Motorola M68000 User's Guide download pdf (Page 191)

The 68010 177

MOVE SR,<ea>

to examine the processor state. This instruction is not privileged on the

68000. The user could then tell that she was really in the user state.

To correct this deficiency, the 68010 makes the MOVE from SR

instruction privileged. The virtual operating system can then “fool” the

program into believing that it is in the supervisor state when it is actually

in the user state. It can simulate the execution of the instruction and return

the status register contents with the supervisor bit set. One minor change

was also made: a new instruction was added, the MOVE from CCR. Its

general form is

MOVE CCR,<ea>

The contents of the condition code register is moved to the destination

operand. This instruction is not available on the 68000 and it is not

privileged on the 68010. It allows a user-mode program on the 68010

to access the CCR without generating a privilege exception. You will

find a summary of the new and changed instructions at the end of this

chapter.

Reference Classifications

Three additional output lines are provided with both the 68000 and

the 68010 CPU. These lines are used to classify the type of memory

reference. They are interpreted as follows:

££2 E£1 ££4 Reference class

o o o N/A

0 0 1 User Data

0 1 0 User Prograir

0 1 1 N/A

1 0 0 N/A

1 0 1 Supervisor Data

1 1 0 Supervisor Program

1 1 1 Interrupt Acknowledge

N/A = not generated by a normal instruction

A computer system based on the 68000 family can use these lines to con

trol access to certain areas of memory. Separate areas can be reserved

for user and supervisor states. A further distinction can be made between

data and program references. Bus errors can be generated if incorrect

references are made. All this must be accomplished with external hard

ware.

With the 68000 there is no way to override the use of the three function

class bits. However, the 68010 provides two new registers and two new

1 2 ... 186 187 188 189 190 191 192 193 194 195 196 ... 255 256

No comments

Motorola M68000 User's Guide Page 191