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;throttle_multiply.asm

;

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

;* mpy8u                                                                         *

;* Second Level Subroutine                                                       *

;*                                                                               *

;* Program from Atmel file avr200.asm                                            *

;*                                                                               *

;* Since the 25kHz pwm cycle is 64 clock cycles long, this subroutine            *

;* requires just under 1 25kHz clock cycles.                                     *

;*                                                                               *

;* A single line was added which adds 3 to Cycle_count                           *

;*                                                                               *

;* Inputs:  HILOCAL1 and B_TEMPLOCAL                                             *

;*                                                                               *

;* Returns: HILOCAL1 x B_TEMPLOCAL = B_TEMPLOCAL1:B_TEMPLOCAL                    *

;*                                                                               *

;* Changed: B_TEMPLOCAL2                                                         *

;*                                                                               *

;* Calls:   Not allowed                                                          *

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

   HILOCAL1       mc8u                 ; multiplicand

   B_TEMPLOCAL    mp8u                 ; multiplier

   B_TEMPLOCAL    m8uL                 ; result Low byte

   B_TEMPLOCAL1   m8uH                 ; result High byte

   B_TEMPLOCAL2   mcnt8u               ; loop counter

;

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

;*

;* "mpy8u" - 8x8 Bit Unsigned Multiplication

;*

;* This subroutine multiplies the two register variables mp8u and mc8u.

;* The result is placed in registers m8uH, m8uL

;*

;* Number of words   :9 + return

;* Number of cycles  :58 + return

;* Low registers used   :None

;* High registers used  :4 (mp8u,mc8u/m8uL,m8uH,mcnt8u)

;*

;* Note: Result Low byte and the multiplier share the same register.

;* This causes the multiplier to be overwritten by the result.

;*

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

;***** Subroutine Register Variables

;.def mc8u  =r16     ;multiplicand

;.def mp8u  =r17     ;multiplier

;.def m8uL  =r17     ;result Low byte

;.def m8uH  =r18     ;result High byte

;.def mcnt8u   =r19     ;loop counter

;***** Code

mpy8u:

   clr   m8uH                          ;clear result High byte

   ldi   mcnt8u,8                      ;init loop counter

   lsr   mp8u                          ;rotate multiplier

m8u_1:

   brcc  m8u_2                         ;carry set

   add   m8uH,mc8u                     ;add multiplicand to result High byte

m8u_2:

   ror   m8uH                          ;rotate right result High byte

   ror   m8uL                          ;rotate right result L byte and multiplier

   dec   mcnt8u                        ;decrement loop counter

   brne  m8u_1                         ;if not done, loop more

   ret

;