Arithmetic, VHDL and CPUs


Put simply, VHDL provides support for a number of arithmetic operators;
but 
it can appear confusing at first;
which libraries,
which types,
which tools
it says nothing about how the operations might synthesise;

Historically, synthesis of these operators depended on libraries supplied by the tool vendor.
Evolved into  
ieee.std_logic_unsigned
ieee.std_logic_signed
ieee.std_logic_arith*
*not to be confused with the Synopsys package of the same name!