Motorola M68000 User's Guide download pdf (Page 152)

158 Assembly Language Programming for the 68000 Family

<bitop> = BTSTr BSET, BCLR, BCBG

The effective address can be a data register or memory location, but

not an address register. For each of these instructions, you specify the

number of the particular bit that you want to manipulate. The bits are

numbered starting with the low-order bit as bit 0. The bit number may be

placed in a register for the first form of the instruction, or specified as an

immediate value for the second form of the instruction. If the effective

address is a data register, then the instruction operates on any of the 32

bits of the data register. If the effective address is a memory location,

then the instruction operates only on the byte at the specified address. In

other words, the range of the bit number must be between 0 and 7.

BSET and BCLR will set the specified bit to a 1 or a 0 respectively.

BCHG will complement the specified bit—in other words, a 1 becomes

a 0, and vice versa. BTST will test the specified bit to determine if it is

a 1 or a 0. It does not change it. The Z-condition bit is set accordingly.

If the bit is zero, then Z will be set. It doesn’t matter what the values of

the other bits are in the data item.

Exercises

1. What is NOT 55AA16?

2. What is AAAA16 or 555516?

3. Write a logical instruction that will clear register DO.

4. Write a logical instruction that will ensure that the high-order three

bits of register DO are ones.

5. Write a logical instruction that will ensure that the low-order four bits

of register DO are zeros.

6. Write a logical instruction that will set the bits of register Dl such that

a bit is set to one if it differs from the corresponding bit in register

DO.

7. Give the bit number of the high-order bit for

a) a byte

b) a word

c) a longword

8. Where does the bit go that is shifted off the end in a left or right shift?

9. What is the difference between a logical shift right and an arithmetic

right?

10. Is there a difference between the ASL and LSL instructions?

11. What register can be used to specify a shift count?

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Motorola M68000 User's Guide Page 152