Example

   entity counter is
        port (Incr, Load, Clock: in     bit;
              Carry:             out    bit;
              Data_Out:          buffer bit_vector(7 downto 0);
              Data_In:           in     bit_vector(7 downto 0));
   end counter;

Generics allow static information to be communicated to a block from its environment for all architectures of a design unit. These include timing information (setup, hold, delay times), part sizes, and other parameters.

Example

    entity and_gate is
        port(a,b: in  bit;
             c:   out bit);
        generic (gate_delay: time := 5ns);
    end and_gate;

Architecture - Back To Top

An architecture defines one particular implementation of a design unit, at some desired level of abstraction.

  architecture arch_name of entity_name is
       ...  declarations ...
   begin
       ...  concurrent statements  ...
   end

Declarations include data types, constants, signals, files, components, attributes, subprograms, and other information to be used in the implementation description. Concurrent statements describe a design unit at one or more levels of modeling abstraction, including dataflow, structure, and/or behavior.

• Behavioral Model: No structure or technology implied. Usually written in sequential, procedural style.
• Dataflow Model: All datapaths shown, plus all control signals.
• Structural Model: Interconnection of components.

VHDL PACKAGES - Back To Top

A VHDL package contains subprograms, constant definitions, and/or type definitions to be used throughout one or more design units. Each package comprises a "declaration section", in which the available (i.e. exportable) subprograms, constants, and types are declared, and a "package body", in which the subprogram implementations are defined, along with any internally-used constants and types. The declaration section represents the portion of the package that is "visible" to the user of that package. The actual implementations of subroutines in the package are typically not of interest to the users of those subroutines.

Package declaration format:

   package package_name is
     ... exported constant declarations
     ... exported type declarations
     ... exported subprogram declarations
   end package_name;

Example:

    package ee530 is
       constant maxint: integer := 16#ffff#;
       type arith_mode_type is (signed, unsigned);
       function minimum(constant a,b: in integer) return integer;
    end ee530;

Package body format:

   package body package_name is
       ... exported subprogram bodies
       ... other internally-used declarations
   end package_name;

Example:

   package body ee530 is
      function minimum (constant a,b: integer) return integer is
         variable c: integer; -- local variable
             begin
                if a < b then
                    c := a;  -- a is min
                else
                    c := b;  -- b is min
                end if;
                return c;  -- return min value
             end;
    end ee530;

Package Visibility

To make all items of a package "visible" to a design unit, precede the desired design unit with a "use" statement:

Example:

   use library_name.package_name.all

A "use" statement may precede the declaration of any entity or architecture which is to utilize items from the package. If the "use" statement precedes the entity declaration, the package is also visible to the architecture.

User-Developed Packages

Compile user-developed packages in your current working library. To make it visible:

    use package_name.all;

Note: 'std' and 'work' (your current working library) are the two default libraries. The VHDL 'library' statement is needed to make the 'ieee' library and/or additional libraries visible.

Example

   library lib_name;            -- make library visible
   use lib_name.pkg_name.all;   -- make package visible

VHDL Standard Packages

STANDARD - basic type declarations (always visible by default)
TEXTIO - ASCII input/output data types and subprograms

To make TEXTIO visible: use std.textio.all;

IEEE Standard 1164 Package

This package contained in the 'ieee' library supports multi-valued logic signals with type declarations and functions. To make visible:

   library ieee;      -- VHDL Library stmt
      use ieee.std_logic_1164.all;

Special 12-valued data types/functions to interface with QuickSim II and schematic diagrams.

   library mgc_portable;            -- Special Mentor Graphics Library
   use mgc_portable.qsim_logic.all; -- Quicksim portable data types

VHDL IDENTIFIERS, NUMBERS, STRINGS, AND EXPRESSIONS - Back To Top

Identifiers

Identifiers in VHDL must begin with a letter, and may comprise any combination of letters, digits, and underscores. Note that VHDL internally converts all characters to UPPER CASE.

