;* Module    : BIGMULT.SUB
;* Programmer: Tony Papadimitriou <tonyp@acm.org>
;* Language  : Motorola/Freescale/NXP HC08/9S08 Assembly Language (aspisys.com/ASM8)
;* Purpose   : General-purpose re-entrant multi-byte multiplication (example code)
;* Status    : Public Domain (released on 2009.06.26)
;* Original  : http://www.aspisys.com/code/hc08/bigmult.html
;* History   : 09.06.28: Optimized for size and speed
;*           : 09.11.05: Optimized by using X index instead of SP where possible
;*           : 09.11.18: Optimized by two cycles per loop (same size)
;*           : 09.12.05: Optimized by one byte and one cycle per carryover loop
;*           : 10.01.26: ?AnRTS also uses RTC instruction, renamed to Done@@
;*           : 10.03.26: Added #SPAUTO and ::
;*           : 10.04.06: Minor optimizations for size (same speed)
;*           : 10.05.14: Fixed ?AAX subroutine to return with RTS, not RTC
;*           : 10.08.19: Renamed routine to BigMultiply and added related macro
;*           : 11.06.05: Minor optimization (SP => SPX), saved one byte
;*           : 11.10.06: Minor optimization (two bytes), minor changes in test code
;*           : 13.05.13: New COMMON.INC and minor optimizations (ClrVar)

#ifmain ;-----------------------------------------------------------------------
                    #Uses     mcu.inc
#endif ;------------------------------------------------------------------------
                    #Error    Coded for 9S08 only

; Purpose: Multiply two numbers of (almost) any byte size
;        : (For simplicity, both operands are of the same size)
; Input  : A = size for multiplier or multiplicand (range 1..127)
;        :       +--------------+------------+---------+
;        : HX -> | multiplicand | multiplier | product |
;        :       +--------------+------------+---------+
; Output : product filled with product of multiplication
; Size   : 123 bytes

BigMultiply         macro     PointerToMultiplicandMultiplierProduct,ByteSize
                    mreq      1,2:PointerToMultiplicandMultiplierProduct,ByteSize
                    #spauto   :sp
                    lda       ~2~                 ;;A = operand size
                    ldhx      ~1~                 ;;HX -> parm tuple
                    call      ~0~



BigMultiply         proc
                    cbeqa     #0,Done@@           ;zero size won't work

                    cmpa      #127
                    bhi       Done@@              ;over 127-byte number won't work

                    psha      opsize@@
                    pshhx     .a@@                ;multiplicand pointer


                    bsr       ?AAX                ;HX -> multiplier pointer
                    pshhx     .b@@                ;multiplier pointer

                    bsr       ?AAX                ;HX -> product
                    pshhx     .ans@@              ;pointer to product (answer)
                    psha      index_a@@           ;index to current multiplicand digit
                    psha      index_b@@           ;index to current multiplier digit

                    lsla                          ;product is twice the size
ZeroAns@@           clr       ,x                  ;initialize product to zero
                    aix       #1
                    dbnza     ZeroAns@@

Loop@@              ldhx      .a@@,sp
                    lda       index_a@@,sp
                    bsr       ?DecaAAX
                    lda       ,x                  ;A = current multiplicand digit

                    ldhx      .b@@,sp
                    lda       index_b@@,sp
                    bsr       ?DecaAAX
                    ldx       ,x                  ;X = current multiplier digit

                    mul                           ;XA = sub-product
                    pshx                          ;save sub-product temporarily
                    psha                          ; (in little-endian order)


                    lda       index_a@@,spx
                    deca                          ;DECA instead of AIX #-2 below
                    add       index_b@@,spx
                    ldhx      .ans@@,sp
                    bsr       ?DecaAAX            ;DECA instead of AIX #-2 below

                    pula                          ;update product with sub-product
                    add       1,x
                    sta       1,x

                    adc       ,x
                    sta       ,x

          ;cascade possible Carry all the way to beginning of product

                    bcc       Cont@@              ;skip over Carry cascade

                    lda       index_b@@,sp
                    add       index_a@@,sp
                    sub       #2                  ;zero-based index & less the one we're at
                    beq       Cont@@              ;skip over Carry cascade

                    sec                           ;always a Carry from here
CarryOver@@         aix       #-1

                    adc       ,x
                    sta       ,x
          #if SPEED_SIZE = 1
                    bcc       Cont@@              ;done early with Carry [minor avg speed improvement for longer operands]
                    dbnza     CarryOver@@

          ;done with current multiplier digit

Cont@@              @cop                          ;in case of many iterations
                    tsx                           ;HX -> stack frame
                    dbnz      index_b@@,spx,Loop@@

                    lda       opsize@@,spx        ;restore multiplier digit counter
                    sta       index_b@@,spx

          ;done with current multiplicand digit

                    dbnz      index_a@@,spx,Loop@@

                    ais       #:ais               ;de-allocate local variables
                    pull                          ;restore caller's registers
Done@@              rtc


?DecaAAX            proc
                    deca                          ;make it zero-based
;                   bra       ?AAX


?AAX                proc
                    @aax                          ;Add A to HX (uses several bytes,
                    rts                           ;so I made it a local subroutine)

                    #sp                           ;cancel all SP offsets

;                     T E S T   C O D E

MULT_OPERAND_SIZE   def       4                   ;default operand size

          #ifz MULT_OPERAND_SIZE
                    #Error    MULT_OPERAND_SIZE must be greater than zero
          #else if MULT_OPERAND_SIZE > 127
                    #Error    Current coding supports up to 127 byte operands

multiplicand        rmb       MULT_OPERAND_SIZE   ;
multiplier          rmb       MULT_OPERAND_SIZE   ;keep together in this order
product             rmb       MULT_OPERAND_SIZE*2 ;

                    #size     MyVars


Start               proc

                    @ClrVar   MyVars

                    @move.s   #$12345600,multiplicand
                    @move.s   #$123400,multiplier

                    @BigMultiply #multiplicand,#MULT_OPERAND_SIZE
Test                bra       *                   ;product: $14B60AD780000

                    @vector   Vreset,Start

                    end       :s19crc