Motorola M68000 User's Guide download pdf (Page 35)

Microcomputer Architecture 21

operations allow manipulation of the individual bits of the data. You will

soon see how logical operations can be very useful.

Some instructions do not manipulate data but are instead used to

control the flow of your program. Often you will desire to repeat an

operation many times. Rather than repeat the instructions over and over

when you write your program, you can use the control instructions to

cause the microprocessor to automatically repeat a group of instructions

that you have written only once.

Memory

The memory used with the 68000 consists of a number of locations or

cells, each holding one 8-bit number or byte. Memory cells are numbered

from zero up to the maximum allowable amount of memory. The 68000

allows a maximum of 16 megabytes of memory. A megabyte is equal

to 220 or 1,048,57610. Therefore, 16 megabytes is actually 16,777,21610

locations or addresses. Figure 3 shows the concept of memory cells and

their corresponding addresses.

A program consists of instructions and data. Since everything in mem

ory is a number, careful organization is required to prevent the computer

from interpreting instructions as data, or data as instructions. This is nor

mally the responsibility of the programmer.

One of the reasons for using assembly language is to free the program-

ADDRESS MEMORY

MAX

Figure 3 Memory Organization.

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 255 256

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Motorola M68000 User's Guide Page 35