Online Computer Terms Dictionary - R

random-access memory

<storage> (RAM) (Previously "direct-access memory"). A data storage device for which the order of access to different locations does not affect the speed of access. This is in contrast to, say, a magnetic disk, magnetic tape or a mercury delay line where it is very much quicker to access data sequentially because accessing a non-sequential location requires physical movement of the storage medium rather than just electronic switching.

The most common form of RAM in use today is built from semiconductor integrated circuits, which can be either static (SRAM) or dynamic (DRAM). In the 1970s magnetic core memory was used. RAM is still referred to as core by some old-timers.

The term "RAM" has gained the additional meaning of read-write. Most kinds of semiconductor read-only memory (ROM) are actually "random access" in the above sense but are never referred to as RAM. Furthermore, memory referred to as RAM can usually be read and written equally quickly (approximately), in contrast to the various kinds of programmable read-only memory. Finally, RAM is usually volatile though non-volatile random-access memory is also used.

Interestingly, some DRAM devices are not truly random access because various kinds of "page mode" or "column mode" mean that sequential access is faster than random access.

(1995-12-05)

Random Access Memory Digital-to-Analog Converter

<hardware> (RAMDAC) A combination of three fast DACs with a small SRAM used in graphics display adapters to store the colour palette and to generate the analog signals to drive a colour monitor. The logical colour number from the display memory is fed into the address inputs of the SRAM to select a palette entry to appear on the output of the SRAM. This entry is composed of three separate values corresponding to the three components (red, green, and blue) of the desired physical colour. Each component value is fed to a separate DAC, whose analog output goes to the monitor, and ultimately to one of its three electron guns (or equivalent in non-CRT displays).

DAC word lengths range usually from 6 to 10 bits. The SRAM's wordlength is three times the DAC's word length. The SRAM acts as a colour lookup table. It usually has 256 entries (and thus an 8-bit address). If the DAC's word length is also 8 bits, we have a 256 x 24-bit SRAM which allows a selection of 256 out of 16777216 possible colours for the display. The contents of the SRAM can be changed while the display is not active (during display blanking times). The SRAM can usually be bypassed and the DACs can be fed directly by display data (for true colour modes).

(1996-03-24)