Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 193)

The 68010 179

MOVE.L #$4000,DO

M0VEC DO,VBR

The VBR can be used for many purposes. The implementation of

a virtual machine is made much easier by the use of the VBR. Each

different user of the virtual machine must think that he/she has his/her

own set of exception vectors. These should appear to be located at

address zero. However, when the program runs, these vectors really can’t

be used since the virtual machine operating system requires complete

control. The virtual machine operating can use the VBR to set up an

alternate address for the actual vectors. If an exception occurs that the

virtual machine operating systems wants to pass to the user it can re

vector through the address found in the vectors based at zero. The VBR

can also be used to make a debugger appear more transparent to the

program it is debugging.

RTD and Loop Mode

Two other minor features that exist for the 68010 are the RTD in

struction and loop mode. The RTD instruction works exactly like the

RTS instruction except that a constant is added to the stack pointer af

ter the return address is fetched. This constant value is normally used

to clear the stack of any parameters passed to the subroutine. As dis

cussed in Chapter 8, this is normally the responsibility of the caller. To

see how this instruction works, let’s say that subroutine MYSUB is called

with three longword arguments. The following call would be used:

MOVE.L ARG3,-(SP)

MOVE.L ARG2,-(SP)

MOVE.L ARG1,-(SP)

JSR MYSUB

Normally the caller would use an ADDA.L #12,SP following the JSR to

clean up the stack. With the 68010, the following instructions can be used

such that the subroutine can clean up the stack upon its return:

MYSUB: .

RTD 12

Loop mode is an enhancement to the operating speed of the 68010 that

the user doesn’t even have to be aware of. To understand the advantage

of loop mode, we will have to take a closer look at the factors that govern

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Motorola M68000 Betriebsanweisung Seite 193