multiplication vs shifting

[tenaja]

PBP uses a subroutine for performing the multiplication. (For all 16F * calculations, and for 18F * calcs involving more than 8 bits.) If you perform the calculation multiple times, an individual line may be shorter than shifting. However, on just a one-time multiplication, the PBP subroutine will add significant code.

[Ioannis]

No other code, just Variable declaration and END,for a F877:

For BYTE variable:

x=x*25            '40 bytes
x=x<<4+x<<3+x     '43 bytes

For WORD variable:

x=x*25             '43 bytes
x=x<<4+x<<3+x      '49 bytes
x=x<<25            '27 bytes!!!

Ioannis

[Kamikaze47]

whats the point of the last one? shift a word variable 25 times either way and you will just get 0.

i was multiplying a word var and saving into a long var

[Ioannis]

I know 25 shift will not give the result you want.

Just tested to see how much less program space and how faster a shift will be. Useless for you but quite impressive!

Which PIC are you using? Is there any chance to use one with hardware multiplier?

Ioannis

[Darrel Taylor]

To me, the time it takes to execute is more important than the size of the code.
And the fastest way will usually <strike>be the smallest too</strike> not be the smallest.

On a 16F @ 4Mhz with WORD variables ...
<table cellpadding=6 border=1><tr><td align=center>Formula</td><td align=center>Execution Time</td><td align=center>Words per<br>Instance</td><td align=center>Words added<br>to Library</td></tr><tr><td>x = x*25</td><td align=center>230 - 250uS<br>depends on X value</td><td align=center>12</td><td align=center>27</td></tr><tr><td>x = x<<4 + x<<3 + x</td><td align=center>110uS</td><td align=center>34</td><td align=center>13</td></tr><tr><td> T = X << 1
T = T << 1
T = T << 1
X = T << 1 + T + X </td><td align=center>31uS</td></td><td align=center>30</td><td align=center>13<br>adds SHIFTL to library<br>but doesn't use it.</td></tr></table>

I wonder if ASM will help

[Kamikaze47]

Nice work DT.

It's amusing that doing a single shift twice takes less time than a double shift.

I have run into a problem though. Does anyone know why this gives me an incorrect result?

X VAR WORD
T VAR LONG
Y VAR LONG

X=8100

T = X << 1
T = T << 1
T = T << 1
Y = T << 1 + T + X

LCDOUT $FE,$80,"T=",DEC T
LCDOUT $FE,$C0,"Y=",DEC Y

T is 64800 (as expected), but Y is 2516719364 ?!

I'm guessing that its something about long variables that I am missing.

[Darrel Taylor]

Nice find Kamikaze!

There's a small bug in the pbppi18l.mac file.
The SHIFTL?NCN macro should be changed as follows. (3's in red)

SHIFTL?NCN macro Nin, Cin, Nout
    if ((Cin) == 1)
        bcf     STATUS, C
      if ((Nout) == (Nin))
        CHK?RP  Nout
        rlcf    Nout, F
        rlcf    (Nout) + 1, F
        rlcf    (Nout) + 2, F
        rlcf    (Nout) + 3, F
      else
        CHK?RP  Nin
        rlcf    Nin, W
        MOVE?AB Nout
        CHK?RP  Nin
        rlcf    (Nin) + 1, W
        MOVE?AB (Nout) + 1
        CHK?RP  Nin
        rlcf    (Nin) + 2, W
        MOVE?AB (Nout) + 2
        CHK?RP  Nin
        rlcf    (Nin) + 3, W     ; was 2
        MOVE?AB (Nout) + 3
      endif
    else
        MOVE?NN Nin, R0
        movlw   Cin
        L?CALL  SHIFTL
        MOVE?ANN R0, Nout
    endif
    endm
SHIFTL_USED = 1
  endmod

I'll let the guys know.
<br>