Advanced Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 168

Step 2
temp

← sar OR mask

Step 3
If ((temp ** 2) > num) sar

← temp;

Step 4
mask

← mask srl 1 (shift right logically one place);

Step 5
lpcnt

← lpcnt 2 1

Step 6
If (lpcnt 55 0) stop; else go to step 2.

Start

SAR[n − 1, . . . , 0] 0

←

i ← n −1

SAR[i] 1

←

Yes

i ← i −1

SAR * SAR > num?

SAR[i] ← 0

No

No

i = 0?

Yes

Stop

Figure 4.12 ■ Successive-approximation method for finding square root

Example 4.8
▼ Write a subroutine to implement the square root algorithm. This subroutine must be
able to find the square root of a 32-bit unsigned integer. The parameter q (for which we want
to find the square root) is pushed into the stack and the square root is returned in double
accumulator D.
Solution: Four local variables are needed for implementing the square root algorithm.
• mask . set a bit in the SAR register to be 1
• sar . successive approximation register
• temp . the value of ( mask OR sar )
• lpcnt . number of loop iterations to be performed
The stack frame of this algorithm is shown in Figure 4.13.