Examples

     Memory1, Adder_Module, Bus_16_Bit

Numeric Constants

Numeric contants can be defined, and can be of any base (default is decimal). Numbers may include embedded underscores to improve readability.

Format: base#digits# -- base must be a decimal number

Examples

     16#9fba#           (hexadecimal)
     2#1111_1101_1011#  (binary)
     16#f.1f#E+2        (floating-point, exponent is decimal)

Bit String Literals

Bit vector constants are are specified as literal strings.

Examples

     x"ffe"            (12-bit hexadecimal value)
     o"777"            (9-bit octal value)
     b"1111_1101_1101" (12-bit binary value)

Arithmetic and Logical Expressions

Expressions in VHDL are similar to those of most high-level languages. Data elements must be of the type, or subtypes of the same base type. Operators include the following:

• Logical: and, or, nand, nor, xor, not (for boolean or bit ops)
• Relational: =, /=, <, <=, >, >=
• Arithmetic: +, -, *, /, mod, rem, **, abs
  (a mod b takes sign of b, a rem b takes sign of a)
• Concatenate: &
  (ex. a & b makes one array)

Examples

   a <= b nand c;
   d := g1 * g2 / 3;
   Bus_16 <= Bus1_8 & Bus2_8;

VHDL DATA TYPES - Back To Top

Each VHDL objects must be classified as being of a specific data type. VHDL includes a number of predefined data types, and allows users to define custom data types as needed.

Predefined Scalar Data Types (single objects)

VHDL Standard:

• bit values: '0', '1'
• boolean values: TRUE, FALSE
• integer values: -(231) to +(231 - 1) {SUN Limit}
• natural values: 0 to integer'high (subtype of integer)
• positive values: 1 to integer'high (subtype of integer)
• character values: ASCII characters (eg. 'A')
• time values include units (eg. 10ns, 20us)

IEEE Standard 1164 (package ieee.std_logic_1164.all)

• std_ulogic values: 'U','X','1','0','Z','W','H','L','-'
  'U' = uninitialized
  'X' = unknown
  'W' = weak 'X'
  'Z' = floating
  'H'/'L' = weak '1'/'0'
  '-' = don't care
• std_logic resolved "std_ulogic" values
• X01 subtype {'X','0','1'} of std_ulogic
• X01Z subtype {'X','0','1','Z'} of std_ulogic
• UX01 subtype {'U','X','0','1'} of std_ulogic
• UX01Z subtype {'U','X','0','1','Z'} of std_ulogic

Predefined VHDL Aggregate Data Types

• bit_vector array (natural range <>) of bit
• string array (natural range <>) of char
• text file of "string"

IEEE Standard 1164 Aggregate Data Types

(From package: ieee.std_logic_1164.all)

• std_ulogic_vector array (natural range <>) of std_ulogic
• std_logic_vector array (natural range <>) of std_logic

Examples

    signal dbus: bit_vector(15 downto 0);
    dbus (7 downto 4) <= "0000"; (4-bit slice of dbus)
    signal cnt:  std_ulogic_vector(1 to 3);
    variable message: string(0 to 20);

User-Defined Enumeration Types

An enumerated data type can be created by explicitely listing all possible values.

Example

   type opcodes is (add, sub, jump, call);  -- Type with 4 values
   signal instruc: opcodes;                 -- Signal of this type
     ...
   
   if instruc = add then   -- test for value 'add'
     ...

Other user-defined types

Custom data types can include arrays, constrained and unconstrained, and record structures.

• Constrained array: Upper and lower indexes are specified.

  Example

     type word is array (0 to 15) of bit;

• Unconstrained array: Indexes are specified when a signal or variable of that type is declared.
  

  Examples

     type memory is array (integer range <>) of bit_vector(0 to 7);
    -- a type which is an arbitrary-sized array of 8-bit vectors
     variable memory256: memory(0 to 255); -- a 256-byte memory array
     variable stack: memory(15 downto 0);  -- a 16-byte memory array

• Subtype: A selected subset of values of a given type. Elements of different subtypes having the same base type may be combined in expressions (elements of different types cannot). Subtypes can be used to detect out-of-range values during simulation.
  

