Scalar Data Types and Operations 大同大學資訊工程系副教授鄭福炯

Scalar Data Typesand

Operations

大同大學資訊工程系副教授鄭福炯 ([email protected])

© Fu-Chiung ChengVHDL Ch022

Type

Type of a data object defines– the set of values that the object can assume

and– the set of operations that can be performed

on those valuesType: scalar types, access types, file

types and composite types


Lexical Elements of VHDL

Lexical elements of VHDL: comments, identifiers, reserved words, special symbols, numbers, characters, strings, bit strings

Comments:– A comment line in VHDL is represented

by two successive dashes “--”.– A comment extends from “--” to the end

of the line.– See page 192 examples



Identifiers:– Identifiers are used to name items– Use meaningful name – Identifiers can be arbitrarily long– Some Rules:

must start with an alphabetic letter.can contain alphabetic letters, decimal digits and underline

character “_”.can not end with “_”.can not contain successive “_”.



Legal and illegal examples:– See page 18

Extended identifiers– Can contain any sequence of characters– Is written by enclosing the characters

between ‘\’ characters– Examples: \data bus\, \global.clock\, \923\



Reserved Words– Reserved words are used to denote special

constructs that form a model– Can not be used as identifiers– Is listed in Figure 1-15 on page 19



Special Symbols:– Operators: +, -, &, *, /, ., <, =, >, |, /=, :=, >=, <=, <>– To delimit parts of language constructs and as

punctuation: “, ‘, (, ), “,”, :, ;, – array indices: []

Numbers:– Integer literals: 23, 0, 146, 56E5, 1E+12,

2#11111101#, 16#12FD”, 2#1111_1101#– Real literals: 23.1, 0.0, 3.14159, 1.234E09



Characters– ‘A’~’Z’, ‘a’-’z’, ‘,’,

Strings– A sequence of characters– “data bus”, “923” (use & operator)– Binary digits: B for binary, O for octal, X for

hexadecimal (B”01010111”, X”97”)


Syntax Descriptions

Extended Backus-Naur Form to describe the rules of VHDL syntax

EBNF divides a language into syntactic categories:

Variable assignment: – Variable_Assignment target := Expression;– x := y + 1;

Function_Call name [( Association_List) ] – zero or one


Syntax Descriptions

Optional operator “{}”– process is

{ process_declarative_item }begin { sequential_statement }end process;

– zero or more Iterative operator “{…}”

– identifier_list <= identifier, {…} | identifier– one or more


Syntax Descriptions

Choice operator “|”– mode <=in | out | inout– one of the many

Parenthetic grouping ( )– Term Factor { (*|/|mod|rem)

Factor)}


Constant Declaration

EBNF: constant_decl constant id { ,…}: subtype_indication [:= expr];

Examples:– constant NUMBER_OF_BYTE: integer := 4;– constant SIZE, COUNT: integer := 255;


Variable Declaration andAssignment Variables act as placeholders for quantities that change during simulation. EBNF: variable_decl variable id { ,…}: subtype_indication [:= expr]; Examples:

– variable index, sum : integer := 0;– variable_assign <= [label: ] id:= expr;

Can be used in process block only– pc := 1;– index := index + 1;


Type

Every name or id in VHDL has an associated “type”. The “type” determines the operations that can be applied to the name. VHDL along with its packages provides pre-defined types. Additionally the user can define new types.


Type Declarations EBNF” type_decl <=type id is type_definition; Useful when pre-defined types are insufficient. Examples:

– type apples is range 0 to 100;– type oranges is range 0 to 100;– apples may not be assigned to variable oranges

Default value is left hand side of range.


Type Declarations

User defined type– If we define our own types for ports, the type names

must be declared in a package (similar to a header file).

– Example:package int_types is

type Small_Int is range 0 to 255;end package int_type


Type Declarations

User defined type– Example

use work.int_type.all;entity SmallAdder is

Port (a,b: in small_int; s: out small_in) end enitity small_adder


Integer type

“integer” is a pre-defined type used to represent whole numbers.– variable x, y : integer ;

VHDL standard requires that the implementation be able to represent numbers from –2^31 + 1 to 2^31 – 1.

User can define new “integer” types.


Integer type

EBNF:– type_decl <= type identifier is int_type_defn;– int_type_defn <= range expr (to | downto )

expr Examples:

– type month is range 1 to 12 ;– type count_down is range 10 downto 0;


Integer type : operations Addition: +, Subtraction or negation: -, Multiplication:

*, Division: / Modulo: mod

– a = b*n + (a mod b), sign of b, n: integer– (-5) mod 3 = 1

Remainder: rem– a = (a/b)*b + (a rem b), sign of a– (-5) rem 3 = 1

Absolute value: abs Exponentiation: ** Logical: =, /=, <, >, <=, >=


Floating-point type

“real” is a pre-defined type used to represent floating-point numbers– variable x, y : real ;

Similar to integers the user can also define new real types with limited range.

