** Data types:
**
** Eterm: A tagged erlang term (possibly 64 bits)
** BeamInstr: A beam code instruction unit, possibly larger than Eterm, not smaller.
** UInt:  An unsigned integer exactly as large as an Eterm.
** SInt:  A signed integer exactly as large as an eterm and therefor large
**        enough to hold the return value of the signed_val() macro.
** UWord: An unsigned integer at least as large as a void * and also as large
**          or larger than an Eterm
** SWord: A signed integer at least as large as a void * and also as large
**          or larger than an Eterm
** Uint32: An unsigned integer of 32 bits exactly
** Sint32: A signed integer of 32 bits exactly
** Uint16: An unsigned integer of 16 bits exactly
** Sint16: A signed integer of 16 bits exactly.

See erts/emulator/beam/big.c for the conversion between types. For example uint_to_big(Uint x, Eterm *y). term_to_Uint(Eterm term, Uint *up).
An Eterm can contain any erlang term like atoms, integers, etc.

== BIFs ==

The bifs are summed up in the bif.tab file. For example:
bif 'erl.lang':exit/1
ebif_exit_1
bif erlang:exit/2

This means the exit bif is mapped to the exit_1 method in the bif.c file.
The bif.c file holds the bif implementations like: 

BIF_RETTYPE spawn_3(BIF_ALIST_3)
{...

BIF_ALIST_3 means you have BIF_P, BIF_ARG1 to 3 and 
The arguments BIF_ARG1 are Eterms, so you have to check them with 

is_number(BIF_ARG_1), 
is_atom, 
is_tuple, 
is_list
etc

As defined in bif.h:

#define BIF_RETTYPE Eterm

#define BIF_P A__p

#define BIF_ALIST_0 Process* A__p
#define BIF_ALIST_1 Process* A__p, Eterm A_1
#define BIF_ALIST_2 Process* A__p, Eterm A_1, Eterm A_2
#define BIF_ALIST_3 Process* A__p, Eterm A_1, Eterm A_2, Eterm A_3

#define BIF_ARG_1  A_1
#define BIF_ARG_2  A_2
#define BIF_ARG_3  A_3

BIF_RETTYPE process_flag_2(BIF_ALIST_2)
{
   Eterm old_value;
  ...

   else if (BIF_ARG_1 == am_priority) {
       erts_smp_proc_lock(BIF_P, ERTS_PROC_LOCK_STATUS);
       old_value = erts_set_process_priority(BIF_P, BIF_ARG_2);
       erts_smp_proc_unlock(BIF_P, ERTS_PROC_LOCK_STATUS);
       if (old_value == THE_NON_VALUE)
	   goto error;
       BIF_RET(old_value);
   }