Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 122)

108 Assembly Language Programming for the 68000 Family

reference. With call by reference, the address of the parameter is passed

on the stack. This allows the subroutine not only to access the parameter

in the caller’s domain, but actually to change its value—in other words,

the subroutine can have both input and output parameters. Call by value

only allows input parameters.

The PEA instruction discussed in Chapter 7 is used to pass the address

of each parameter that is to be a call by reference. Note that it is certainly

possible to have a mix of call by value and call by reference in the same

subroutine call. The only restriction is that both the caller and the callee

agree on the order and type of the parameters. Let’s say we want to

call subroutine ALPHA with the single parameter COUNT, using call by

reference. We would call ALPHA as follows:

PEA COUNT

JSR ALPHA

Furthermore, assume that subroutine ALPHA does something simple, like

adding 10 to its only parameter. Here’s how we can do it:

ALPHA: MOVEA.L 4(SP),A0 GETS ADDRESS OP COUNT IN AO

ADD.L #10,(AO) ADD 10

RTS

Keep in mind that the data object to which the parameter points can

be as simple as a single integer or as complicated as the programmer

desires—an array of records, or even a record consisting of several arrays.

Anything that can be located by a single address can be passed as a

parameter using call by reference.

One problem still remains: the parameters are still on the stack when

the subroutine returns. The subroutine can’t pop the parameters, since

the return address is at the top of the stack. One method is to have the

caller clean up the stack. The caller has several choices as to what to do.

The caller can pop the parameters:

MOVE.L (SP)+,D0 PARM1

MOVE.L (SP)+,D0 PARM2

MOVE.L (SP)+,D0 PARM3

This technique is most useful when one or more parameters are return

values. The caller can also modify SP directly:

ADDA. L # 1 2 ,SP

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Motorola M68000 Betriebsanweisung Seite 122