Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 162)

148 Assembly Language Programming for the 68000 Family

We can sign-extend a byte to a word, or a word to a longword, using

the EXT instruction. The general form is:

EXT[.<size>] Dn

<size> = W, L

So, if we want to divide two signed words, we can use the following

instructions:

MOVE.W VAL1,D0

EXT.L DO

DIVS VAL2,D0

MOVE.W DO,RESULT

VAL1: DS.W 1

VAL2: DS.W 1

RESULT: DS.W 1

Before leaving division, there are a few more loose ends to clear up.

In signed division, the sign of the remainder is always the same as that of

the dividend. This means that if a negative number is divided by another

negative number, even though the quotient is positive, any remainder

will be negative. If we divide —23 by —10, the execution of the DIVS

instruction will yield a quotient of +2 and a remainder of —3.

In division, signed or unsigned, it is possible for the quotient to be

larger than the destination register can hold. This overflow condition,

always the case when we try to divide by zero, can occur for divisors

other than zero. The overflow bit, V, is used to detect this situation. The

result of such a division leaves the operands unchanged. A divide by zero

is a special case: a special exception condition is generated by an attempt

to divide by zero. This causes a trap to exception vector number 5. The

discussion of traps in the next chapter will show how this capability can

be utilized.

Decimal Arithmetic

Up to this point, all our numerical values have been represented as

binary data consisting of one or more bytes. Decimal numbers that are

input to a program must be converted to binary. Likewise, a decimal

number that is output must be converted from the internal binary repre

sentation. The INDEC and OUTDEC subroutines introduced in Chapter

4 perform these operations. As it turns out, the 68000 has a number of in

structions that allow the represention of decimal numbers internally in a

special way that makes the input and output conversions much simpler.

This representation is called binary coded decimal or BCD.

1 2 ... 157 158 159 160 161 162 163 164 165 166 167 ... 255 256

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 162