Datasheet

Identifying Purposes and Characteristics of Memory 49
TABLE 1.2 How Some Memory Types Transfer 3200MBps per Channel
Memory Type
Actual/Effective (FSB) Clock
Frequency (MHz)
Bytes per Transfer
SDR SDRAM PC400* 400/400 8
DDR SDRAM PC3200 200/400 8
DDR2 SDRAM PC2-3200 100/400 8
DDR3 SDRAM PC3-3200** 50/400 8
DRDRAM PC800 400/800 4***
* SDR SDRAM PC400 does not exist.
**PC3-3200 does not exist and is too slow for DDR3.
***Assuming requisite 32-bit dual-channel mode
SRAM
Static random access memory (SRAM) doesn’t require a refresh signal like DRAM does.
The chips are more complex and are thus more expensive. However, they are considerably
faster. DRAM access times come in at 40 nanoseconds (ns) or more; SRAM has access
times faster than 10ns. SRAM is often used for cache memory.
ROM
ROM stands for read-only memory. It is called read-only because the original form of this
memory could not be written to. Once information had been etched on a silicon chip and
manufactured into the ROM package, the information couldn’t be changed. If you ran
out of use for the information or code on the ROM, you added little eyes and some cute
fuzzy extras and you had a bug that sat on your desk and looked back at you. Some form
of ROM is normally used to store the computer’s BIOS, because this information normally
does not change very often.
The system ROM in the original IBM PC contained the power-on self-test (POST), Basic
Input/Output System (BIOS), and cassette BASIC. Later IBM computers and compatibles
include everything but the cassette BASIC. The system ROM enables the computer to “pull
itself up by its bootstraps,” or boot (find and start the operating system).
Through the years, different forms of ROM were developed that could be altered, later
ones more easily than earlier ones. The first generation was the programmable ROM (PROM),
which could be written to for the first time in the field using a special programming device,
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