COMPUTER SYSTEMS ORGANIZATION - Structured Computer Organization - page 76

Computer Bits/cell

Burroughs B1700 1

IBM PC 8

DEC PDP-8 12

IBM 1130 16

DEC PDP-15 18

XDS 940 24

Electrologica X8 27

XDS Sigma 9 32

Honeywell 6180 36

CDC 3600 48

CDC Cyber 60

Figure 2-10. Number of bits per cell for some historically interesting commer-
cial computers.

whereas a 64-bit machine will have 64-bit registers and instructions for moving,
adding, subtracting, and otherwise manipulating 64-bit words.

2.2.3 Byte Ordering

The bytes in a word can be numbered from left to right or right to left. At first
it might seem that this choice is unimportant, but as we shall see shortly, it has
major implications. Figure 2-11(a) depicts part of the memory of a 32-bit com-
puter whose bytes are numbered from left to right, such as the SPARC or the big
IBM mainframes. Figure 2-11(b) gives the analogous representation of a 32-bit
computer using right-to-left numbering, such as the Intel family. The former sys-
tem, where the numbering begins at the ''big'' (i.e., high-order) end is called a big
endian computer, in contrast to the little endian of Fig. 2-11(b). These terms are
due to Jonathan Swift, whose Gulliver's Travels satirized politicians who made war
over their dispute about whether eggs should be broken at the big end or the little
end. The term was first used in computer architecture in a delightful article by
Cohen (1981).
It is important to understand that in both the big endian and little endian sys-
tems, a 32-bit integer with the numerical value of, say, 6, is represented by the bits
110 in the rightmost (low-order) 3 bits of a word and zeros in the leftmost 29 bits.
In the big endian scheme, the 110 bits are in byte 3 (or 7, or 11, etc.), whereas in
the little endian scheme they are in byte 0 (or 4, or 8, etc.). In both cases, the word
containing this integer has address 0.
If computers stored only integers, there would be no problem. However, many
applications require a mixture of integers, character strings, and other data types.
Consider, for example, a simple personnel record consisting of a string (employee