Motorola M68000 User's Guide download pdf (Page 26)

12 Assembly Language Programming for the 68000 Family

10011100

+ 00000001

10011101

Let’s convert 8910 to —8910 using two’s complement. First we must

convert 8910 to binary. 89io = 010110012. Now form the one’s complement,

101001102; finally, to get the two’s complement we add one. Our result

is —8910 = 101001112.

The nice property of two’s complement numbers is that we can add

them together without concern for the sign. We do not have to perform

any conversion. As a simple example, we should be able to add 8910 and

—8910 and obtain a zero result.

01011001 8910

+ 10100111 -89^0

oooooooo o10

We ignore any carry out from the sign bit position.

To subtract in two’s complement, we merely negate the subtrahend

and then add. This operation is performed regardless of whether the

subtrahend is positive or negative.

ASCII Character Codes

In order to represent character information in the computer’s memory,

we must find a way to convert the such as CR (carriage return), LF

(line feed) and HT (horizontal tab). There are other “control character”

codes that are of general interest but are not necessarily available on

all terminals. For example, a BEL (bell) might sound a beep on your

terminal, or a VT (vertical tab) might be implemented. The other codes

with values less than 3210 are used for a variety of purposes including the

protocols used for data communications.

One special character should be mentioned. The DEL (delete) code,

12710, which is sometimes called a rubout, is most commonly used by

software to indicate the deletion of the last character typed. Some soft

ware uses the BS (backspace) character to perform this same operation.

You should note that these are really two different character codes, 810

and 12710, and the interpretation as to what, if anything, these characters

do is up to the software.

Some computers and terminals have incorporated additional charac

ters as an extension to the standard ASCII character set. By allowing

codes above 12710> an additional 12810 characters can be specified. These

might be from a foreign language, or for special graphics used by certain

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Motorola M68000 User's Guide Page 26