Hardware Reference
In-Depth Information
it can hold any one of 2 k different bit combinations. Figure 2-9 shows three dif-
ferent organizations for a 96-bit memory. Note that adjacent cells have consecutive
addresses (by definition).
Address
Address
1 Cell
Address
0
0
0
1
2
3
4
5
1
2
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
16 bits
(c)
12 bits
(b)
8 bits
(a)
Figure 2-9. Three ways of organizing a 96-bit memory.
Computers that use the binary number system (including octal and hexadeci-
mal notation for binary numbers) express memory addresses as binary numbers. If
an address has m bits, the maximum number of cells addressable is 2 m . For ex-
ample, an address used to reference the memory of Fig. 2-9(a) needs at least 4 bits
in order to express all the numbers from 0 to 11. A 3-bit address is sufficient for
Fig. 2-9(b) and (c), however. The number of bits in the address determines the
maximum number of directly addressable cells in the memory and is independent
of the number of bits per cell. A memory with 2 12 cells of 8 bits each and a memo-
ry with 2 12 cells of 64 bits each need 12-bit addresses.
The number of bits per cell for some computers that have been sold commer-
cially is listed in Fig. 2-10.
The significance of the cell is that it is the smallest addressable unit. In recent
years, nearly all computer manufacturers have standardized on an 8-bit cell, which
is called a byte . The term octet is also used. Bytes are grouped into words .A
computer with a 32-bit word has 4 bytes/word, whereas a computer with a 64-bit
word has 8 bytes/word. The significance of a word is that most instructions oper-
ate on entire words, for example, adding two words together. Thus a 32-bit ma-
chine will have 32-bit registers and instructions for manipulating 32-bit words,
 
 
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