32 bit math

[Charles Linquis]

I am writing an SNMP parser. The TRAP command requires that I show elapsed time in 'timer ticks', which are defined as 10mSec in length. I have talked my customer into accepting 1 Second updates, and I have written a 32 bit seconds counting routine (using 2-16 bit words). But now, to get timer ticks, I have to multiply this 32 bit number (actually only 25 bits will be used) by 100.

Can someone tell me how I might do this?

[BigWumpus]

Oh,

it is easy !

Shift left all bits by 2 positions (= x4),
store it to a variable X
Shift left all bits by 3 positions (= x32),
add it to the variable X
Shift left all bits by 1 position (= x64),
add it to the variable X

X is holding the result !

It can easily be done by small assembler-code.

[Charles Linquis]

Unfortunately, I'm "assembly challenged" when it comes to math. Can someone point me to a good source of information on how to accomplish what Mr. Wumpus has explained?

[BigWumpus]

IMHO:

;Here you must place your 32-bit-value
AU var byte BankA ;Upper
AH var byte BankA ;High
AM var byte BankA ;Middle
AL var byte BankA ;low

;Here the result is placed
XU var byte BankA ;Upper
XH var byte BankA ;High
XM var byte BankA ;Middle
XL var byte BankA ;low

asm
;clear X
clrf _XU
clrf _XH
clrf _XM
clrf _XL
;shift A by 1 position left
rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f
[repeat the shift-routine]
;move A to X
movff _AU,_XU
movff _AH,_XH
movff _AM,_XM
movff _AL,_XL
[repeat the shift-routine]
[repeat the shift-routine]
[repeat the shift-routine]
;add A to X
movf _AL,w
addwf _XL,f ;ignore Carry
movf _AM,w
addwfc _XM,w
movf _AH,w
addwfc _XH,w
movf _AU,w
addwfc _XU,w
[repeat the shift-routine]
[repeat the add-routine]
endasm

[Charles Linquis]

Thanks for the code, but when I input 0x64, I get 0x90 as an output.

In the meantime, I try to figure it out at this end.

[Charles Linquis]

This is what I'm using.

If input = $64
Output = $36

;-----------------------------------------------------------------

The code:


Mult32:

asm
;clear X
clrf _XU
clrf _XH
clrf _XM
clrf _XL

;shift A by 1 position left

rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f

;[repeat the shift-routine]

rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f


;move A to X

movff _AU,_XU
movff _AH,_XH
movff _AM,_XM
movff _AL,_XL

;[repeat the shift-routine]

rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f

;[repeat the shift-routine]

rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f

;[repeat the shift-routine]

rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f


;add A to X

movf _AL,w
addwf _XL,f ;ignore Carry
movf _AM,w
addwfc _XM,w
movf _AH,w
addwfc _XH,w
movf _AU,w
addwfc _XU,w

;[repeat the shift-routine]

rlncf _AL,f
rlcf _AM,f
rlcf _AH,f
rlcf _AU,f

;[repeat the add-routine]

movf _AL,w
addwf _XL,f ;ignore Carry
movf _AM,w
addwfc _XM,w
movf _AH,w
addwfc _XH,w
movf _AU,w
addwfc _XU,w



endasm