HCS12 Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 57

ldd $1005 ; place the dividend in D

ldx $1020 ; place the divisor in X

idivs ; perform the signed division

stx $1030 ; save the quotient

std $1032 ; save the remainder

▲

Because most arithmetic operations can be performed only on accumulators, we need to
transfer the contents of index register X to D so that further division on the quotient can be
performed. The HCS12 provides two exchange instructions in addition to the TFR instruction
for this purpose.
• The xgdx instruction exchanges the contents of accumulator D and index register X.
• The xgdy instruction exchanges the contents of accumulator D and index register Y.
The HCS12 provides instructions for performing unsigned 8-bit by 8-bit and both signed
and unsigned 16-bit by 16-bit multiplications. Since the HCS12 is a 16-bit microcontroller, we
expect that it will be used to perform complicated operations in many sophisticated applica-
tions. Performing 32-bit by 32-bit multiplication will be one of them.
Since there is no 32-bit by 32-bit multiplication instruction, we have to break a 32-bit num-
ber into two 16-bit halves and use the 16-bit by 16-bit multiply instruction to synthesize the
operation. Assume that M and N are the multiplicand and the multiplier, respectively. These
two numbers can be broken down as follows:
M 5 M H M L
N 5 N H N L
where M H and N H are the upper 16 bits and M L and N L are the lower 16 bits of M and N, respec-
tively. Four 16-bit by 16-bit multiplications are performed, and then their partial products are
added together, as shown in Figure 2.3.
The procedure is as follows:
Step 1
Allocate 8 bytes to hold the product. Assume these 8 bytes are located at P, P11, . . . , and
P17.
Step 2
Compute the partial product M L N L (in Y:D) and save it at locations P14,P17.
Step 3
Compute the partial product M H N H (in Y:D) and save it at locations P,P13.
Step 4
Compute the partial product M H N L (in Y:D) and add it to memory locations P12,P15.
The C flag may be set to 1 after this addition.
Step 5
Add the C flag to memory location P11 using the adca (or adcb) instruction. This addition
may also set the C flag to 1. So, again, add the C flag to memory location P.
Step 6
Generate the partial product M L N H (in Y:D) and add it to memory locations P12,P15.
The carry flag may be set to 1, so add the C flag to memory location P11 and then add it
to memory location P.