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

[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

[Art]

Best I can think of is 3 assembler instructions, but more time than 3 cycles of course.
Copy the port value to a byte, count how many times you shift the byte right until the byte is zero.

If those times are for a single value,
I think it would be better than either example to check each bit in a verbose fashion still in the compiler.

bytevar var byte ‘ buffer byte for port
bitnum var byte ‘ output bit number that was set

bytevar = porta


btfsc		_bytevar		,07
goto labelone
btfsc		_bytevar		,06
goto labeltwo
btfsc		_bytevar		,05

......

[Art]

Or rotate left until carry is set:

bitcount var byte
bytebuff var byte

NCD8:
bytebuff = portX
clrf	_bitcount
bcf	status	,C
findbit:
incf	_bitcount
rrf	_bytebuff
btfss	status	,C
goto findbit
return

It’s been a while for asm, and might not work straight up, but enough to get the idea.
findbit: is currently an endless loop if the port is zero, so you’d need to check for zero at the beginning.

[Art]

Ok,
In the listing he’s rotating left. I would think that would result in quicker times for the lower values.
I’m pretty sure there’s an error in my asm, as the example uses a different instruction to rotate with carry.
This is the same thing in BASIC but rotating right so the higher values should be found faster.
I’d be interested to enter a race with it.. just not sure some lines are converted the way I’d want them to.

workword var word
workbyte var workword.byte0 ‘ 1st byte
bitcarry var workword.8 ‘ 9th bit
bitcount var byte


NCD8:
bitcount = 8 ‘ mov l to w & w to f
workbyte = portX ‘ mov l to w & w to f

if workbyte = 0 then ‘ portX was zero
goto whatawaste
endif

inloop:
bitcount = bitcount - 1 ‘ decf
workword = workword << 1 ‘ 2 x rrf with carry

if bitcarry = 0 then ‘ btfss
goto inloop ‘ goto
endif
return

whatawaste:
bitcount = 0 ‘ bsf
return