple sectors. Boot Block Flash ROM specially protects one or more blocks from normal erase operations so that special data in them-such as the firmware that defines the operation of the memory-will survive ordinary erase procedures. Altering the boot block typically requires applying the super-voltage to the reset pin of the chip at the same time as performing an ordinary write to the book block.
Although modern Flash ROMs can be erased only in blocks, most support random reading and writing. Once a block is erased, it will contain no information. Each cell will contain a value of zero. Your system can read these blank cells, though without learning much. Standard write operations can change the cell values from zero to one but cannot change them back. Once a given cell has been changed to a logical one with a write operation, it will maintain that value until the Flash ROM gets erased once again, even if the power to your system or the Flash ROM chip fails.
Flash memory is an evolving technology. The first generation of chips required that your PC or other device using the chips handle all the minutiae of the erase and write operations. Current generation chips have their own onboard logic to automate these operations, making Flash ROM act more like ordinary memory. The logic controls the timing of all the pulses used to erase and write to the chip, ensures that the proper voltages reach the memory cells, and even verifies that each write operation was carried out successfully.
On the other hand, the convenience of using Flash ROM has led many developers to create disk emulators from it. For the most effective operation and longest life, however, these require special operating systems (or modified versions of familiar operating systems) that minimize the number of erase and reprogramming cycles. | 
Disclaimer
