Hashrate calculation and BFL asic stats.

[brontosaurus]

Sorry if this has been discussed before, but I'm buggered if I can find any threads that give me the answer.

Lots of pages refer to the 'double' sha256 hash in the bitcoin hashing algorithm. However, there seems to be a concensus that most miners only perform the inner hash once, then use the output digest from it again and again; indeed in the Getwork wiki, it actually says:

"running a full SHA256 round is generally inadvisable. Most miners will precompute the SHA256 "midstate" from the first 512-bit chunk of data, and only repeat the 2nd SHA256 round with the final 512-bit chunk (which contains the nonce). "

This may be out of date, and I'd appreciate some advice as to whether it is true or not.

Question is:

When an asic supplier quotes a hashrate, is it based on a 'true' double hash (taking twice the time) or an 'optimised' one as described above? It makes a big difference, I've only so far seem one supplier - Bitfury - mentioning a double hash. I recently came across the stats of the BFl chip - what a monster! 7.5 x 7.5mm2. That gives a useable chip area of 6.9 x 6.9 or 47.6 mm2. On TSMC's 65nm process, raw gate density is about 850k gates/mm2, so this device probably has a utilisation of 80 - 85% or 32 - 34 million gates.

BFL say there are 16 unrolled pipelines on the chip. If they are 'single' length, ie 64 sections, then each section is roughly 33*10^6/(16 *64), ie 32000 gates (or 16000 if a double length pipeline is used). Yes, I know there is control logic and so on, but it shouldn't take more than 500k gates.

Either way, they seem awfully large, by a factor of 10 or 5. Possibly due to an FPGA conversion?

Anyway, this is bugging me. Can anyone help?

Thanks.

[jhansen858]

You always have to do the full calculation.  If you could just calculate part of it and then do the full 2nd hash, then that makes sha256 completely pointless.

[brontosaurus]

Hi, thanks for the reply. The Getwork wiki stuff did'nt seem to make sense, but then I can't find a central resource for the detailed mining algorithm.

Got any ideas?

[k9quaint]

Hi, thanks for the reply. The Getwork wiki stuff did'nt seem to make sense, but then I can't find a central resource for the detailed mining algorithm.

Got any ideas?

Just look at the code.

[jhansen858]

The simplest miner would work like this:

     1. Request a new [Getwork][1] from a given pool/bitcoind with specified credentials
     2. For nonce=0;nonce<0xFFFFFFFF;nonce++
     - Set Getwork Data nonce
     - SHA-256 hash the Getwork Data ([block header][2])
     - Check if hash result is smaller than Getwork Target, if so, submit a share
     - If more than 1 second passed since you received Getwork, stop the loop
     3. GOTO: 1


Also Found this actual code.

http://pastebin.com/bW3fQA2a