Identifying Purposes and Characteristics of Memory

“More memory, more memory, I don’t have enough memory!” Today, memory is one of the most popular, easy, and inexpensive ways to upgrade a computer.

As the computer’s CPU works, it stores information in the computer’s memory.

The rule of thumb is the more memory a computer has, the faster it will operate.

To identify memory within a computer, look for several thin rows of small circuit boards sitting vertically, packed tightly together near the processor.

Location of memory within a system
Parity












Parity checking is a rudimentary error-checking scheme that lines up the chips in a column and divides them into an equal number of bits, numbered starting at 0.

All the number n bits, one from each chip, form a numerical set. If even parity is used, for example, the number of bits in the set is counted up, and if the total comes out even, then the parity bit is set to 0, because the count is already even. If it comes out odd, then the parity bit is set to 1 to even up the count.

You can see that this is effective only for determining if there was a blatant error in the set of bits, but there is no indication as to where the error is and how to fix it. This is error checking, not error correction.

Finding an error can lock up the entire system and display a memory parity error. Enough of these errors and you need to replace the memory.

If that doesn’t fix the problem, good luck. In the early days of personal computing, almost all memory was parity-based. Compaq was one of the first manufacturers to employ non-parity RAM in their mainstream systems.

As quality has increased over the years, parity checking in the RAM subsystem has become rarer.

If parity checking is not supported, there will generally be fewer chips per module, usually one less per column of RAM.

The next step in the evolution of memory error detection is known as Error Checking and
Correcting (ECC).

If memory supports ECC, check bits are generated and stored with the data. An algorithm is performed on the data and its check bits whenever the memory is accessed. If the result of the algorithm is all zeros, then the data is deemed valid and processing
continues.

ECC can detect single- and double-bit errors and actually correct single-bit errors.

In the following sections, we’ll outline the four major types of computer memory—DRAM, SRAM, ROM, and CMOS—as well as memory packaging.