VHDL IEEE 1076-2008 Grammar

absolute_pathname

. partial_pathname

abstract_literal

decimal_literal
| based_literal

access_type_definition

access subtype_indication

actual_designator

[ inertial ] expression
| signal_name
| variable_name
| file_name
| subtype_indication
| subprogram_name
| instantiated_package_name
| open

actual_parameter_part

parameter_association_list

actual_part

actual_designator
| function_name ( actual_designator )
| type_mark ( actual_designator )

adding_operator

+
| –
| &

aggregate

( element_association { , element_association } )

alias_declaration

alias alias_designator [ : subtype_indication ] is name [ signature ] ;

alias_designator

identifier
| character_literal
| operator_symbol

allocator

new subtype_indication
| new qualified_expression

architecture_body

architecture identifier of entity_name is architecture_declarative_part begin architecture_statement_part end [ architecture ] [ architecture_simple_name ] ;

architecture_declarative_part

{ block_declarative_item }

architecture_statement_part

{ concurrent_statement }

array_constraint

index_constraint [ array_element_constraint ]
| ( open ) [ array_element_constraint ]

array_element_constraint

element_constraint

array_element_resolution

resolution_indication

array_type_definition

unbounded_array_definition
| constrained_array_definition

assertion

assert condition [ report expression ] [ severity expression ]

assertion_statement

[ label : ] assertion ;

association_element

[ formal_part => ] actual_part

association_list

association_element { , association_element }

attribute_declaration

attribute identifier : type_mark ;

attribute_designator

attribute_simple_name

attribute_name

prefix [ signature ] ' attribute_designator [ ( expression ) ]

attribute_specification

attribute attribute_designator of entity_specification is expression ;

base

integer

base_specifier

B
| O
| X
| UB
| UO
| UX
| SB
| SO
| SX
| D

based_integer

extended_digit { [ _ ] extended_digit }

based_literal

base # based_integer [ . based_integer ] # [ exponent ]

basic_character

basic_graphic_character
| format_effector

basic_graphic_character

upper_case_letter
| digit
| special_character
| space_character

basic_identifier

letter { [ _ ] letter_or_digit }

binding_indication

[ use entity_aspect ] [ generic_map_aspect ] [ port_map_aspect ]

bit_string_literal

[ integer ] base_specifier " [ bit_value ] "

bit_value

graphic_character { [ _ ] graphic_character }

block_configuration

for block_specification { use_clause } { configuration_item } end for ;

block_declarative_item

subprogram_declaration
| subprogram_body
| subprogram_instantiation_declaration
| package_declaration
| package_body
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| signal_declaration
| shared_variable_declaration
| file_declaration
| alias_declaration
| component_declaration
| attribute_declaration
| attribute_specification
| configuration_specification
| disconnection_specification
| use_clause
| group_template_declaration
| group_declaration
| PSL_Property_Declaration
| PSL_Sequence_Declaration
| PSL_Clock_Declaration

block_declarative_part

{ block_declarative_item }

block_header

[ generic_clause [ generic_map_aspect ; ] ] [ port_clause [ port_map_aspect ; ] ]

block_specification

architecture_name
| block_statement_label
| generate_statement_label [ ( generate_specification ) ]

block_statement

block_label : block [ ( guard_condition ) ] [ is ] block_header block_declarative_part begin block_statement_part end block [ block_label ] ;

block_statement_part

{ concurrent_statement }

case_generate_alternative

when [ alternative_label : ] choices => generate_statement_body

case_generate_statement

generate_label : case expression generate case_generate_alternative { case_generate_alternative } end generate [ generate_label ] ;

case_statement

[ case_label : ] case [ ? ] expression is case_statement_alternative { case_statement_alternative } end case [ ? ] [ case_label ] ;

case_statement_alternative

when choices => sequence_of_statements

character_literal

' graphic_character '

choice

simple_expression
| discrete_range
| element_simple_name
| others

choices

choice { | choice }

component_configuration

for component_specification [ binding_indication ; ] { verification_unit_binding_indication ; } [ block_configuration ] end for ;

component_declaration

component identifier [ is ] [ local_generic_clause ] [ local_port_clause ] end component [ component_simple_name ] ;

