Motorola M68000 User's Guide download pdf (Page 123)

Subroutines 109

A unique method that you might come across is to use the LEA instruc

tion:

LEA 12(SP),SP

This method is reported to execute somewhat faster than the ADDA.

An alternate approach is to have the subroutine clean up the stack.

This is complicated by the fact that the return address is on the stack.

You can’t just add a value to SP. You must first move the return address

to a safe place, adjust SP, and finally return the return address to the stack

in preparation for the RTS. For the above example, this can be done as

follows:

MOVEA.L (SP)+,AO GET RETURN ADDRESS

ADDA.L #12,SP ADJUST SP

MOVE.L AO,- (SP) PUT RETURN ADDRESS BACK

RTS

A subtle method would be to substitute a JMP instruction for the RTS.

MOVEA.L (SP)+,A0 GET RETURN ADDRESS

ADDA.L #12,SP ADJUST STACK

JMP (AO) RETURN

These last two methods have the disadvantage in that they use an address

Stack Fram es

There are a number of problems introduced when parameters are

passed on the stack. Unless the subroutine keeps track of the value of

SP when it was called, it will not be able to find its parameters. For

example, if a subroutine needs to push one or more values on the stack

during its operation, the value of SP will change and the parameters

will not be located at the same relative offsets that they were originally.

Also, high-level languages use the concept of

local or automatic variables.

These are special storage locations that only exist during the time interval

that the subroutine, or a subroutine that it calls, is executing. These local

variables are normally allocated from the stack. This allows such nice

features as recursive subroutine calls. Each time a subroutine is called it

will create a new and distinct set of local variables.

All of this greatly increases the difficulty in keeping track of where

things are on the stack. Each time the stack pointer SP changes, all the

1 2 ... 118 119 120 121 122 123 124 125 126 127 128 ... 255 256

No comments

Motorola M68000 User's Guide Page 123