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20 Assembly Language Programming for the 68000 Family

nal rate at which operations are performed. The basic interval between

clock pulses is the cycle time for the CPU. It takes a multiple number of

cycles for the CPU to execute an instruction. Not all instructions require

the same number of cycles, and not every cycle requires an access to

memory. Furthermore, the M68000 family supports what is known as an

asynchronous bus: the speed of the bus does not have to be directly re

lated to the CPU clock. This is a major departure from the M6800 family

design.

When you consider this information, together with some more exotic

concepts such as instruction prefetch and pipelining, to be covered in

later chapters, it is a complicated task to determine the exact relationship

between the data bus width and the CPU speed. One thing is clear;

the M68000 is a fast microprocessor. Microprocessor manufacturers are

constantly designing benchmark tests to show the performance edge of

their chips. It is always possible to design a program that shows up

the good features of any chip in comparison with others. I will leave

it up to you to decide for yourself how much faith you want to place in

benchmark programs.

The CPU

Before starting on assembly language programming, it is essential that to

take a look at the 68000 microprocessor architecture. We are not going to

discuss all the details of the actual machine language used by the CPU,

but we must know enough about the structure of memory and the internal

CPU registers to use assembly language.

As you are probably aware from your experience with a high-level

programming language such as BASIC or Pascal, all information in the

computer’s memory and acted on by the CPU must be represented as

numbers. This includes textual information, which is represented by the

numeric equivalents for each character as governed by an appropriate

character set. You will learn more about character manipulation in later

chapters.

The instructions of the 68000 microprocessor are designed to manip

ulate numeric information in a variety of ways. Data can be moved from

one place to another in the computer’s memory, or data can be moved

from memory to registers contained in the microprocessor chip. Registers

are special places to store and manipulate data. They are like memory lo

cations except that they operate at much higher speeds and serve special

purposes for the CPU. The most important use of the registers is in per

forming arithmetic operations. The 68000 is capable of performing the

normal arithmetic operations on integer numbers, such as addition, sub

traction, multiplication and division, as well as logical operations. Logical

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Motorola M68000 Betriebsanweisung Seite 34