RAM is NOT an acronym

by Reggie Huff

I know what they told you but... RAM is NOT an acronym.

SRAM, DRAM, SDRAM, VRAM, NOVRAM... The talk of the town. And they've always said, "It stands for Random Access Memory." Right?

That story is obsolete. The term RAM has become a noun meaning Volatile memory.

Once upon a time, way back when RAM actually was an acronym, it referred to the means of locating particular data-or the architecture-in a storage system. We meant that in a RAM type of system any data location could be selected and accessed next; in other words, chosen and accessed randomly. This characteristic distinguished the information access method from other methods which were not random such as Sequential Access, First-In-First-Out (FIFO), and Last-In-First-Out (LIFO).

SEQUENTIAL ACCESS MEMORY
In Sequential Access Memory architectures, information is stored in a long string. When you want to access some part of that string you have to "travel" through whatever data lies between where you are starting and where you want to go.

Audio and video cassette tapes are good examples of this type of storage. If you happen to be at the end of the third song on a music tape and you want to hear the sixth song, you have to wind through the fourth and fifth songs before you can access song six. This method is so named because the information is stored Sequentially (one after another in sequence: AKA serially) and therefore, needs to be accessed sequentially.

FIFO ACCESS MEMORY

Another access method is First-In-First-Out or FIFO. This storage architecture can be thought of as a queue where the information is available in the same order as it was put into the memory. Looking down from the ceiling of a Savings & Loan we see that there is a line of folks patiently waiting for their turn with one of the tellers. The first person into the queue is the first one out at the teller end. The second one into the line is the second one out, etc. The FIFO method is widely used in semiconductor products in storage blocks such as shift registers as well as for providing the foundational structures for Serial (AKA sequential) data communications.

LIFO ACCESS MEMORY
Last-In-First-Out or LIFO memory architectures operate on the same principle as those plate-stack-carts next to the silverware in a cafeteria. The plates are loaded onto a spring after they are washed and the last plate put onto the stack is the first plate we can access. The first plate put onto the stack is the last one we can access, and so forth.

LIFO Plate-Stack Cart

In semiconductors, this LIFO Memory architecture is used as a block in most microprocessors and microcontrollers. Its most common function is to store the address of some step in a series of instructions (a routine) when that series is interrupted and called upon to tend to some other, more urgent task. If, in turn, that routine is also interrupted midway in its course, then the "stopped at" instruction address is placed on the stack on top of the first one. Once the more urgent task is completed, the system returns to the stack to find the address of the step in the interrupted routine so the machine can continue from where it left off.

RANDOM ACCESS MEMORY
When you approach the counter in a Post Office Box service center, the clerk is able to access any PO Box regardless of its location. The clerk might access PO Box 121 and then Box 108 and then Box 111 and so on. This "free to chose any location next" method is called Random Access since any random location might be the next in a series.

Random Access P.O. Boxes

In electronic systems we use Random Access architectures for most of our data storage. In this case what we mean by Random Access is that any row in the memory array might be chosen next in any sequence of data access instructions.

The variety of Random Access IC-Memory devices includes SRAM, DRAM, SDRAM, VRAM, NOVRAM, ROM, PROM, EPROM,EEPROM, and Flash. The Random Access feature is spread far and wide across the spectrum of IC memory types.

VOLATILITY
To begin to understand the functions and features of the myriad "flavors" of memory chips, we'll separate the volatile from the non-volatile.

In the world of IC memories the term volatile is an adjective whose definition includes: Following no predictable pattern, variable, uncertain, unstable, changeable, inconsistent, erratic, unpredictable, unsteady...

What this all amounts to is that Volatile Memory chips forget everything they were asked to remember whenever power disappears. If you've ever been in the middle of typing a letter or working on a spread sheet when suddenly the lights went out or someone unplugged your computer, you learned two important things:
• That whatever you had been typing was lost forever
• That it's important to "save" your work periodically

You may have had the experience of taking your car in for service and discovering when you get it back that the radio stations you had programmed were lost. If you've replaced the battery in your Buck Rogers home phone you might have found that all of the speed-dial numbers you had programmed so carefully were lost.

What happened in each of these cases is that power was temporarily removed (i.e. the mechanic disconnected your car battery) from the memory device that was storing the information. The memory type in these cases is volatile memory that only "remembers" when there is power present. When the power comes back on, volatile memory types return to service with their content completely scrambled.

When Volatile Memory is powered up it needs to be initialized or "loaded" with data before it may be called upon for anything useful. Examples of Volatile Memory chips include SRAM, DRAM, SDRAM, VRAM, and NOVRAM. Notice: RAM is synonymous with Volatile.

Conversely, the music on your CDs or cassette tapes, reel-to-reel tapes, or vinyl LPs remains intact when they are in the player with power applied as well as when they are in your purse or briefcase without any power whatsoever. These are examples of Non-Volatile Memory.

In semiconductor memory devices, examples of the Non-Volatile Memory characteristic include ROM, PROM, EPROM, EEPROM, NOVRAM, and Flash. With each of these memory devices, whatever data they contained while power was applied remains intact while the power is gone. When they are later powered up, the previous data set is ready for action.

The term RAM, therefore, calls out the distinction of volatility. All RAM involves Volatile memory. However, all RAM is not Random Access (i.e. VRAM includes both Random Access features and Serial Access features). More poignant, for the sake of my argument here, is the fact that most of the other IC memory families also use the Random Access architecture (ROM, PROM, EPROM, EEPROM, etc.), so the random accessibility of the RAM clan does not distinguish it from any of the other families of memories. Hence, RAM has become a noun meaning Volatile and no longer reflects the acronym or the random accessibility which it once represented.

"RAM is NOT an Acronym" first appeared on EBNONLINE.COM on October 11, 1999.

© Copyright 1999, 2000, 2001 Huff Communications. All Rights Reserved.