  Examples

     subtype byte_signed is integer range -128 to 127;
     subtype byte_unsigned is integer range 0 to 255;

Aliases

An alias" defines an alternate name for a signal or part of a signal. Aliases are often used to refer to selected slices of a bit_vector.

Example

   signal instruction: bit_vector(31 downto 0);
   alias opcode: bit_vector(6 downto 0) is instruction(31 downto 25);
    ...
   opcode <= "1010101";  -- Set the opcode part of an instruction code

VHDL OBJECTS: CONSTANTS, VARIABLES, AND SIGNALS - Back To Top

Constants

A constant associates a value to a symbol of a given data type. The use of constants may improve the readability of VHDL code and reduce the likelihood of making errors. The declaration syntax is:

constant symbol: type := value;

Examples

  constant  Vcc:  signal:= '1';   --logic 1 constant
  constant  zero4: bit_vector(0 to 3) := ('0','0','0','0');

Variables

A variable is declared within a blocks, process, procedure, or function, and is updated immediately when an assignment statement is executed. A variable can be of any scalar or aggregate data type, and is utilized primarily in behavioral descriptions. It can optionally be assigned initial values (done only once prior to simulation). The declaration syntax is:

variable symbol: type [:= initial_value];

Examples

   process
       variable count: integer  := 0;
       variable rega: bit_vector(7 downto 0);
   begin
       ...
       count := 7;      -- assign values to variables
       rega  := x"01";
       ...
   end;

Signals

A signal is an object with a history of values (related to "event" times, i.e. times at which the signal value changes).

Signals are declared via signal declaration statements or entity port definitions, and may be of any data type. The declaration syntax is:

signal sig_name: data_type [:=initial_value];

Examples

    signal clock: bit;
    signal GND:   bit := '0';
    signal databus: std_ulogic_vector(15 downto 0);
    signal addrbus: std_logic_vector(0 to 31);

Each signal has one or more "drivers" which determine the value and timing of changes to the signal. Each driver is a queue of events which indicate when and to what value a signal is to be changed. Each signal assignment results in the corresponding event queue being modified to schedule the new event.

signal line x

10ns '0' Driver of

20ns '1' signal x

Event Values

Times

NOTE: If no delay is specified, the signal event is scheduled for one infinitessimally-small "delta" delay from the current time. The signal change will occur in the next simulation cycle.

Examples

(Assume current time is T)

    clock   <= not clock after 10ns;      -- change at T + 10ns
    databus <= mem1 and mem2 after delay; -- change at T + delay
    x       <= '1';                       -- change to '1' at time T + "delta";

Element delay models may be specified as either "inertial" or "transport". Inertial delay is the default, and should be used in most cases.

• Inertial delay: The addition to an event queue of an event scheduled at time T automatically cancels any events in the queue scheduled to occur prior to time T, i.e. any event shorter than the delay time is suppressed.
• Transport delay: Each new event is simply inserted into the event queue, i.e. behavior is that of a delay line. The keyword transport is used to indicate transport delays.

Examples

    B <= A after 5ns;            -- inertial delay
    C <= transport A after 5 ns; -- transport delay

             5______15 17_________30
    A _______|       |_|          |_____________ 
                  ____________________
    B ___________|                    |_________ (Inertial Delay)
                  _______   __________
    C ___________|       |_|          |_________ (Transport Delay)
                10      20 22         35

Where there are multiple drivers for one signal, a "resolution function" must be provided to determine the value to be assigned to the signal from the values supplied by the multiple drivers. This allows simulation of buses with multiple sources/drivers.

NOTE: The std_logic and std_logic_vector types from the ieee library have predefined resolution functions:

Example

    signal data_line: std_logic;
    begin
      block1:  
          data_line <= '1';     -- one driver
          ...
      block2:
          data_line <= 'Z';  -- 2nd driver

The resolved value is '1' since '1' overrides a 'Z' (floating) value. If the two values had been '1' and '0', the resolved value would have been 'X', indicating an unknown result.

