Advanced Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 148

ldab #40

pshb
ldx

#0

pshx

Solution: The first instruction initializes the stack pointer to $1500. The second and the third
instructions together push the 8-bit value $20 into the stack. The fourth and fifth instruc-
tions push the 8-bit value 40 (hex $28) into the stack. The sixth and seventh instructions
push the value 0 to the top 2 bytes of the stack. The contents of the HCS12 stack are shown
in Figure 4.2.

SP (= $14FC)

0
0
$28
$20

Figure 4.2 ■ The contents of the HCS12 stack

▲
One of the main uses of the stack is saving the return address for a subroutine call. Before
the stack can be used, we need to set up the stack pointer. Since the stack can only be imple-
mented in the RAM, we must make sure not to violate this requirement. Because the stack is a
last-in-first-out data structure, it can be used to reverse a data structure.

4.4 What Is a Subroutine?

A subroutine is a sequence of instructions that can be called from many different places in a pro-
gram. A key issue in a subroutine call is to make sure that the program execution returns to the point
immediately after the subroutine call (this address is called the return address ) when the subroutine
completes its execution. This is normally achieved by saving and retrieving the return address in and
from the stack. The program flow change involved in a subroutine call is illustrated in Figure 4.3.

Caller

<call> subroutine_x

subroutine_x

<return>

Figure 4.3 ■ Program flow during a subroutine call