HCS12 Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 78

chk_end dec lp_cnt

bne again ; have we tested all 16 bits yet?

forever bra forever

end

▲
Sometimes we need to shift a number larger than 16 bits. However, the HCS12 does not
have an instruction that does this. Suppose the number has k bytes and the most significant byte
is located at loc . The remaining k 2 1 bytes are located at loc 1 1, loc 1 2, . . . , loc 1 k 2 1, as
shown in Figure 2.16.

loc loc + 1 loc + k - 1

...

msb lsb

Figure 2.16 ■ k bytes to be shifted

The logical shift-one-bit-to-the-right operation is shown in Figure 2.17.

loc loc + 1 loc + k - 1

0

msb lsb

Figure 2.17 ■ Shift-one-bit-to-the-right operation

As shown in Figure 2.17,
• Bit 7 of each byte will receive bit 0 of the byte on its immediate left with the
exception of the most significant byte, which will receive a 0.
• Each byte will be shifted to the right by one bit. Bit 0 of the least significant byte
will be shifted out and lost.
The operation can therefore be implemented as follows:
Step 1
Shift the byte at loc to the right one place (using the lsr <opr> instruction).
Step 2
Rotate the byte at loc 1 1 to the right one place (using the ror <opr> instruction).
Step 3
Repeat step 2 for the remaining bytes.
By repeating this procedure, the given k -byte number can be shifted to the right as many bits as
desired. The operation to shift a multibyte number to the left should start from the least signifi -
cant byte and rotate the remaining bytes toward the most signifi cant byte.