ALU_Op is the basic instructions of Z80 coded here. T80_ALU.vhdl is a slave, a service exposed to T80_MCode.vhdl throw T80.vhdl
[[http://www.z80.info/decoding.htm § Disassembly tables]] shall make a cool ALU_Op quick reference card, doesn't it ?
===== Z80 architecture : T80_MCode.vhdl =====
First 5 pages, and last 2 pages of source codes to read. Others pages are "always the same" architectually speaking.