Editing Binary prefix (section)

==== Main memory ====
Early computers used one of two addressing methods to access the system memory; binary (base 2) or decimal (base 10).<ref name="weik1961" /> For example, the [[IBM 701]] (1952) used a binary methods and could address 2048 [[Word (computer architecture)|word]]s of 36 [[bit]]s each, while the [[IBM 702]] (1953) used a decimal system, and could address ten thousand 7-bit words.

By the mid-1960s, binary addressing had become the standard architecture in most computer designs, and main memory sizes were most commonly powers of two. This is the most natural configuration for memory, as all combinations of states of their [[address line]]s map to a valid address, allowing easy aggregation into a larger block of memory with contiguous addresses.

While early documentation specified those memory sizes as exact numbers such as 4096, 8192, or {{val|16384}} units (usually [[word (computer architecture)|word]]s, bytes, or bits), computer professionals also started using the long-established metric system prefixes "kilo", "mega", "giga", etc., defined to be powers of 10,<ref name="BIPM8" /> to mean instead the nearest powers of two; namely, 2<sup>10</sup> = 1024, 2<sup>20</sup> = 1024<sup>2</sup>, 2<sup>30</sup> = 1024<sup>3</sup>, etc.<ref name="blais1930" /><ref name="liny1972" /> The corresponding metric prefix symbols ("k", "M", "G", etc.) were used with the same binary meanings.<ref name="real1959" /><ref name="gruen1960" /> The symbol for 2<sup>10</sup> = 1024 could be written either in lower case ("k")<ref name="horak2008" /><ref name="dodd1997" /><ref name="laver1989" /> or in uppercase ("K"). The latter was often used intentionally to indicate the binary rather than decimal meaning.<ref name="amda1964" /> This convention, which could not be extended to higher powers, was widely used in the documentation of the [[IBM&nbsp;360]] (1964)<ref name="amda1964" /> and of the [[IBM System/370]] (1972),<ref name="IBM1972" /> of the [[CDC&nbsp;7600]],<ref name="CDC7600" /> of the DEC [[PDP-11]]/70 (1975)<ref name="bell1975" /> and of the DEC [[VAX-11/780]] (1977).{{citation needed|date=August 2023}}

In other documents, however, the metric prefixes and their symbols were used to denote powers of 10, but usually with the understanding that the values given were approximate, often truncated down. Thus, for example, a 1967 document by [[Control Data Corporation]] (CDC) abbreviated "2<sup>16</sup> = {{nowrap|64 × 1024}} = {{val|65536}} words" as "65K words" (rather than "64K" or "66K"),<ref name="CDC1967" /> while the documentation of the [[HP 2100|HP 21MX]] real-time computer (1974) denoted {{nowrap|3 × 2<sup>16</sup>}} = {{nowrap|192 × 1024}} = {{val|196608}} as "196K" and 2<sup>20</sup> = {{val|1048576}} as "1M".<ref name="frank1974" />

These three possible meanings of "k" and "K" ("1024", "1000", or "approximately 1000") were used loosely around the same time, sometimes by the same company. The [[HP 3000]] business computer (1973) could have "64K", "96K", or "128K" bytes of memory.<ref name="HP1973" /> The use of SI prefixes, and the use of "K" instead of "k" remained popular in computer-related publications well into the 21st century, although the ambiguity persisted. The correct meaning was often clear from the context; for instance, in a binary-addressed computer, the true memory size had to be either a power of 2, or a small integer multiple thereof. Thus a "512 megabyte" RAM module was generally understood to have {{nowrap|512 × 1024<sup>2</sup>}} = {{val|536870912}} bytes, rather than {{val|512000000}}.