Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 157)

Advanced Arithmetic 143

the high-order longwords. The carry condition, C, is set/reset as a result

of our first addition. All we have to do is somehow add the carry bit to

our high-order addition. But wait, the MOVE instruction used to save the

low-order result will clear the carry bit. In fact, we can’t even get a hold

of the high-order longword without destroying the carry bit. The 68000

designers anticipated this problem. They provided the extend condition

or X bit in the condition code register. This bit is always a copy of the

carry bit for arithmetic operations. However, it is not cleared, or modified

in any way, as a result of a MOVE instruction.

The ADDX, SUBX and NEGX instructions are provided to utilize the

value in the extend bit. Their general forms are:

ADDX[.<size>] Dy,Dx

ADDXl.<size>] -(Ay),-(Ax)

SUBXt.<size>] Dy,Dx

SUBX[.<size>] -(Ay),-(Ax)

NEGX[.<size>] <ea>

<size> = B, W, L

They work like ADD, SUB and NEG, except that the extend bit is

factored into the calculation, and the addressing modes are restricted

to those shown. For ADDX, the extend bit is added to the result; for

SUBX and NEGX it is subtracted—this is equivalent to a borrow for a

subtraction. The ADDX, SUBX, and NEGX instructions all set the C and

X bits after a calculation. The remaining instructions needed to complete

the calculation are:

MOVE.L A,DO

MOVE.L B,D1

ADDX.L D1,D0

MOVE.L D0,C

The above procedure can be used to add numbers of any precision. An

ADD instruction is used for the lowest order byte, word, or longword,

and all other additions are performed with the ADDX instruction.

Multiple precision subtraction can be performed in a similar manner,

using the SUB instruction and the subtract-with-extend instruction, SUBX.

The following instructions will subtract the three-byte variable A from B

and store the result in C:

MOVE.B B+2,D0

SUB.B A+2,D0

MOVE.B DO,C+2

MOVE.B B+l,D0

MOVE.B A+l,D1

SUBX.B D1,D0

MOVE.B D0,C+1

MOVE.B B,D0

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