Advanced Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 164

4.7 Mathematical Subroutines

Mathematical computation is one of the most important application areas for the micro-
controller. Several examples will be used to illustrate parameter passing, local variable alloca-
tion, and result returning.

4.7.1 Subroutine for Performing Multiple-Byte Division
The HCS12 provides instructions for 16-bit by 16-bit signed and unsigned divisions and
also for 32-bit by 16-bit signed and unsigned divisions. However, the HCS12 does not have
instructions for performing higher-precision divisions; say, for example, 32-bit by 32-bit division.
We will need to write subroutines to synthesize such operations.
The most popular method for performing high-precision divide operations is the repeated
subtraction method. The conceptual hardware for implementing this algorithm is shown in
Figure 4.10.

m s l s

R register Q register

C 32-bit 32-bit

Set lsb

Shift left

Write R

Controller

ALU

P register

32-bit

Figure 4.10 ■ Conceptual hardware for implementing the repeated subtraction method

Step 1
icnt

← n; R ← 0; Q ← dividend; P ← divisor.

Step 2
Shift the register pair (R, Q) 1 bit left.
Step 3
Subtract the register P from the register R; put the result back to R if the result is
nonnegative.
Step 4
If the result of step 2 is negative, then set the least significant bit of Q to 0. Otherwise, set
the least significant bit of Q to 1.
Step 5
icnt ← icnt 2 1.

Step 6
If (icnt 55 0), then stop;
else go to step 2.