Motorola M68000 User's Guide download pdf (Page 36)

22 Assembly Language Programming for the 68000 Family

mer from having to worry about the exact representation of instructions

and data in memory. However, a programmer usually finds the occasion

when such knowledge is useful.

Recall that memory consists of an array of individually addressable

bytes. If the data we wish to store in memory is only a single byte, there

is no question as to how it is represented, only where. If, however, the

data is a word or instruction consisting of more than one byte, it is not

clear how this information is stored. Word data (16 bits) are always stored

with the high-order byte stored in the lower memory address. This means

that if we were reading a dump of memory, word data would be read

directly. Many microprocessors have this order reversed, making it much

harder to interpret the contents of memory. Figure 4 shows how byte,

word and longword values are stored.

Instructions consist of one or more words. The first word always con

tains the operation code, or opcode. This specifies what the particular

Integer Data

1 Byte -8 Bits

IS 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

MSB Bvt®0 LSB

Byte 1

Byte 2 Byte 3

1 Word-16 Bits

IS 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

MSB

WordO

LSB

Word 1

Word 2

Even Bytes I Odd Bytes

7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0

1 Long Word*32 Bits

15 14 13 12 11 10 9376543210

MSB

— — Long Word 0—

---------

—

High Order

Low Order

LSB

— — Long Word 1

---------------

—

— — Long Word 2 —

---------

—

Figure 4 Bytes, words, and longwords memoiy. (Courtesy of Motorola, Inc.)

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 255 256

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Motorola M68000 User's Guide Page 36

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