Examples– type temp is range –273.0 to 1000.0 ;


Floating point type:operations

Addition: + Subtraction or negation: -• Multiplication: * Division: / Absolute value: abs Exponentiation: ** Logical: =, /=, <, >, <=, >=


Time type Predefined physical type.

type time is range implementation definedunits fs;ps = 1000 fs;ns = 1000 ps;us = 1000 ns;ms = 1000 us;sec = 1000 ms;min = 60 sec;hr = 60 min;

end units;


Enumerated types Useful for giving names to a values of an object

(variable or signal). Example:

– type alu_func is (disable, pass, add, sub, mult, div); Predefined enum types

– type character is ( ‘a’, ‘b’, ‘c’, ……….);Operations: =, /=, <, >, <=, >=

– type boolean is ( false,true);Operations: and, or, nand, nor, xor, xnor, not,=, /=, <, >, <=, >=


Characters ISO 8859-Latin-1 8-bit character set

– type character is (null, soh, ….……….

);

– Example:variable command_char, terminator: character;

command := ‘P’;terminator := cr;


Boolean

Boolean is one of the most important predefined types in VHDL– type boolean is (false, true);– Operations

and, or, nand, nor, xor, xnor, not– and, or, nand, nor are called “short-

circuit” operators


Bit type

Bit is also a predefined enumerated type– type bit is (‘0’, ‘1’);– Operations

Logical: =, /=, <, >, <=, >=Boolean: and, or, nand, nor, xor, xnor, not

Shift:sll, srl, sla, sra, rol, ror


Standard logic Since VHDL is designed to modeling digital hardware, it is

necessary to include types to represent digitally encoded values

type std_logic is (’U’, -- Uninitialized ‘X’, -- Forcing Unknown ‘0’, -- Forcing zero ‘1’, -- Forcing one ‘Z’, -- High impedance ‘W’, -- Weak Unknown ‘L’, -- Weak zero ‘H’, -- Weak one ‘_’); -- don’t care


Subtypes Sub types are useful for limiting the range of base type Examples:

type month is 1 to 31;subtype working_day is month range 1 to 3;

variable x,y : month;variable z : working_day;y = x + z;

subtype natural is integer range 0 to biggest_integer;subtype positive is integer range 1 to biggest_integer;


Type Conversion

Integer to real: read(123) Real to integer: integer(3.6) See Chap 4 for more detail.


Attributes of Scalar Types T’left : first (leftmost) value of T T’right : last (rightmost) value of T T’low : least value of T T’high : highest value of T T’ascending : true if T is ascending, false

otherwise T’image(x) : A string representing the value of x T’value(s) : The value in T that is represented by

s.


Example

type set_index is range 21 downto 11;set_index’left = 21set_index’right = 11set_index’low = 11set_index’high = 21set_index’ascending = falseset_index’image(14) = “14”set_index’value(“20”) = 20Restrictions on coding

style for RTL model


Attributes of Scalar Types

Discrete types are integer and all enumerated types.– T’pos(x): position of x in T– T’val(n): value in T at position n– T’succ(x): successor of x in T– T’pred(x): predecessor of x in T– T’leftof(x): value in T at position one left of x– T’rightof(x): value in T at position one right of x


Example

type logic_level is (unknown, low, undriven, high);

logic_level’pos(unknown) = 0logic_level’val(3) = highlogic_level’succ(unknown) = lowlogic_level’pred(undriven) = low


Expression and Operators

Primary values that can be used in expressions:– Literal values– Identifiers representing data objects (constants,

variables)– Attributes that yield values– Qualified expressions– Type-converted expressions and– Expressions in parentheses


Expression and Operators

VHDL operators– See page 53~54

Scalar Data Types and Operations 大同大學資訊工程系副教授鄭福炯

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Transcript of Scalar Data Types and Operations 大同大學資訊工程系副教授鄭福炯

Scalar Data Types and Operations 大同大學 資訊工程系 副教授 鄭福炯

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Transcript of Scalar Data Types and Operations 大同大學 資訊工程系 副教授 鄭福炯

Scalar Data Types and Operations 大同大學資訊工程系副教授鄭福炯

Transcript of Scalar Data Types and Operations 大同大學資訊工程系副教授鄭福炯