Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 175)

Exception Processing, System Control Operations, and I/O 161

PC location. Normally this is to an area of read only memory (ROM) that

contains a small bootstrap program to load the complete operating system

from disk. In some cases, the bulk of the operating system is contained in

ROM, and little or no information is obtained from disk. In either case,

the program flow ends up in your operating system, usually at command

level.

Interrupts that are generated by external devices cause exceptions in

either of two ways. The first method is one in which the particular device

provides the vector number ranging from 64 to 255. The particular vectors

used for particular devices is system-dependent. The second method

is to use the autovector feature. The autovectors are 25 through 31.

The particular vector selected is determined from the device’s interrupt

priority.

Bus error, address error, illegal instruction, zero divide, and privilege

violation are exceptions caused by errors in a program. A bus error is

generated when you try to reference a non-existent area of memory. An

address error results when you try to access a word, longword, or in

struction from an odd memory address. The illegal instruction exception

is generated if the CPU tries to execute a word bit pattern that doesn’t

correspond to any valid instruction. One very specific instruction, IL

LEGAL, always generates this exception. In a sense it is the only legal

instruction that generates an illegal instruction exception. A zero divide

exception results from a DIVU or DIVS instruction when you divide by

zero. Certain instructions are executable only in the supervisor mode. If

one of these instructions is executed in the user mode, the privilege vio

lation exception occurs.

Traps

Exception vectors numbered 32 through 47 are associated with the

TRAP instruction. Its format is:

TRAP #<vector>

where <vector> is a number from 010 through 1510. The operand value of

the TRAP instruction is added to 3210 to determine the actual exception

vector. Execution of this instruction causes an immediate exception to be

generated, with control passed to the appropriate exception handler. If

the handler subsequently executes an RTE instruction, the program will

continue with the instruction following the TRAP.

TRAPS are sometimes called software interrupts and are the method

that many operating systems use to provide user services. There are a

couple of advantages to this method:

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Motorola M68000 Betriebsanweisung Seite 175