HCS12 Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 48

Start

A ← [$1001]

A ← [A] + [$1002]

A ← [A] − [$1005]

$1010 ← [A]

Stop

Figure 2.2 ■ Logic flow of program 2.4

Example 2.4
▼

Write a program to subtract the contents of the memory location at $1005 from the sum of
the memory locations at $1000 and $1002 and store the result at the memory location $1010.
Solution: The logic flow of this program is illustrated in Figure 2.2. The assembly program is as
follows:

org $1500 ; starting address of the program

ldaa $1000 ; A ⇐ [$1000]

adda $1002 ; A ⇐ [A] 1 [$1002]

suba $1005 ; A ⇐ [A] 2 [$1005]

staa $1010 ; $1010 ⇐ [A]

end

▲

Example 2.5
▼ Write a program to subtract 5 from four memory locations at $1000, $1001, $1002, and
$1003.
Solution: In the HCS12, a memory location cannot be the destination of an ADD or SUB in-
struction. Therefore, three steps must be followed to add or subtract a number to or from a
memory location.
Step 1
Load the memory contents into an accumulator.
Step 2
Add (or subtract) the number to (from) the accumulator.