Motorola M68000 User's Guide download pdf (Page 61)

Getting Started 47

MOVE DO,AO

the assembler would use the MOVEA form of the instruction. Check your

assembler manual to be sure you can do this. Even if allowed, it is better

to use the correct instruction mnemonic.

Quite often we may want to set a register or memory location equal

to zero. We can always move a zero value into the destination operand.

Or we can subtract a register from itself. There are other ways to zero a

(CLR). This instruction is provided for just this purpose. Its general form

[<label>] CLRl.<size>] <ea> [<comment>]

The size may be B, W, or L. We can clear a data register or memory

location, but not an address register. Here are some examples:

CLR.L DO CLEARS DO

CLR.B CHAR CLEARS A BYTE IN MEMORY

CLR.W D5 CLEARS LOW ORDER WORD IN D5

Input and Output

While we can write many programs that do not require data to be

entered by the user, we certainly do not want to limit ourselves in this

way. Programs can be written that manipulate data that is included in

the assembly language source itself. This would be of limited use if the

data required frequent modification: each time the program is to be run

with different data the source program would have to be edited and then

reassembled, a time-consuming and unreasonable requirement for many

users. What we seek is the ability to obtain data entered from the user’s

terminal or keyboard at the time the program is executed.

Similarly, a method is normally required to obtain output from the

program during or after its execution. Unless there is some way of

displaying the program output on a terminal or a printer, a program’s

action can only be determined by looking at the contents of variables or

registers that may have changed during the programs execution. While

this may be possible with programmer utilities, such as a debugger, it is

certainly not the best way to start programming in assembly language.

Unfortunately, input and output is always dependent on the partic

ular computer you are using. Not all computer systems using the 68000

processor are equipped with the same input/output devices. Some sys

tems have a built in video display and others may have a separate CRT

1 2 ... 56 57 58 59 60 61 62 63 64 65 66 ... 255 256

No comments

Motorola M68000 User's Guide Page 61