component_instantiation_statement

instantiation_label : instantiated_unit [ generic_map_aspect ] [ port_map_aspect ] ;

component_specification

instantiation_list : component_name

composite_type_definition

array_type_definition
| record_type_definition

compound_configuration_specification

for component_specification binding_indication ; verification_unit_binding_indication ; { verification_unit_binding_indication ; } end for ;

concurrent_assertion_statement

[ label : ] [ postponed ] assertion ;

concurrent_conditional_signal_assignment

target <= [ guarded ] [ delay_mechanism ] conditional_waveforms ;

concurrent_procedure_call_statement

[ label : ] [ postponed ] procedure_call ;

concurrent_selected_signal_assignment

with expression select [ ? ] target <= [ guarded ] [ delay_mechanism ] selected_waveforms ;

concurrent_signal_assignment_statement

[ label : ] [ postponed ] concurrent_simple_signal_assignment
| [ label : ] [ postponed ] concurrent_conditional_signal_assignment
| [ label : ] [ postponed ] concurrent_selected_signal_assignment

concurrent_simple_signal_assignment

target <= [ guarded ] [ delay_mechanism ] waveform ;

concurrent_statement

block_statement
| process_statement
| concurrent_procedure_call_statement
| concurrent_assertion_statement
| concurrent_signal_assignment_statement
| component_instantiation_statement
| generate_statement
| PSL_PSL_Directive

condition

expression

condition_clause

until condition

condition_operator

??

conditional_expression

expression when condition { else expression when condition } [ else expression ]

conditional_force_assignment

target <= force [ force_mode ] conditional_expression ;

conditional_signal_assignment

conditional_waveform_assignment
| conditional_force_assignment

conditional_variable_assignment

target := conditional_expression ;

conditional_waveform_assignment

target <= [ delay_mechanism ] conditional_waveforms ;

conditional_waveforms

waveform when condition { else waveform when condition } [ else waveform ]

configuration_declaration

configuration identifier of entity_name is configuration_declarative_part { verification_unit_binding_indication ; } block_configuration end [ configuration ] [ configuration_simple_name ] ;

configuration_declarative_item

use_clause
| attribute_specification
| group_declaration

configuration_declarative_part

{ configuration_declarative_item }

configuration_item

block_configuration
| component_configuration

configuration_specification

simple_configuration_specification
| compound_configuration_specification

constant_declaration

constant identifier_list : subtype_indication [ := expression ] ;

constrained_array_definition

array index_constraint of element_subtype_indication

constraint

range_constraint
| array_constraint
| record_constraint

context_clause

{ context_item }

context_declaration

context identifier is context_clause end [ context ] [ context_simple_name ] ;

context_item

library_clause
| use_clause
| context_reference

context_reference

context selected_name { , selected_name } ;

decimal_literal

integer [ . integer ] [ exponent ]

delay_mechanism

transport
| [ reject time_expression ] inertial

design_file

design_unit { design_unit }

design_unit

context_clause library_unit

designator

identifier
| operator_symbol

direction

to
| downto

disconnection_specification

disconnect guarded_signal_specification after time_expression ;

discrete_range

discrete_subtype_indication
| range

element_association

[ choices => ] expression

element_constraint

array_constraint
| record_constraint

element_declaration

identifier_list : element_subtype_definition ;

element_resolution

array_element_resolution
| record_resolution

element_subtype_definition

subtype_indication

entity_aspect

entity entity_name [ ( architecture_identifier ) ]
| configuration configuration_name
| open

entity_class

entity
| architecture
| configuration
| procedure
| function
| package
| type
| subtype
| constant
| variable
| component
| label
| literal
| units
| group
| file
| property
| sequence

entity_class_entry

entity_class [ <> ]

entity_class_entry_list

entity_class_entry { , entity_class_entry }

entity_declaration

entity identifier is entity_header entity_declarative_part [ begin entity_statement_part ] end [ entity ] [ entity_simple_name ] ;

entity_declarative_item

subprogram_declaration
| subprogram_body
| subprogram_instantiation_declaration
| package_declaration
| package_body
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| signal_declaration
| shared_variable_declaration
| file_declaration
| alias_declaration
| attribute_declaration
| attribute_specification
| disconnection_specification
| use_clause
| group_template_declaration
| group_declaration
| PSL_Property_Declaration
| PSL_Sequence_Declaration
| PSL_Clock_Declaration

