Motorola M68000 User's Guide download pdf (Page 67)

Getting Started 53

SPA CE: EQU

$2 0

MOVE•W

# 3 0 ,D1

MLOOP:

MOVE•L

EXP,DO

JS R

OUTDEC

ADDQ.L

# 1 ,E X P

MOVE•B

# SPACE,DO

JS R

PUTC

MOVE•L

POWER,DO

JS R

OUTDEC

AD D.L DO, DO

MOVE. L

DO,POWER

JS R

NEWLINE

DBRA

Dl,M LOOP

• • •

E X P :

D C .L 1

POWER:

D C .L

A S C II SPACE

SET UP LOOP COUNT

GET CURRENT EXPONENT

OUTPUT

INCREMENT EXPONENT

OUTPUT A SPACE

GET CURRENT POWER

OUTPUT

DOUBLE IT

AND SAVE

GO TO A NEW LIN E

LOOP UNTIL DONE

WE CONTINUE HERE

Figure 7 Program to output powers of two.

Putting It All Together

Figure 7 shows a complete program that will output a table of the

powers of 2 from 21 to 231. As you learned in Chapter 1, 231 is the largest

power of 2 that can be represented with a 32-bit unsigned number. The

program is written as one counting loop. The D1 register is initialized

to 30, which is the number of powers that we wish to output minus one.

MLOOP is the label referenced by the DBRA instruction, which is the last

instruction of the program prior to the return to the operating system. We

also declared two variables, EXP and POWER, to represent the current

exponent and the actual power of 2 for the value of EXP. These two

variables are initialized prior to entering the main loop. Each successive

power is computed by doubling the previous power with the ADD.L

DO,DO instruction. Since the DO register is used for several functions,

POWER and EXP are updated after their new values are computed.

Formatting of the output lines is accomplished by inserting a space

after outputting the exponent, and an advance to the next line after

outputting the power. The ASCII character value for the space character

is 20 in hexadecimal. The standard procedure, PUTC, is called with this

value in the DO register. The output from this program should look like

the following:

1 2 ... 62 63 64 65 66 67 68 69 70 71 72 ... 255 256

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Motorola M68000 User's Guide Page 67

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