Erlang is dynamically typed. That is, types will be checked at runtime and if a type error occurs an exception is thrown. The compiler does not check the types at compile time, unlike in a statically typed language like C or Java where you can get a type error during compilation.

These aspects of the Erlang type system, strongly statically typed with an order on the types puts some constraints on the implementation of the language. In order to be able to check and compare types at runtime each Erlang term has to carry its type with it. This is solved by tagging the terms.

In the memory representation of an Erlang term a few bits are reserved for a type tag. For performance reasons the terms are divided into immediate and boxed terms. An immediate term can fit into a machine word, that is, in a register or on a stack slot. A boxed term consists of two parts: a tagged pointer and a number of words stored on the process heap. The boxes stored on the heap have a header and a body, unless it is a list. Currently ERTS uses a staged tag scheme, the history and reasoning behind the this scheme is explained in a technical report from the HiPE group. (See http://www.it.uu.se/research/publications/reports/2000-029/)

The tagging scheme is implemented in erl_term.h. The basic idea is to use the least significant bits for tags. Since most modern CPU architectures aligns 32- and 64-bit words, there are at least two bits that are “unused” for pointers. These bits can be used as tags instead. Unfortunately those two bits are not enough for all the types in Erlang, more bits are therefore used as needed.

The current (R15-R16) tagging scheme looks like this:

 aaaaaaaaaaaaaaaaaaaaaaaaaatttt00 HEADER (see below)
 pppppppppppppppppppppppppppppp01 CONS
 pppppppppppppppppppppppppppppp10 BOXED (pointer to header)
 iiiiiiiiiiiiiiiiiiiiiiiiiiii0011 PID
 iiiiiiiiiiiiiiiiiiiiiiiiiiii0111 PORT
 iiiiiiiiiiiiiiiiiiiiiiiiii001011 ATOM
 iiiiiiiiiiiiiiiiiiiiiiiiii011011 CATCH
 iiiiiiiiiiiiiiiiiiiiiiiiii111011 NIL (i always zero...)
 iiiiiiiiiiiiiiiiiiiiiiiiiiii1111 SMALL_INT

 aaaaaaaaaaaaaaaaaaaaaaaaaa000000 ARITYVAL (Tuple)
 vvvvvvvvvvvvvvvvvvvvvvvvvv000100 BINARY_AGGREGATE       |
 vvvvvvvvvvvvvvvvvvvvvvvvvv001x00 BIGNUM with sign bit   |
 vvvvvvvvvvvvvvvvvvvvvvvvvv010000 REF                    |
 vvvvvvvvvvvvvvvvvvvvvvvvvv010100 FUN                    | THINGS
 vvvvvvvvvvvvvvvvvvvvvvvvvv011000 FLONUM                 |
 vvvvvvvvvvvvvvvvvvvvvvvvvv011100 EXPOR                  |
 vvvvvvvvvvvvvvvvvvvvvvvvvv100000 REFC_BINARY |          |
 vvvvvvvvvvvvvvvvvvvvvvvvvv100100 HEAP_BINARY | BINARIES |
 vvvvvvvvvvvvvvvvvvvvvvvvvv101000 SUB_BINARY  |          |
 vvvvvvvvvvvvvvvvvvvvvvvvvv101100 Not used
 vvvvvvvvvvvvvvvvvvvvvvvvvv110000 EXTERNAL_PID  |        |
 vvvvvvvvvvvvvvvvvvvvvvvvvv110100 EXTERNAL_PORT | EXT    |
 vvvvvvvvvvvvvvvvvvvvvvvvvv111000 EXTERNAL_REF  |        | 
 vvvvvvvvvvvvvvvvvvvvvvvvvv111100 Not used

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