Motorola M68000 User's Guide download pdf (Page 150)

156 Assembly Language Programming for the 68000 Family

we always went through the loop 32 times. Also note that this routine

does not check for overflow. The 68000 has a multiply instruction, so this

routine should never be required.

Rotates

Rotates are very similar to shifts. We can rotate to the left or right. The

difference is that rather than shift in zeros, as for left shifts and logical

right shifts, the bit that would normally fall off the end is shifted back

into the longword, word, or byte at the opposite end. The ROL (rotate

left) and ROR (rotate right) instructions work as follows:

ROL

A corresponding pair or rotates, ROXL and ROXR, work in the same

way as ROL and ROR except that the extend bit is included as an extra

bit to be included in the rotate. Here is how they work:

ROXL

ROXR

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Motorola M68000 User's Guide Page 150