How to get first bit '0' position in certain register?

Dear all,

For certain register, say stream = 16'b1111010011111111; the position of first bit '0' is: 8 (from LSB); if stream = 16'b1111000100010011, then the position is 2;

what I need is to get the position.

Since it should be done in one clock cycle, combination logic in Verilog might be a better choice.

Could anybody give me some pieces of suggestion? Thanks in advance.

Newhand

This function is called "find first one" and is fairly common. You could use a for loop or if-else construct in Verilog. Try Googling on "find first one" - you may find something you can use.

Robert

Another term to search on is priority encoder.

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Two possible approaches:

always @(posedge CLOCK)begin if(~A[0]) OUT

If you can restrict your search to the first 24 bits, and if you have a Virtex-II BlockRAM available, you can feed the upper 12 bits as address to one port, and the lower 12 bits as address to the other port, and make the BlockRAM give you the 4-bit position information within the 12 address bits. You then have to combine the two port outputs. You can get the answer in one clock cycle (faster than all the suggested software solutions) at well over 100 MHz, ocnservatively.

Peter Alfke, Xilinx Applications (answering from home) Merry Christmas to everybody on this newsgroup, and an interesting and interactive 2004!