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Guys I have a few points that are not clear for me. 1) In the link below it says there are some fields we have to combine and hash. However when I get a work by saying getwork from a pool. What I get is in the below either. The problem is that I cannot find the fields listed in the first link, Version, hashPrevBlock etc.

https://en.bitcoin.it/wiki/Block_hashing_algorithm

Getwork result.

{   'data':'0000000271112b7a1cf8e97b8367102f7e155499bf5c7303cbcdd4360000000000000000427719b35a84bc7732edd34ff3a6278dc4ac0ec2a9544e6088bc0bd719e17a8e52db3b211902666600000000000000800000000000000000000000000000000000000000000000000000000000000000000000000000000080020000',
'hash1':'00000000000000000000000000000000000000000000000000000000000000000000008000000000000000000000000000000000000000000000000000010000',
'target':'0000000000000000000000000000000000000000000000000000ffff00000000',
'midstate':'45aebb01c0fdb704ee7d2bbf02665943f50f299ff8bb252e614100065bc5c20c'
}

2) It says in the first link "Whenever Nonce overflows (which it does frequently), the extraNonce portion of the generation transaction is incremented, which changes the Merkle root." Should I put extra 32 bits for extranonce?

3) What is the reason reversing the header?

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  1. the link you posted is referring to for the full process of mining, as you would need if you're using an ASIC or some other hardware solution capable of generating 4 gigahashes or more per second. if you're using getwork, most of that information is irrelevant and confusing.

  2. see (1) above. you don't worry about extraNonce when using getwork; you can just request getwork again and get a new data string from the server.

  3. the reason for reversing the header (actually reversing each 32-bit word in the header) is that it has already been reversed (converted to little-endian) before sending it to you and you need to undo that reversal, putting it back in big-endian form.

all you need to do when using getwork is:

  • convert the data to big-endian
  • get the sha256 hash of the sha256 hash of the 80-byte header
  • if that hash is below the threshold, send your header with current nonce to the server using the appropriate getwork RPC call with data
  • keep trying again with different nonces each time (the last 4 bytes of the 80 bytes are the nonce)
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  • you mean the only thing I care about will be the last 4 bytes of the data, right? But whose last bytes, the little-endian one or the other? :)
    – Eray Tuncer
    Commented Jan 26, 2014 at 10:42
  • it really doesn't matter. typically, at least on x86 computers, people increment the little-endian number, then convert the block to big-endian for hashing. I just use random numbers so I couldn't care less about endianness; just have to make sure to put it in the right order for submission to the server.
    – jcomeau_ictx
    Commented Jan 26, 2014 at 18:06
  • remember, it's the same 4 bytes in each case; you're not reversing the 80 bytes, you're reversing every 4 bytes within the 80 bytes, so 0x12345678 becomes 0x78563412.
    – jcomeau_ictx
    Commented Jan 26, 2014 at 18:13
  • In the example of 0x12345678 you are reversing every 1 byte chunks within a 4 byte value right? 0x12345678 = 00010010-00110100-01010110-01111000. 0x78563412 = 01111000-01010110-00110100-00010010
    – Eray Tuncer
    Commented Jan 27, 2014 at 1:03
  • yes, that's the idea.
    – jcomeau_ictx
    Commented Jan 27, 2014 at 1:07

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