Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 184)

170 Assembly Language Programming for the 68000 Family

set operation can be performed in an indivisible manner. In other words,

only one CPU at a time can perform a TAS in a multiple CPU system.

No two TAS’s can overlap.

This instruction is normally used for locking operations. If a processor

wants to place a lock on some item, it can execute the following code:

Ls TAS LOCK

BNE L

It might look as if this loop would go on forever. This is not the case if the

lock was zero to begin with. Remember, the test operation is performed

before the set takes place. After a processor is finished with the locked

item, it must reset the lock and therefore allow other CPU’s to access the

data, one at a time. This unlock operation is performed by:

CLR.B LOCK

The byte variable LOCK must be initially cleared or no CPU will ever

be able to access the item.

The particular items protected by locks can vary all over the place.

A lock could be placed on a record of a file, an area of memory,

and so on. Since the use of a TAS involves a busy wait, an operating

system normally uses this instruction to implement higher level methods

of ensuring mutually exclusive access. Consult a good operating systems

text for these details.

Exercises

1. How large is the status register?

2. In addition to the CCR bits, what extra bits are in the SR?

3. Can the status register be accessed when in user mode?

4. What is the range of values for the interrupt mask?

5. Write an instruction to clear the carry bit in the CCR. All other bits

should remain the same.

6. What is the purpose of the RTR instruction?

7. Write an instruction to set the byte at location MINUS if the result of

a calculation is negative.

8. Assume your program is in supervisor mode. Write the instructions

necessary to initialize the USP to location USTACK.

9. What is an exception?

10. Where are the exception vectors located?

11. What is the difference between an RTE instruction and an RTS

instruction?

12. What happens when you try to access a word or longword at an odd

memory address?

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Motorola M68000 Betriebsanweisung Seite 184