Help needed on value translation of dissected bits

["Jarolin, Robert" <Robert.Jarolin () trueposition com>]

Normally with the translation of specific bits to a decimal value is
easy using the definition in the hf structure, but I am trying to find a
way to properly display 1 or more bytes that have disjoint bits
associated to a specific value.

Let me give an example:
Assume that you have 2 bytes to dissect that are formatted as follows:
- There are 2 values, A and B that are encoded into 2 Bytes
- Value A has a maximum size of 127 and is encoded using the lower 7
bits of Byte 1
- Value B has a maximum size of 511 and is encoded using all 8 bits in
Byte 2 along with bit 8 in Byte 1 representing the MSB for Value B

Normally, I would define the dissection in the hf structure as follows:
{ &hf_myproto_B,
  { "B Value", "myproto.b",
       FT_UINT16, BASE_DEC, NULL, 0x80ff, NULL, HFILL
  }
}

In this case, it will mask of the bits correctly, but it will translate
it to decimal as if there the extra bits that are masked off still
exist, but are a value of 0
So, for instance, the following encoding:
 1000 0000
 0000 0000

Would be translated as follows:
A = 0
B = 32768

Instead of what I want:
A = 0
B = 256

Any way that I can do it?  If not, can I somehow use "proto_add_text" or
something similar to make the addition to the tree look similar to how
it would have looked with the bits that are masked off being shown?

Thanks for any help.


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