CONCURRENT STATEMENTS - Back To Top

Concurrent statements are included within architecture definitions and within "block" statements, representing concurrent behavior within the modelled design unit. These statements are executed in an asynchronous manner, with no defined order, modeling the behavior of independent hardware elements within a system.

Concurrent Signal Assignment

A signal assignment statement represents a process that assigns values to signals. It has three basic formats.

1. A <= B; A <= B when condition1 elseC when condition2 else D when condition3 else E;
2. with expression select A <= B when choice1, C when choice2, D when choice3, E when others;

For each of the above, waveforms (time-value pairs) can also be specified.

Examples

    A <= B after 10ns when condition1 else
         C after 12ns when condition2 else
         D after 11ns;

    -- 4-input multiplexer (Choice is a 2-bit vector)
    with Choice select 
         Out <=  In0 after 2ns when "00",
                 In1 after 2ns when "01",
                 In2 after 2ns when "10",
                 In3 after 2ns when "11";

    -- 2-to-4 decoder (Y = 4-bit and A = 2-bit vectors)
    Y <= "0001" after 2ns when A = "00" else
         "0010" after 2ns when A = "01" else
         "0100" after 2ns when A = "10" else
         "1000" after 2ns ;

    -- Tri-state driver: (Y is logic4; X is bit_vector)
    Y <= '0' after 1ns when En = '1' and X = '0' else
         '1' after 1ns when En = '1' and X = '1' else
         'Z' after 1ns;

    -- A is a 16-bit vector
    A <= (others => '0');   -- set all bits of A to '0'

The keyword "others" in the last example indicates that all elements of A not explicitly listed are to be set to '0'.

Process Statement - Back To Top

An independent sequential process represents the behavior of some portion of a design. The body of a process is a list of sequential statements.

Syntax:

    label: process (sensitivity list)   
           ... local declarations ...    
           begin
           ... sequential statements ...
           end process label;

Example

   DFF: process (clock)
          begin
             if clock = '1' then
                Q  <= D after 5ns;
                QN <= not D after 5ns;
             end if;
        end process DFF;

The sequential statements in the process are executed in order, commencing with the beginning of simulation. After the last statement of a process has been executed, the process is repeated from the first statement, and continues to repeat until suspended. If the optional sensitivity list is given, a wait on ... statement is inserted after the last sequential statement, causing the process to be suspended at that point until there is an event on one of the signals in the list, at which time processing resumes with the first statement in the process.

Block Statement - Back To Top

A block is a grouping of related concurrent statements that can be used in representing designs in a hierarchical manner.

Syntax:

   label: block (guard expression)
          ... local declarations ...
          begin
          ... concurrent statements ...
          end block label;

If a guard expression is given, "guarded" a boolean variable GUARD is automatically defined and set to the boolean value of the guard expression. GUARD can then be tested within the block, to perform selected signal assignments or other statements only when the guard condition evaluates to TRUE.

Examples

    -- D Latch: Transfer D input to Q output when Enable = '1' 
    block (Enable = '1')
    begin
       Q <= guarded D after 5ns;

    end block;

    -- D Flip-flop: Transfer D to Q on falling edge of Clock
    block (Clock'EVENT and Clock = '0')
    begin
       Q <= guarded D after 5ns;
    end block;

    -- Tristate driver with input B and output A 
    block (Enable = '1')
    begin
        A <= B when GUARD = '1' else 'Z';
    end block;

In the last example, B is assigned to signal A only when GUARD is true, which implies Enable = '1'.

Concurrent Procedure Call - Back To Top

An externally defined procedure/subroutine can be invoked, with parameters passed to it as necessary. This serves the same function and behaves in the same manner as a "process" statement, with any signals in the passed parameters forming a sensitivity list.

Example

   ReadMemory (DataIn, DataOut, RW, Clk);
   (where the ReadMemory procedure is defined elsewhere)

Component instantiation - Back To Top

Instantiates (i.e. create instances of) predefined components within a design architecture. Each such component is first declared in the declaration section of that architecture, and then "instantiated" one or more times in the body of the architecture.

