Motorola M68000 User's Guide download pdf (Page 176)

162 Assembly Language Programming for the 68000 Family

1. Execution of the TRAP instruction will result in the CPU being

placed into supervisor mode.

2. By use of a vector number, rather than the specific address of the

operating system service, the actual address within the operating

system can change without the user reassembling his programs. The

operating system merely ensures that the proper address is placed

into the exception vector at the time the system starts up.

A special trap instruction, TRAPV, can be used to test the result of a

computation for an overflow condition. If the overflow condition is set,

the TRAPV instruction will cause an exception to vector number 7. If

we have an appropriate exception handler, this instruction is quite useful.

For example, if we perform an addition and then wish to trap on an

overflow, all we have to do is write the following instructions:

ADD.L DO fDl

TRAPV

Along the same line as the TRAPV instruction is the CHK (check

CHK <ea>,Dn

The word in register Dn is first checked to see if it is below zero. If

it is, an exception is generated. If the word in Dn is zero or greater,

it is compared with the source operand. If it is greater than the source

operand, an exception is generated. The exception vector is number 6.

If the T bit in the status register is set to 1, we enter the trace mode. In

trace mode, every instruction generates an exception after it completes,

but before the next instruction begins. This is very useful for debugging a

program. In effect, we can single-step a program. Remember that the T

bit is reset before the exception handler gets control so that the handler

itself will not generate further exceptions. When the RTE is executed,

the T bit will assume its value before the exception, and execution will

continue with the next instruction in the program. The trace exception is

vector number 9.

Setting up an exception handler is quite simple. All that is needed is to

place its address in the appropriate exception vector location. However,

this requires access to the vector area of memory. Some systems restrict

access to this area in the user mode. In this case, we must somehow get

into supervisor mode. That means an exception of some sort. Usually an

operating system has a specific service that allows entry into supervisor

mode for a short time. If the operating system is very secure, or is

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Motorola M68000 User's Guide Page 176