On Sep 2, 2010, at 12:35 PM, Andres Vahter wrote:
mov.w R12,R13 ; The operand "input" in
register R12
rla.w R13
add.w R12,R13 ; X1=X*2^1+X
rla.w R13
rla.w R13
add.w R12,R13 ; X2=X1*2^2+X
rla.w R13
add.w R12,R13 ; X3=X2*2^1+X
rla.w R13
add.w R12,R13 ; X4=X4*2^1+X
rla.w R13
rla.w R13
rla.w R13
add.w R12,R13 ; Final Result=X5=X4*2^3+X
It computes (((X*2 + X)*4 + X)*2 + X)*2 + X)*8 + X
= (12X) + X)*2 + X)*2 + X)*8 + X
= 26X + X)*2 + X)*8 + X
= 54X + X)*8 + X
= 440X + X
= 441*X
Which is what it said at the top; multiplies 41 * 441.
It looks to me like a pretty standard multiplication algorithm, only
since one argument is a know constant, you get to leave out the steps
that would involve adding 0.
I've seen code generators for other microcontrollers that claim to
generate the optimal sequence for multiplying a register by any
constant. It should be possible to do for MSP430 too. Perhaps the C
compiler already does so? (probably not; I've also seen complaints
that gcc does a poor job of multiplying by constants.)
BillW