• In the declaration section: list the "component declaration" and one or more "configuration specifications".

  The "component declaration" defines the component interface, which corresponds to the component's entity declaration. This allows the VHDL compiler to check signal compatibilities.

  Example

      component adder
         port(a,b:  in  bit_vector(7 downto 0);
              s:    out bit_vector(7 downto 0);
              cin:  in  bit;
              cout: out bit);
      end component;

• The "configuration specification" identifies specific architecture(s) to be used for each instance of the component. (There may be multiple architectures for a given component.)
  

  Examples

      for ALL:     comp1 use entity work.comp1 (equations);
      for ADDER1:  adder use entity work.adder (equations);
      for ADDER2:  adder use entity work.adder (dataflow);

  In all three examples, the prefix work. indicates that the current working library contains the indicated component models. In the first example, architecture equations of entity comp1 is used for all instances of comp1. In the other examples, architecture equations is to be used for instance ADDER1 of component adder, and architecture dataflow is to be used for instance ADDER2 of component adder.

Component Instantiation Each instance of a declared component is listed, an instance name assigned, and actual signals connected to its ports as follows:

instance_name: component_name port map (port list);

The port list may be in either of two formats:

• v,w, and y must be bit_vectors, y and z bits

Example

A1: adder port map(a=>v, b=>w, s=>y, cin->x, cout->z);

(The signal ordering is not important in this format)

Example:

    architecture r1 of register is
       component jkff
          port(J,K,CLK: in bit;
               Q,QN:    out bit);
       end component;
       for ALL: jkff use entity work.jkff (equations);
       -- Use architecture equations of entity jkff
          for all instances

       component dff
          port(D,CLK: in bit;
               Q,QN:  out bit);
       end component; 
       for DFF1: dff  use entity work.dff  (equations);
       for DFF2: dff  use entity work.dff  (circuit);
       --Use different architectures of dff for instances
         DFF1 and DFF2
    begin
       JKFF1: jkff port map (j1,k1,clk,q1,qn1);
       JKFF2: jkff port map (j2,k1,clk,q2,qn2);
       DFF1:  dff  port map (d1,clk,q4,qn4);
       DFF2:  dff  port map (d2,clk,q5,qn5);
    end.

Concurrent assertion - Back To Top

A concurrent assertion statement checks a condition (occurrence of an event) and issues a report if the condition is not true. This can be used to check for timing violations, illegal conditions, etc. An optional severity level can be reported to indicate the nature of the detected condition.

Syntax:

    assert  (clear /= '1') or (preset /= '1')
    report "Both preset and clear are set!"
    severity warning;

Generate statement - Back To Top

A generate statement is an iterative or conditional elaboration of a portion of a description. This provides a compact way to represent what would ordinarily be a group of statements.

Example

Generate a 4-bit full adder from 1-bit full_adder stages:

  add_label:        -- Note that a label is required here
    for i in 4 downto 1 generate
        FA: full_adder port map(C(i-1), A(i), B(i), C(i), Sum(i));
    end generate;

The resulting code would look like:

    FA4: full_adder port map(C(3), A(4), B(4), C(4), Sum(4));
    FA3: full_adder port map(C(2), A(3), B(3), C(3), Sum(3));
    FA2: full_adder port map(C(1), A(2), B(2), C(2), Sum(2));
    FA1: full_adder port map(C(0), A(1), B(1), C(1), Sum(1));

SEQUENTIAL STATEMENTS - Back To Top

Sequential statements are used to define algorithms to express the behavior of a design entity. These statements appear in process statements and in subprograms (procedures and functions).

Wait statement - Back To Top

- suspends process/subprogram execution until a signal changes, a condition becomes true, or a defined time period has elapsed. Combinations of these can also be used.

Syntax:

    wait [on signal_name {,signal_name}]
         [until condition]
         [for time expression]

Example

Suspend execution until one of the two conditions becomes true, or for 25ns, whichever occurs first.

    wait until clock = '1' or enable /='1' for 25ns;

Signal assignment statement - Back To Top

Assign a waveform to one signal driver (edit the event queue).