entity_declarative_part

{ entity_declarative_item }

entity_designator

entity_tag [ signature ]

entity_header

[ formal_generic_clause ] [ formal_port_clause ]

entity_name_list

entity_designator { , entity_designator }
| others
| all

entity_specification

entity_name_list : entity_class

entity_statement

concurrent_assertion_statement
| passive_concurrent_procedure_call_statement
| passive_process_statement
| PSL_PSL_Directive

entity_statement_part

{ entity_statement }

entity_tag

simple_name
| character_literal
| operator_symbol

enumeration_literal

identifier
| character_literal

enumeration_type_definition

( enumeration_literal { , enumeration_literal } )

exit_statement

[ label : ] exit [ loop_label ] [ when condition ] ;

exponent

E [ + ] integer
| E – integer

expression

condition_operator primary
| logical_expression

extended_digit

digit
| letter

extended_identifier

\ graphic_character { graphic_character } \

external_name

external_constant_name
| external_signal_name
| external_variable_name

external_constant_name

<< constant external_pathname : subtype_indication >>

external_signal_name

<< signal external_pathname : subtype_indication >>

external_variable_name

<< variable external_pathname : subtype_indication >>

external_pathname

package_pathname
| absolute_pathname
| relative_pathname

factor

primary [ ** primary ]
| abs primary
| not primary
| logical_operator primary

file_declaration

file identifier_list : subtype_indication [ file_open_information ] ;

file_logical_name

string_expression

file_open_information

[ open file_open_kind_expression ] is file_logical_name

file_type_definition

file of type_mark

floating_type_definition

range_constraint

for_generate_statement

generate_label : for generate_parameter_specification generate generate_statement_body end generate [ generate_label ] ;

force_mode

in
| out

formal_designator

generic_name
| port_name
| parameter_name

formal_parameter_list

parameter_interface_list

formal_part

formal_designator
| function_name ( formal_designator )
| type_mark ( formal_designator )

full_type_declaration

type identifier is type_definition ;

function_call

function_name [ ( actual_parameter_part ) ]

function_specification

[ pure | impure ] function designator subprogram_header [ [ parameter ] ( formal_parameter_list ) ] return type_mark

generate_specification

static_discrete_range
| static_expression
| alternative_label

generate_statement

for_generate_statement
| if_generate_statement
| case_generate_statement

generate_statement_body

[ block_declarative_part begin ] { concurrent_statement } [ end [ alternative_label ] ; ]

generic_clause

generic ( generic_list ) ;

generic_list

generic_interface_list

generic_map_aspect

generic map ( generic_association_list )

graphic_character

basic_graphic_character
| lower_case_letter
| other_special_character

group_constituent

name
| character_literal

group_constituent_list

group_constituent { , group_constituent }

group_declaration

group identifier : group_template_name ( group_constituent_list ) ;

group_template_declaration

group identifier is ( entity_class_entry_list ) ;

guarded_signal_specification

guarded_signal_list : type_mark

identifier

basic_identifier
| extended_identifier

identifier_list

identifier { , identifier }

if_generate_statement

generate_label : if [ alternative_label : ] condition generate generate_statement_body { elsif [ alternative_label : ] condition generate generate_statement_body } [ else [ alternative_label : ] generate generate_statement_body ] end generate [ generate_label ] ;

if_statement

[ if_label : ] if condition then sequence_of_statements { elsif condition then sequence_of_statements } [ else sequence_of_statements ] end if [ if_label ] ;

incomplete_type_declaration

type identifier ;

index_constraint

( discrete_range { , discrete_range } )

index_subtype_definition

type_mark range <>

indexed_name

prefix ( expression { , expression } )

instantiated_unit

[ component ] component_name
| entity entity_name [ ( architecture_identifier ) ]
| configuration configuration_name

instantiation_list

instantiation_label { , instantiation_label }
| others
| all

integer

digit { [ _ ] digit }

integer_type_definition

range_constraint

interface_constant_declaration

[ constant ] identifier_list : [ in ] subtype_indication [ := static_expression ]

