Motorola M68000 User's Guide download pdf (Page 116)

102 Assembly Language Programming for the 68000 Family

The BSR instruction operates identically to the JSR instruction except

that it has the same limited addressing range as the conditional branch

instructions discussed in Chapter 5. The target address of the BSR instruc

tion must be a label. The JSR instruction allows the use of a wider range

of addressing modes. For example, we can place the address of the sub

routine in an address register and then use register indirect addressing.

The instructions

LEA SUBR,AO

JSR (AO)

have the same effect as the single instruction

JSR SUBR

You should use the BSR instruction whenever possible, since it takes up

less space in memory for subroutine calls that are relatively close to the

address of the BSR itself.

Passing Param eters

An assembly language subroutine is normally free to access any vari

ables within a program. You might want a subroutine to use specific

variables of your program as data for its operation. Some variables might

only be examined and not modified by the subroutine. Others might be

modified. An important point is that if a subroutine specifically refer

ences variables other than ones it uses for its own internal purposes, the

programmer is stuck with always using these variables to pass data to

and from the subroutine. From your programming in a high-level lan

guage, you are most likely wondering if parameters or arguments can be

passed to subroutines in assembly language. The answer is yes. By allow

ing parameters, we can make a subroutine more general by not always

requiring the same specific variables to be used to pass data to or from

the subroutine.

Parameters or arguments can be passed to subroutines in a great

variety of ways. There are no standard methods as would be used by

high-level languages. The choice is up to the programmer. In fact, it is

common practice for several different methods to be used in the same

program. The simplest method is to place the parameters to be passed

to the subroutine in one or more of the registers. We did this with the

BIGEST subroutine. Under normal circumstances one or more values are

returned by the subroutine. These values can be placed in the same or

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Motorola M68000 User's Guide Page 116