Retrieving 32bit Multiply Result

[Darrel Taylor]

hehe,

DT

[Josuetas]

I just saw this link. it is of great interest to me right now...

I have this question... how do you add two of these Dwords?

how do i know if the add of two words overflows?
i.e. $1FFFE + $2E

my guess is to add the lower words $FFFE+$2E. But how do i know if there is an overflow? so that i can add one to the higher WORD? even worse..

i.e. $EFFF8 + $6FFE4

My guess again: add two lower words... if over flow add two higher words +1, else add two higher words....

I think i can manage myself once i am sure abour the overflow

Thanks,

[Darrel Taylor]

For words and bytes, you can detect an overflow by testing if the result is less than what you added.

Due to trucation of a 16-bit word ...
$FF00 + $0180 = $0080

So the result will always be less than either of the values added if an overflow occurs.

I think i can manage myself once i am sure abour the overflow

I'm sure you could. But that's just not my way ...

;Initialize your hardware first

A         VAR WORD[2]
B         VAR WORD[2]
Result    VAR WORD[2]
OVRFLOW32 VAR BIT

ASM
; ===== Load 32bit Constant into DWORD =============================
MOVE?CD macro C32in, Dout                      ; Max= 4,294,967,296
    MOVE?CB   low C32in, Dout
    MOVE?CB   low (C32in >> 8), Dout + 1
    MOVE?CB   low (C32in >> 16), Dout + 2
    MOVE?CB   low (C32in >> 24), Dout + 3
  endm
ENDASM

@  MOVE?CD  0xEFFF8, _A  ; Load 983,032 into A
@  MOVE?CD  0x6FFE4, _B  ; Load 458,724 into B

GOSUB Add32

LCDOUT  $FE,1,  "A=   ",HEX4 A[1],":",HEX4 A[0]
LCDOUT  $FE,$C0,"B=   ",HEX4 B[1],":",HEX4 B[0]
LCDOUT  $FE,$90,"Res= ",HEX4 Result[1],":",HEX4 Result[0]
LCDOUT  $FE,$D0,"OVRFLOW32 = ", BIN1 OVRFLOW32
stop

; ===== Add 2 32bit variables ======================================
Add32:
    OVRFLOW32 = 0
    Result[0] = A[0] + B[0]
    IF Result[0] < B[0] then Result[1] = 1
    Result[1] = Result[1] + A[1]
    IF Result[1] < A[1] then OVRFLOW32 = 1
    Result[1] = Result[1] + B[1]
    IF Result[1] < B[1] then OVRFLOW32 = 1
RETURN

Which displays ...

A=   000E:FFF8
B=   0006:FFE4
Res= 0015:FFDC
OVRFLOW32 = 0

HTH,

[Josuetas]

Checking for a smaller result is a simple trick, thanks,

right now my code is never suposed to exced the Max= 4,294,967,296
so i am ignoring the OVRFLOW32 = 0.

This is the code that i am using

Bigword   var word [2]
BigwordL  var BigWord(0)
BigwordH  var BigWord(1)
Bigword1  var word [2]
Bigword1L var BigWord(0)
Bigword1H var BigWord(1)


AddTwoBigWords:   'Adds Bigword1 and BigWord2 result in BigWord
   BigWordH = BigWordH + BigWord1H  ;ignoring any overflow
   BigWordL = BigWordL + BigWord1L
   if BigWordL < BigWord1L then
         BigWordH = BigWordH+1          ;ignoring any overflow
   endif
return

Would this be ok? i made some tests and it seems ok but your experience might see anything wrong here.

[Darrel Taylor]

Yup,

Looks great to me.

Nice reduction.
<br>

[Josuetas]

I was so confused about the Multiply operation that i posted here but.... i am now just using ** and * :s sorry

[Josuetas]

Well i am really proud of this, i Hope its ok.

For anyone in need of a Division of a BigWord by some number i found this problem:

Lets think of a BigWord as considered before ^

Now you might want to Divide a value stored in tBigWord1 by some other value, in this case (EXAMPLE CODE) the value is stored in factoriva.

One could use the DIV32 function as proposed by Darrel in this or other post BUT if factoriva is too small the result CAN be higher than a word size FFFF, and the the result is no good.

For example:

4:01F5 DIV32 2 the result of DIV32 is

Result = FFFF, Remainder = 01F7 !! Wrong i guess

This code is possible just thanks to Darrel`s @ PutMulResult?D MACRO

Bigword   var word [2]
BigwordL  var BigWord(0)
BigwordH  var BigWord(1)
Bigword1  var word [2]
Bigword1L var BigWord(0)
Bigword1H var BigWord(1)

MainLoop:
    lcdout $fe,1
    tbigword1H = 4         ' My Dollars Value High Word
    tbigword1L = 501      ' My Dollars value Low Word
    cents = 24              ' My cents value

    GOSUB DIVBIGWORD
    HSEROUT [#FactorIva," ",HEX4 tBigWordH,":", HEX4 tBigWordHL,10,13]

    FactorIva= FactorIva+ 1
    IF FactorIva= 101 THEN
         END
    ENDIF    
    GOTO MainLoop

'This SUB receives  tBigWord1(Dword) a value to be divided by
'FactorIva (word), Result in tBigWord, works for any value of FactorIva
'and tBigWord1 in their limits.
DIVBigWord:  
    tBigWordH= tBigWord1H / FactorIva
    tBigWord1H = tBigWord1H // FactorIva
    @ PutMulResult?D  _tBigWord1
    tBigWordL = DIV32 FactorIva
    Remainder = R2
    RETURN

This code in the serial port os The value tBigWord1 Divided for Factors 1 to 100
Seems to work.
Factor Result
2 0002:00FA

Remember to Add the @ PutMulResult?D MACRO posted by Darrel at the begining of the Code.