Page 221 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

198 Chapter 7 Arithmetic Operations

* SUBROUTINE COMPAR subtracts the next word on the stack from L, and pulls the
* next word into L. Condition codes reflect top - next
Jf!
COMPAR: SUED 4 , SP ; Compare with low 16 bits of operand
XGDY ; Get high 16-bits
SBCB 3, SP ; Compare with next 8 bits of operand
SBCA 2 , SP ; Compare with high 8 bits of operand
CPZRO: TBNE D, Ll ; If high 16 bits of result are nonzero, go to clear Z
TBEQ Y, L2 ; If low 16 bits of result are zero, leave Z alone
Ll: ANDCC #$FB ; Clear Z bit
L2: PULX ; Pull return address
PULY ; Remove operand from stack
PULD ; Remove operand from stack
JMP 0,X ; Return to caller
Figure 7.15. 32-Bit Compare Subroutine

Multiple-precision multiplication takes advantage of the EMUL instruction. The
general 32-bit by 32-bit unsigned case is illustrated in Figure 7.16 and handled by the
subroutine in Figure 7.17. A signed multiplication subroutine can be easily written that
combines Figures 7.17 and 7.3. Unsigned division is shown in Figure 7.18. It can be
modified to leave the remainder, rather than the quotient. For signed division, recall that
the sign of the remainder is the same as the sign of the dividend and that the sign of the
quotient is positive if the signs of the dividend and divisor are equal; otherwise, it is
negative. A signed divide can be implemented with the unsigned divide and sign
modification using the above rule. These subroutines can also be optimized. When a
PUSH macro precedes an ADD subroutine, the combined operation can be done simply as
seen in Figure 7.19.
We have completed our examination of multiple-precision arithmetic for both signed
and unsigned integers. With the techniques developed in this section and in the examples
of the earlier chapters, you should be prepared to handle any arithmetic calculation with
signed or unsigned long (32-bit) integers.

Figure 7.16. Multiplication of 32-Bit Numbers