Motorola M68000 User's Guide download pdf (Page 159)

Advanced Arithmetic 145

ADDX.L -(A0),-(A1)

RTS

Moving the constant 4 into the CCR will clear all the bits except for the

Z condition. Remember, when the Z bit is set, there is a zero value. I will

get back to this point shortly. Notice that the subroutine first adjusts the

addresses to point to the longword just beyond the number, and then uses

address register indirect with predecrement addressing for the additions.

The values in AO and A1 will be restored to their original values when

the subroutine returns. This subroutine could be called as follows:

LEA A,AO

LEA B,A1

JSR DBADD

This call would result in adding double-precision value A to double

precision value B.

I mentioned that there is a reason to make sure the Z condition is set

in the CCR before starting a multiple precision calculation. The reason is

that we may want to test the result of our calculation to determine if it is

zero. Since this is a multiple precision calculation, what we really require

is that all the intermediate results are zero, as well as the final addition or

subtraction. ADDX, SUBX, and NEGX all have an interesting property

when it comes to the Z condition: the Z condition is cleared if a result is

non-zero, and unchanged otherwise. This means that once it is cleared it

will remain cleared. This indicates a non-zero result for our calculation.

A BEQ or BNE instruction can then be used to conditionally branch,

depending on the state of the Z bit.

The NEGX instruction is used to negate (subtract from zero) a

multiple precision value. The following instructions will negate the

double-precision longword in location COUNT:

MOVE.W #4,CCR

NEGX.L COUNT+4

NEGX.L COUNT

Notice that the Z bit must be set prior to execution of the first NEGX.

Address register indirect with predecrement addressing can also be used:

LEA COUNT+8/AO

MOVE.W #4/CCR

NEGX.L -(AO)

Multiplication and Division

The 68000 has two multiply and two divide instructions. One pair of

multiply and divide instructions is used for unsigned values and the other

1 2 ... 154 155 156 157 158 159 160 161 162 163 164 ... 255 256

No comments

Motorola M68000 User's Guide Page 159