Inline Assembler

;***************************************************************************

;*

;* "mpy16u" - 16x16 Bit Unsigned Multiplication

;*

;* This subroutine multiplies the two 16-bit register variables 

;* mp16uH:mp16uL and mc16uH:mc16uL.

;* The result is placed in m16u3:m16u2:m16u1:m16u0.

;*  

;* Number of words  :14 + return

;* Number of cycles  :153 + return

;* Low registers used  :None

;* High registers used  :7 (mp16uL,mp16uH,mc16uL/m16u0,mc16uH/m16u1,m16u2,

;*                          m16u3,mcnt16u)  

;*

;***************************************************************************

;***** Subroutine Register Variables

.def  mc16uL  =r16    ;multiplicand low byte

.def  mc16uH  =r17    ;multiplicand high byte

.def  mp16uL  =r18    ;multiplier low byte

.def  mp16uH  =r19    ;multiplier high byte

.def  m16u0  =r18    ;result byte 0 (LSB)

.def  m16u1  =r19    ;result byte 1

.def  m16u2  =r20    ;result byte 2

.def  m16u3  =r21    ;result byte 3 (MSB)

.def  mcnt16u  =r22    ;loop counter

;***** Code

mpy16u:  clr  m16u3    ;clear 2 highest bytes of result

  clr  m16u2

  ldi  mcnt16u,16  ;init loop counter

  lsr  mp16uH

  ror  mp16uL

m16u_1:  brcc  noad8    ;if bit 0 of multiplier set

  add  m16u2,mc16uL  ;add multiplicand Low to byte 2 of res

  adc  m16u3,mc16uH  ;add multiplicand high to byte 3 of res

noad8:  ror  m16u3    ;shift right result byte 3

  ror  m16u2    ;rotate right result byte 2

  ror  m16u1    ;rotate result byte 1 and multiplier High

  ror  m16u0    ;rotate result byte 0 and multiplier Low

  dec  mcnt16u    ;decrement loop counter

  brne  m16u_1    ;if not done, loop more

  ret