Page 97 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

74 Chapter 3 Addressing Modes

Figure 3.14. Nonreentrant Subroutine to Clear Memory

This program fails to work correctly if it is interrupted after the STAA instruction is
executed, and before the RTS instruction is executed and the interrupt handler calls this
subroutine. The second call to this subroutine will wipe out the counter at location $822
because the second call will also use this same location and will leave it cleared when the
first call to this subroutine resumes execution. The first call to this subroutine will not
work the same way if the interrupt occurs as it does if the interrupt doesn't occur.
However, the subroutine in Figure 3.15 will work correctly if it is similarly interrupted.
The key idea behind both recursion and reentrancy is to keep data on the stack. The
stack provides new storage locations for each instantiation of the subroutine to keep its
variables separate from the variables of other instantiations, so they aren't destroyed,
Note that the decrement instruction accesses the counter on the stack without
pulling the byte. If three 1-byte items were pushed on the stack, the instruction LDAA
2, SP will read into accumulator A the first byte pushed without removing it from the
stack. In general, items can be pushed on the stack at the beginning of a procedure and
pulled off at the end of the procedure. Within the procedure the items can be read and
written using offsets from the stack pointer.

Figure 3.15. Reentrant Subroutine to Clear Memory