Example

    A <= B after 10ns;
    C <= A after 10ns;  -- value of C is current A value

Variable assignment statement - Back To Top

Update a process/procedure/function variable with an expression. The update takes affect immediately.

Example

    A := B and C;
    D := A;        -- value of D is new A value

Procedure call - Back To Top

Invoke an externally-defined subprogram in the same manner as a concurrent procedure call.

Conditional Statements - Back To Top

Standard if..then and case constructs can be used for selective operations.

    if condition then
       ... sequence of statements... 
    elsif condition then
       ... sequence of statements...
    else 
       ... sequence of statements...
    end if;

NOTE: elsif and else clauses are optional.

    case expression is
       when choices => sequence of statements
       when choices => sequence of statements
       ...
       when others => sequence of statements
    end case;

NOTE: case choices can be expressions or ranges.

Loop statements - Back To Top

Sequences of statements can be repeated some number of times under the control of while or for constructs.

 label: while condition loop
    ... sequence of statements ...
    end loop label;

 label:  for loop_variable in range loop
  ... sequence of statements...
   end loop label;

NOTE: the label is optional.

Loop termination statements - allow termination of one iteration, loop, or procedure.

next [when condition]; -- end current loop iteration

exit [when condition]; -- exit innermost loop entirely

return expression; -- exit from subprogram

NOTES: 1. The next/exit condition clause is optional.

2. The return expression is used for functions.

PROCEDURES - Back To Top

A procedure is a subprogram that is passed parameters and may return values via a parameter list.

Example

   procedure  proc_name (signal clk: in vlbit;
                         constant d: in vlbit;
                         signal data: out vlbit) is
       ... local variable declarations ...
   begin
       ... sequence of statements ...
   end proc_name;

Procedure call: proc_name(clk1, d1, dout);

FUNCTIONS - Back To Top

A function is a subprogram that is passed parameters and returns a single value. Unlike procedures, functions are primarily used in expressions.

Example

   -- Convert bit_vector to IEEE std_logic_vector format
   -- (attributes LENGTH and RANGE are described below)
   function bv2slv (b:bit_vector) return std_logic_vector is
       variable result: std_logic_vector(b'LENGTH-1 downto 0);
   begin
       for i in result'RANGE loop
           case b(i) is
               when '0' => result(i) := '0';
               when '1' => result(i) := '1';
           end case;
       end loop;
       return result;
   end;

   -- Convert bit_vector to unsigned (natural) value 
   function b2n (B: bit_vector) return Natural is
       variable S: bit_vector(B'Length - 1 downto 0) := B;
       variable N: Natural := 0;
   begin
       for i in S'Right to S'Left loop
           if S(i) = '1' then
               N := N + (2**i);
           end if;
       end loop;
       return N;
   end;

Function Calls:

       signal databus:  vector4(15 downto 0);
       signal internal: bit_vector (15 downto 0);
       variable x: integer;
        ....
       databus <= bv2slv (internal);
       x := b2n(internal);

Data conversion between ieee types and bit/bit_vector (functions in "ieee.std_logic_1164")

To_bit(sul) - from std_ulogic to bit
To_bitvector(sulv) - from std_ulogic_vector/std_logic_vector
To_StdULogic(b) - from bit to std_ulogic
To_StdLogicVector(bv) - from bit_vector or std_ulogic_vector
To_StdULogicVector(bv)- from bit_vector or std_logic_vector
To_X01(v) - from bit, std_ulogic, or std_logic to X01
To_X01Z(v) - from bit, std_ulogic, or std_logic to X01Z
To_UX01(v) - from bit, std_ulogic, or std_logic to UX01

Other "ieee.std_logic_1164" functions - Back To Top

rising_edge(s) - true if rising edge on signal s (std_ulogic)
falling_edge(s) - true if falling edge on signal s (std_ulogic)

Additional Mentor Graphics-supplied functions for elements of types Bit_vector (implemented as overloaded operator definitions):

library mgc_portable;
use mgc_portable.qsim_logic.ALL;

