Quickest way to get highest set bit on port in to a variable?

[HenrikOlsson]

This is what the NCD routine looks like in the pbppic18.lib file when stripping out the non runtime stuff:

;****************************************************************
;* NCD        : Priority encode                                 *
;*                                                              *
;* Input      : R0 = 16 bit value                               *
;* Output     : W = bit number                                  *
;*                                                              *
;* Notes      :                                                 *
;****************************************************************

NCD     clrf    R0 + 1
NCDL    movwf   R0
        movlw   17              ; Set result
ncdloop addlw   -1              ; Count down result - sets C so loop will end
        rlcf    R0, F           ; Shift upper bit to carry
        rlcf    R0 + 1, F
        bnc     ncdloop         ; If carry set then done
        goto    DUNN

I'll say what I usually say which is that I pretty much suck at assembly language stuff but that looks pretty tight to me. Obviously the execution time will vary depending on how many times it has to iterate thru the loop before it finds a bit that is set. Then there are helper macros which puts the byte/Word in question into the correct register (R0) and so on, if interested they are in the pbppic18.mac file.

Is there a faster way? Perhaps but I'd give NCD a try and get some performance data on that first - otherwise you don't know what to beat (or if you even need to beet it) - IMHO of course.

/Henrik.

[rsocor01]

Like Archangel mentioned above, you should also try the LOOKUP command. This command takes only a few cycles to execute.

[towlerg]

Henric, never the less, not as quick as 8 rotates and 8 jump on carry in-line. Worst case, with no bits set in the target thats only 16 cycles.

rscor01, I didn't realize LOOKUP would do that. How would that go?

George

[rsocor01]

rscor01, I didn't realize LOOKUP would do that. How would that go?

Nevermind. You would need too many entries in the LOOKUP constant fields. That would be 64 entries to be precise since your variable PORT has 6 bits. It will be something like this

LOOKUP MyPort, [0,1,2,2,3,3,3,3,4,........], MyVar

Still, it might be faster than NCD, but I don't know.

[Archangel]

Henric, never the less, not as quick as 8 rotates and 8 jump on carry in-line. Worst case, with no bits set in the target thats only 16 cycles.

rscor01, I didn't realize LOOKUP would do that. How would that go?

George

lookdown2 index, >= [%00000000,%00000001,%00000010],temp

[rsocor01]

lookdown2 index, >= [%00000000,%00000001,%00000010],temp

Nice! Very clever indeed . I have never used this command before, but now I can see it's potential.

[Tabsoft]

I may be wrong but I don't think this will work.
LOOKDOWN2 Search,{Test}[Value{,Value...}],Var

Lookdown2 compares the value of "Search" with the values in the list from left to right (index values 0 up to 255) with the "Test" comparison.

So if "Search" = 5 and "Test" = "=>" then using your values for the Value in the list, the logic tests should look like this.
(Values: 0, 1, 2, 4, 8, 16, 32, 64, 128) bits 0-7
Test 1: is 5 => 0? : Yes, (0 will be stored in Var)
The testing would stop right here for every value of "Search" (0-255) for bytes (0-65535) for words because the "Search" value will always be greater than or equal 0 (unless you're using Long variables)

Maybe I'm wrong?

As I see it, to use Lookdown2 you would still need to enter all the values from 0 to 255 inclusive into the list.
And "Test" would need to be set to "="
Lookdown2 supports up to 85 values in the list or up to 256 when using a PIC18.

[rsocor01]

I may be wrong but I don't think this will work.
LOOKDOWN2 Search,{Test}[Value{,Value...}],Var

Lookdown2 compares the value of Search with the values in the list from left to right (index values 0 up to 255) with the "Test" comparison.

So if Search = 5 and Test = "=>" then using your values for the Value in the list, the logic tests should look like this.
(Values: 0, 1, 2, 4, 8, 16, 32, 64, 128)
Test 1: is 5 => 0? : Yes, (0 will be stored in Var)
The testing would stop right here for every value of Search (0-255) for bytes (0-65535) for words.

Maybe I'm wrong?

As I see it, to use Lookdown2 you would still need to enter all the values from 0 to 255 inclusive into the list.
And
Lookdown2 supports up to 85 values in the list or up to 256 when using a PIC18.

No, I don't think that you are wrong. I think there is a minor change that needs to be done to Dave's code,

lookdown2 index, <= [%00000000,%00000001,%00000010,%00000100,%00001000,%00010000,%00100000,%01000000,%10000000,255],temp