interface_declaration

interface_object_declaration
| interface_type_declaration
| interface_subprogram_declaration
| interface_package_declaration

interface_element

interface_declaration

interface_file_declaration

file identifier_list : subtype_indication

interface_function_specification

[ pure | impure ] function designator [ [ parameter ] ( formal_parameter_list ) ] return type_mark

interface_incomplete_type_declaration

type identifier

interface_list

interface_element { ; interface_element }

interface_object_declaration

interface_constant_declaration
| interface_signal_declaration
| interface_variable_declaration
| interface_file_declaration

interface_package_declaration

package identifier is new uninstantiated_package_name interface_package_generic_map_aspect

interface_package_generic_map_aspect

generic_map_aspect
| generic map ( <> )
| generic map ( default )

interface_procedure_specification

procedure designator [ [ parameter ] ( formal_parameter_list ) ]

interface_signal_declaration

[ signal ] identifier_list : [ mode ] subtype_indication [ bus ] [ := static_expression ]

interface_subprogram_declaration

interface_subprogram_specification [ is interface_subprogram_default ]

interface_subprogram_default

subprogram_name
| <>

interface_subprogram_specification

interface_procedure_specification
| interface_function_specification

interface_type_declaration

interface_incomplete_type_declaration

interface_variable_declaration

[ variable ] identifier_list : [ mode ] subtype_indication [ := static_expression ]

iteration_scheme

while condition
| for loop_parameter_specification

label

identifier

letter

upper_case_letter
| lower_case_letter

letter_or_digit

letter
| digit

library_clause

library logical_name_list ;

library_unit

primary_unit
| secondary_unit

literal

numeric_literal
| enumeration_literal
| string_literal
| bit_string_literal
| null

logical_expression

relation { and relation }
| relation { or relation }
| relation { xor relation }
| relation [ nand relation ]
| relation [ nor relation ]
| relation { xnor relation }

logical_name

identifier

logical_name_list

logical_name { , logical_name }

logical_operator

and
| or
| nand
| nor
| xor
| xnor

loop_statement

[ loop_label : ] [ iteration_scheme ] loop sequence_of_statements end loop [ loop_label ] ;

miscellaneous_operator

**
| abs
| not

mode

in
| out
| inout
| buffer
| linkage

multiplying_operator

*
| /
| mod
| rem

name

simple_name
| operator_symbol
| character_literal
| selected_name
| indexed_name
| slice_name
| attribute_name
| external_name

next_statement

[ label : ] next [ loop_label ] [ when condition ] ;

null_statement

[ label : ] null ;

numeric_literal

abstract_literal
| physical_literal

object_declaration

constant_declaration
| signal_declaration
| variable_declaration
| file_declaration

operator_symbol

string_literal

package_body

package body package_simple_name is package_body_declarative_part end [ package body ] [ package_simple_name ] ;

package_body_declarative_item

subprogram_declaration
| subprogram_body
| subprogram_instantiation_declaration
| package_declaration
| package_body
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| variable_declaration
| file_declaration
| alias_declaration
| attribute_declaration
| attribute_specification
| use_clause
| group_template_declaration
| group_declaration

package_body_declarative_part

{ package_body_declarative_item }

package_declaration

package identifier is package_header package_declarative_part end [ package ] [ package_simple_name ] ;

package_declarative_item

subprogram_declaration
| subprogram_instantiation_declaration
| package_declaration
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| signal_declaration
| variable_declaration
| file_declaration
| alias_declaration
| component_declaration
| attribute_declaration
| attribute_specification
| disconnection_specification
| use_clause
| group_template_declaration
| group_declaration
| PSL_Property_Declaration
| PSL_Sequence_Declaration

package_declarative_part

{ package_declarative_item }

package_header

[ generic_clause [ generic_map_aspect ; ] ]

package_instantiation_declaration

package identifier is new uninstantiated_package_name [ generic_map_aspect ] ;

package_pathname

@ library_logical_name . { package_simple_name . } object_simple_name

parameter_specification

identifier in discrete_range

partial_pathname

{ pathname_element . } object_simple_name

pathname_element

entity_simple_name
| component_instantiation_label
| block_label
| generate_statement_label [ ( static_expression ) ]
| package_simple_name