Arithmetic between bit_vectors: use normal binary operator tokens

a + b, a - b, a * b, a / b, a mod b, a rem b

Logical operations between all signal types and vectors of signal types in the "ieee" library.

and, or, nand, nor, xor, xnor, not

Shift/rotate left/right logical/arithmetic operators:

sll, srl, sra, rll, rrl

Ex. a := x sll 2; -- "shift left logical" bit_vector x by 2 bits

Relational operations: =,/=,<,>,<=,>=

Type conversion:

to_bit (from integer)
to_integer (from bit_vector)

OBJECT ATTRIBUTES - Back To Top

An object attribute returns information about a signal or data type.

Signal Condition Attributes (for a signal S)

S'DELAYED(T) - value of S delayed by T time units
S'STABLE(T) - true if no event on S over last T time units
S'QUIET(T) - true if S quiet for T time units
S'LAST_VALUE - value of S prior to latest change
S'LAST_EVENT - time at which S last changed
S'LAST_ACTIVE - time at which S last active
S'EVENT - true if an event has occurred on S in current cycle
S'ACTIVE - true if signal S is active in the current cycle
S'TRANSACTION - bit value which toggles each time signal S changes

Examples

    if (clock'STABLE(0ns)) then  -- change in clock?
        ...      -- action if no clock edge
    else
        ...      -- action on edge of clock
    end if;

    if clock'EVENT and clock = '1' then
        Q <= D after 5ns;       -- set Q to D on rising edge of clock
    end if;

Data Type Bounds (Attributes of data type T)

T'BASE - base type of T
T'LEFT - left bound of data type T
T'RIGHT - right bound
T'HIGH - upper bound (may differ from left bound)
T'LOW - lower bound

Enumeration Data Types (Variable/signal x of data type T)

T'POS(x) - position number of value of x of type T
T'VAL(x) - value of type T whose position number is x
T'SUCC(x) - value of type T whose position is x+1
T'PRED(x) - value of type T whose position is x-1
T'LEFTOF(x) - value of type T whose position is left of x
T'RIGHTOF(x) - value of type T whose position is right of x

Array Indexes for an Array A (Nth index of array A)

A'LEFT(N) - left bound of index
A'RIGHT(N) - right bound of index
A'HIGH(N) - upper bound of index
A'LOW(N) - lower bound of index
A'LENGTH(N) - number of values in range of index
A'RANGE(N) - range: A'LEFT to A'RIGHT
A'REVERSE_RANGE(N) - range A'LEFT downto A'RIGHT

NOTE: For multi-dimensional array, Nth index must be indicated in the attribute specifier. N may be omitted for a one-dimensional array.

Examples

    for i in (data_bus'RANGE) loop
        ...
    for i in (d'LEFT(1) to d'RIGHT(1)) loop
        ...

Block Attributes (of a block B)

B'BEHAVIOR - true if block B contains no component instantiations
B'STRUCTURE - true if no signal assignment statements in block B

THE TEXTIO PACKAGE - Back To Top

TEXTIO is a package of VHDL functions that read and write text files. To make the package visible:

use std.textio.all;

Data Types:

text - a file of character strings
line - one string from a text file

Example Declarations

   file Prog: text is in "file_name"; --text file "file_name"
   variable L: line;              -- read lines from file to L

Reading Values From a File:

readline(F, L)

Read one line from "text" file F to "line" L
 

read(L, VALUE, GOOD);

Read one value from "line" L into variable VALUE

• GOOD is TRUE if successful
• Data_type of VALUE can be bit, bit_vector, integer, real, character, string, or time.

Writing values to a file:

writeline(F, L);

Write one line to "text" file F from "line" L
 

write(L, VALUE, JUSTIFY, FIELD);

Write one value to "line" L from variable VALUE

• Data_type of VALUE can be bit, bit_vector, integer, real, character, string, or time.
• JUSTIFY is "left" or "right" to justify within the field
• FIELD is the desired field width of the written value
   

   

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