Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 24)

10 Assembly Language Programming for the 68000 Family

or hexadecimal

55P2

- 4A63

0B8F

Hand calculations involving multiplication or division are rarely per

formed by programmers. However, conventional hand methods can be

used. The basic principles we used for addition and subtraction are ap

plied. Although I will not explain multiplication or division, those readers

who desire can try some examples and verify their results by convert

ing to decimal and repeating the multiplication or division in the decimal

number base.

Bits, Bytes, Words and Longwords

So far in our discussion of numbers we have not indicated how large

our numbers can be. If you want to write down a very large number on

paper, the size of the number is only limited by the size of the paper.

This is not the case for computers. Internally the computer must represent

numbers by electrical signals. These signals represent the binary values

0 and 1. The maximum size of a number inside the computer is limited

to the number of binary digits, or bits, used to represent the number.

Theoretically we could use all the bits inside the computer to represent a

single number. This, of course, is not practical. Internally it is convenient

to limit the number of bits used for each number.

Many computers are organized around groups of eight bits, called

bytes. The size of memory on many computers is measured in bytes.

We might say a computer has 64 thousand bytes of memory. This is

equivalent to 512 thousand bits. Modem computers often have memory

sizes in the millions of bytes. A megabyte (MB) is equal to approximately

one million bytes. As we will discuss shortly, a single byte is normally used

to represent a single character of textual information. If we have a 2 MB

memory, we can store 2 million characters of information. If we assume

approximately 60 characters per line of printed material, and 50 lines per

page, this is equivalent to over 650 pages.

Bytes can be grouped together. For most computers, including the

68000 family, two bytes grouped together form a word. A word is there

fore equal to 16 bits. This is also equivalent to four hexadecimal digits.

We can also have longwords, made up of four bytes or 32 bits. Larger

groupings are possible but are not normally handled as a single value ex

cept by much larger computers. We will be dealing primarily with bytes,

words, and longwords in 68000 assembly language programming.

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 255 256

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 24