physical_literal

[ abstract_literal ] unit_name

physical_type_definition

range_constraint units primary_unit_declaration { secondary_unit_declaration } end units [ physical_type_simple_name ]

port_clause

port ( port_list ) ;

port_list

port_interface_list

port_map_aspect

port map ( port_association_list )

prefix

name
| function_call

primary

name
| literal
| aggregate
| function_call
| qualified_expression
| type_conversion
| allocator
| ( expression )

primary_unit

entity_declaration
| configuration_declaration
| package_declaration
| package_instantiation_declaration
| context_declaration
| PSL_Verification_Unit

primary_unit_declaration

identifier ;

procedure_call

procedure_name [ ( actual_parameter_part ) ]

procedure_call_statement

[ label : ] procedure_call ;

procedure_specification

procedure designator subprogram_header [ [ parameter ] ( formal_parameter_list ) ]

process_declarative_item

subprogram_declaration
| subprogram_body
| subprogram_instantiation_declaration
| package_declaration
| package_body
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| variable_declaration
| file_declaration
| alias_declaration
| attribute_declaration
| attribute_specification
| use_clause
| group_template_declaration
| group_declaration

process_declarative_part

{ process_declarative_item }

process_sensitivity_list

all
| sensitivity_list

process_statement

[ process_label : ] [ postponed ] process [ ( process_sensitivity_list ) ] [ is ] process_declarative_part begin process_statement_part end [ postponed ] process [ process_label ] ;

process_statement_part

{ sequential_statement }

protected_type_body

protected body protected_type_body_declarative_part end protected body [ protected_type_simple_name ]

protected_type_body_declarative_item

subprogram_declaration
| subprogram_body
| subprogram_instantiation_declaration
| package_declaration
| package_body
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| variable_declaration
| file_declaration
| alias_declaration
| attribute_declaration
| attribute_specification
| use_clause
| group_template_declaration
| group_declaration

protected_type_body_declarative_part

{ protected_type_body_declarative_item }

protected_type_declaration

protected protected_type_declarative_part end protected [ protected_type_simple_name ]

protected_type_declarative_item

subprogram_declaration
| subprogram_instantiation_declaration
| attribute_specification
| use_clause

protected_type_declarative_part

{ protected_type_declarative_item }

protected_type_definition

protected_type_declaration
| protected_type_body

qualified_expression

type_mark ( expression ) | type_mark aggregate

range

range_attribute_name
| simple_expression direction simple_expression

range_constraint

range range

record_constraint

( record_element_constraint { , record_element_constraint } )

record_element_constraint

record_element_simple_name element_constraint

record_element_resolution

record_element_simple_name resolution_indication

record_resolution

record_element_resolution { , record_element_resolution }

record_type_definition

record element_declaration { element_declaration } end record [ record_type_simple_name ]

relation

shift_expression [ relational_operator shift_expression ]

relational_operator

=
| /=
| <
| <=
| >
| >=
| ?=
| ?/=
| ?<
| ?<=
| ?>
| ?>=

relative_pathname

{ ^ . } partial_pathname

report_statement

[ label : ] report expression [ severity expression ] ;

resolution_indication

resolution_function_name
| ( element_resolution )

return_statement

[ label : ] return [ expression ] ;

scalar_type_definition

enumeration_type_definition
| integer_type_definition
| floating_type_definition
| physical_type_definition

secondary_unit

architecture_body
| package_body

secondary_unit_declaration

identifier = physical_literal ;

selected_expressions

{ expression when choices , } expression when choices

selected_force_assignment

with expression select [ ? ] target <= force [ force_mode ] selected_expressions ;

selected_name

prefix . suffix

selected_signal_assignment

selected_waveform_assignment
| selected_force_assignment

selected_variable_assignment

with expression select [ ? ] target := selected_expressions ;

selected_waveform_assignment

with expression select [ ? ] target <= [ delay_mechanism ] selected_waveforms ;

selected_waveforms

{ waveform when choices , } waveform when choices

sensitivity_clause

on sensitivity_list

sensitivity_list

signal_name { , signal_name }

sequence_of_statements

{ sequential_statement }

sequential_statement

wait_statement
| assertion_statement
| report_statement
| signal_assignment_statement
| variable_assignment_statement
| procedure_call_statement
| if_statement
| case_statement
| loop_statement
| next_statement
| exit_statement
| return_statement
| null_statement

