questions about writing network drivers

[M.C.Shao]

hi,all
merry christmas& happy new year.
I am trying to write a network driver for my rtl8139 chip.For I didn't find an available network dirver for it under rtems.And I am refferring to dec21140's and sonic's drivers. when I read sonic's code, it seems that it use "sonic_write_register_t and sonic_read_register_t" strutcture to write/read sonice chip's register. But I didn't find where the code is implemented to acctually do the "write/read"?
Only those two structures are defined, but no real implementation code.
The second question, where is the "xxx_softc" stucture initiated? I am puzzled about this.
And the last question, what's the difference between memory access and I/O access to the chip registers? I am wondering which method to use now. 
Any suggestions are appriciated. Thank you very much. 
--
Shao Mingchao
Engineering Physics Department
Tsinghua University
Beijing,100084,China
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