Motorola M68000 User's Guide download pdf (Page 115)

Subroutines 101

JSR BIGEST

Each time the JSR instruction is executed, the instructions of the subrou

tine BIGEST are executed. How does the program find these instructions?

The label BIGEST is used to tell the JSR instruction where to find the

subroutine. This can be thought of as equivalent to a simple JMP BIGEST

instruction. So far this is easy. But now consider what happens after the

steps of BIGEST are executed. How do we get back to the instruction

after the JSR? Which JSR? There may be quite a few. The answer lies

with the use of the stack and the RTS instruction.

Prior to transferring control to the subroutine, the JSR instruction

places the address of the instruction that immediately follows the JSR

instruction itself on the stack. This is the instruction that should be

executed when the instructions of the subroutine are completed. When

the RTS instruction is executed in the subroutine, this return address is

popped from the stack and placed in the program counter (PC). This

acts like another JMP instruction. Now control returns to the part of

the program that called the subroutine, with execution continuing at the

proper instruction. Figure 8 shows the sequence of events for a typical

subroutine call.

Figure 8 Subroutine call. END

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Motorola M68000 User's Guide Page 115