shift_expression

simple_expression [ shift_operator simple_expression ]

shift_operator

sll
| srl
| sla
| sra
| rol
| ror

sign

+
| –

signal_assignment_statement

[ label : ] simple_signal_assignment
| [ label : ] conditional_signal_assignment
| [ label : ] selected_signal_assignment

signal_declaration

signal identifier_list : subtype_indication [ signal_kind ] [ := expression ] ;

signal_kind

register
| bus

signal_list

signal_name { , signal_name }
| others
| all

signature

[ [ type_mark { , type_mark } ] [ return type_mark ] ]

simple_configuration_specification

for component_specification binding_indication ; [ end for ; ]

simple_expression

[ sign ] term { adding_operator term }

simple_force_assignment

target <= force [ force_mode ] expression ;

simple_name

identifier

simple_release_assignment

target <= release [ force_mode ] ;

simple_signal_assignment

simple_waveform_assignment
| simple_force_assignment
| simple_release_assignment

simple_waveform_assignment

target <= [ delay_mechanism ] waveform ;

simple_variable_assignment

target := expression ;

slice_name

prefix ( discrete_range )

string_literal

" { graphic_character } "

subprogram_body

subprogram_specification is subprogram_declarative_part begin subprogram_statement_part end [ subprogram_kind ] [ designator ] ;

subprogram_declaration

subprogram_specification ;

subprogram_declarative_item

subprogram_declaration
| subprogram_body
| subprogram_instantiation_declaration
| package_declaration
| package_body
| package_instantiation_declaration
| type_declaration
| subtype_declaration
| constant_declaration
| variable_declaration
| file_declaration
| alias_declaration
| attribute_declaration
| attribute_specification
| use_clause
| group_template_declaration
| group_declaration

subprogram_declarative_part

{ subprogram_declarative_item }

subprogram_header

[ generic ( generic_list ) [ generic_map_aspect ] ]

subprogram_instantiation_declaration

subprogram_kind identifier is new uninstantiated_subprogram_name [ signature ] [ generic_map_aspect ] ;

subprogram_kind

procedure
| function

subprogram_specification

procedure_specification
| function_specification

subprogram_statement_part

{ sequential_statement }

subtype_declaration

subtype identifier is subtype_indication ;

subtype_indication

[ resolution_indication ] type_mark [ constraint ]

suffix

simple_name
| character_literal
| operator_symbol
| all

target

name
| aggregate

term

factor { multiplying_operator factor }

timeout_clause

for time_expression

tool_directive

` identifier { graphic_character }

type_conversion

type_mark ( expression )

type_declaration

full_type_declaration
| incomplete_type_declaration

type_definition

scalar_type_definition
| composite_type_definition
| access_type_definition
| file_type_definition
| protected_type_definition

type_mark

type_name
| subtype_name

unbounded_array_definition

array ( index_subtype_definition { , index_subtype_definition } ) of element_subtype_indication

use_clause

use selected_name { , selected_name } ;

variable_assignment_statement

[ label : ] simple_variable_assignment
| [ label : ] conditional_variable_assignment
| [ label : ] selected_variable_assignment

variable_declaration

[ shared ] variable identifier_list : subtype_indication [ := expression ] ;

verification_unit_binding_indication

use vunit verification_unit_list

verification_unit_list

verification_unit_name { , verification_unit_name }

wait_statement

[ label : ] wait [ sensitivity_clause ] [ condition_clause ] [ timeout_clause ] ;

waveform

waveform_element { , waveform_element }
| unaffected

waveform_element

value_expression [ after time_expression ]
| null [ after time_expression ]

Lexical elements and others

format_effector

horizontal tab
| vertical tab
| cariage return
| line feed
| form feed

upper_case_letter

upper_case_letter

digit

digit

special_character

special_character

space_character

space_character

lower_case_letter

lower_case_letter

other_special_character

other_special_character

PSL_Property_Declaration

external

PSL_Sequence_Declaration

external

PSL_Clock_Declaration

external

PSL_Directive

external

PSL_Verification_Unit

external