Page 186 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Memory ICs BE 667

This leads to an interesting quirk when referencing the size of a memory
device, In SI ~nits,~ the qualifier k (kilo)lo represents one thousand (1,000),
but the closest power of two to one thousand is 21°, which equals 11,024.

Therefore, a 1 kilobit (1 kb or 1 Kb) memory actually refers to a device
containing 1,024. bits.
Similarly, the qualifier M (mega)” is generally taken to represent one
million (l,OOO,OOS), but the closest power of two to one million is 2”, which
equals 1,048,576, Therefore, a 1 megabit (1 Mb) memory actually refers to a
device containing 1,048,576 bits. In the case of the qualifier C (giga),12 which
is now generally taken to represent one billion ( 1,000,000,000),13 the closest
power of two is z3O, which equals 1,073,741,824. Therefore, a 1 gigaabit (1 Gb)
memory actually refers to a device containing 1,073,741,824 bits.
If the width of the memory is equal to a byte or a multiple of bytes, then
the size of the memory may also be referenced in terms of bytes. For example,
a memory containing 1,024 words, each 8 bits wide, may be referred to as being
either an 8 kilobit (8 Kb) or a 1 kilobyte (1 KB) device (note the use of “‘b” an
“‘B” to represent bit and byte, respectively).
Because multiple memory devices are usually connected to a single data
the data corning out of the internal array is typically buffered from the external
system by means of tri-state gates.14 Enabling the tri-state gates allows the
device to drive data onto the data bus, while disabling them allows other
devices eo drive the data bus.
In addition to its address and data buses, a ROM requires a number
of control signals, the two most common being -chip-select and -read.

9 The metric system of measurement was developed during the French Revolution and its use
was legalized in the US. in 1866. The International System of Units (SI) is a modernized
version of the metric system.
10 The term kilo comes from the Greek khiloi, meaning “thousand” (strangely enough, this is the
only prefix with an. actual numerical meaning).
11 The term mega comes from the Greek mega, meaning “great” (hence the fact that Alexander
the Great was known as Alexundros Megos in those days).
12 The term gigu comes from the Latin gigus, meaning “giant.”
13 See the discussions in Chapter 3 on why we now take “one billion” to represent one thousand
million rather than one million million.
14 Tri-state gates were introduced in Chapter 11.