A random access machine is an abstract machine with integer-valued memory registers whose addresses can themselves be computed. An instruction can access a register by its address without traversing all intervening registers (CRC Press LLC 2004).
A word-based variant stores bounded-length sequences of binary digits in its registers. In a logarithmic-word
model for an input of length 
, each memory word has 
binary digits, and
the specified arithmetic and logical operations on a constant number of words have
unit cost. An operation on a number spanning several words must be charged according
to the operations it requires. This distinction matters when specifying the computation
time of an algorithm.
