Hello Alex

All informations on 8051 ALU architecture could be found on Intel's site.
Data of other MCS51 producers are avatars of Intel ones.
I use also MHS (Matra Harris) and Philips architecture datasheets.
You must perform a sort of electronical archeology, or retroengineering,
to deduce the real implementation from known informations. Schematics
found in documentations give you only blocks. It is necessary to
complete this with all missing signals. If your goal is implementation
on a FPGA chip of a model of 8051, a comportemental description
is sufficient. Outside behaviour of a black box wich mimick exactly a true
chip is equivalent of a real one. The most acurate description of an
ALU is found in Mayer's VHDL 8051 implementation. Microcycles are
followed. You must transform this architecture to remove all not
synthetizable elements.
Good work.
J.M.