Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 96)

82 Assembly Language Programming for the 68000 Family

ADDA.L #RECSIZ,A0 GET NEXT RECORD

BRA LOOP BACK FOR MORE

FINI:

Notice that the age is not added directly to the sum in DO. This is because

the age is a byte value, and adding a byte to DO would only allow a sum

as large as a byte. To get around this, we clear the high-order bytes of

Dl and then move the age byte into the low-order byte of Dl. Then we

can perform a full longword addition.

We can also take advantage of this addressing mode to rewrite the

string compare previously presented.

LEA SI,AO

AO -> SI

LEA S2/A1

Al -> S2

LOOP:

CMPM.B

(A0)+, <A1) +

COMPARE BYTES

BNE

DIFF

DIFFERENT

TST.B

-1 (AO)

FINISHED?

BEQ

SAME

YES, THEY MATCH

BRA

LOOP

LOOP FOR NEXT CHAR.

DIFF:

•

HERE WHEN STRINGS DIFFERENT

SAME:

HERE WHEN STRINGS MATCH

Address Register Indirect With Index

This mode is very similar to address register indirect with displace

ment. In fact, it is address register indirect with displacement plus the

addition of a value contained in any one of the address or data registers.

The general form is d8(An,Rn.W) or d8(An,Rn.L). d8 is an 8-bit displace

ment. Its range is —128 to +127. This is not as great a range as with

address register indirect with displacement mode. Rn is any one of the

address or data registers. Either the sign-extended word or the complete

longword is taken from the index register, depending on the suffix the

programmer specifies. The effective address is formed by adding the dis

placement, the contents of the address register, and the contents of the

index register. This can be expressed as <ea> = (An)+(Ri)+d, where Ri

is the appropriate value from Rn.

This addressing mode is especially useful for two-dimensional arrays.

For a simple two-dimensional array of bytes, words, or longwords, the

address of the appropriate row can be placed in the address register and

the index of the column in the index register. In this case the displacement

would be set to zero. The effective operand would then be the location

1 2 ... 91 92 93 94 95 96 97 98 99 100 101 ... 255 256

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