Page 223 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

200 Chapter 7 Arithmetic Operations

ORG LCSAVE
*
* SUBROUTINE DIV
* DIV divides the unsigned next word on the stack into L, and pulls the next word
*
DIV: PULX ; unstack return address
LEAS - 4, SP ; room for remainder right above dividend
PSHD ; save low 16 bits of divisor
PSHY ; save high 16 bits of divisor
MOVB #32,1, -SP ; count for 32 bits
PSHX ; put back return address
CLRA
CLRB
STD REM,SP
STD REM+2,SP
*
DIV1: CLC ; divide loop
LDAA #8 ; shift remainder and divisor: shift 8 bytes
LEAK QUOT+3, SP ; pointer for bottom of quotient-remainder
DIV2: ROL 1, X-
DBNE A,DIV2
*
LDY REM, SP ; subtract from partial product
LDD REM+2 , SP ; (note: 4 extra bytes on stack)
*
SUED DVS+2,SP
XGDY
SBCB DVS+1,SP
SBCA DVS,SP
XGDY
*
BCS DIV3 ; if borrow
STD REM+2 , SP ; then put it back
STY REM,SP
INC QUOT+3, SP ; and put 1 into Isb of quotient
DIV3: DEC COUNT, SP ; counter is high byte of last operand
BNE DIV1 ; count down - 32 bits collected
*
PULX ; pull return
LEAS 9, SP ; balance stack - remove divisor
PULY
PULD ; pop quotient
DIVEXIT: JMP 0,X ; return to caller

Figure 7.18. Continued.