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

[rsocor01]

Thanks for the feedback.

I still think there are issues with this approach.
I ran a test and printed the output.
I ran a For/Next loop from 0 to 255.
e.g.
For i = 0 to 255
Lookdown2 i, <= [%00000000,%00000001,%00000010,%00000100,%00001000, %00010000,%00100000,%01000000,%10000000,%11111111], j
Next i

The output resulted in 247 wrong values out of 256.

I am attaching the output here.

Again, perhaps I'm wrong and missing something?
Attachment 7724

Yes, you are right. We should always use truth tables just to make sure it is correct. We were just giving Matias a general idea. Try this now,

For i = 0 to 255
Lookdown2 i, <= [%00000000,%00000001,%00000011,%00000111,%00001111, %00011111,%00111111,%01111111,%11111111], j
Next i

[Tabsoft]

Thanks. That works great.

To find the port pin state using this, you would just need to decrement the return value by 1 if the value is >0 ( same for NCD).

When I get some time I will run this against NCD and check the instruction cycles to see which is quicker.

Thanks again.

[towlerg]

Not wishing to be rude, but how can 255 iterations be quicker than NCD or the in-line assembler equivalent?

George

[rsocor01]

Not wishing to be rude, but how can 255 iterations be quicker than NCD or the in-line assembler equivalent?

George

The 255 iterations is just to test the LOOKDOWN2 command line.

[Archangel]

As I see it, . . . . you only need 8 iterations with lookdown and using < or > as it jumps to the first "iteration" that tests as true.

[Tabsoft]

For those who are curios as to the testing results of NCD vs. Lookdown2 in the context of this discussion.
I ran the following code in the MPLAB Simulator.

NCDSUB:
    for i = 0 to 255
	mystatus = i
	''Start MPLAB SIM Stopwatch
    	mybyte = NCD mystatus
    	''Stop MPLAB SIM Stopwatch
	next i
    lcdout $FE,1,"NCD: ", idec mybyte
    pause 500
    
    return
    
LOOKSUB:

    for i = 0 to 255
	mystatus = i
	''Start MPLAB SIM Stopwatch
    	lookdown2 mystatus, <= [%00000000, %00000001, %00000011, %00000111, %00001111, %00011111, %00111111, %01111111, %11111111], mybyte
    	''Stop MPLAB SIM Stopwatch
    next i
        
    lcdout,1, "Lookdown: ", idec mybyte
    pause 500

    return

and here are the results.

[Archangel]

Lookdown2 makes 2x code over lookup, have you tested to confirm? Thanks