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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Sunday 9 September 2012

Register Data Types

Formal Definition

Registers provide means for modeling data storage elements.

Simplified Syntax:

reg range list_of_identifiers;

integer list_of_identifiers;

real list_of_identifiers;

time list_of_identifiers;

realtime list_of_identifiers;


Registers are data types that store assigned values until a new assignment occurs. A new value can be assigned to registers only by using procedural assignments.

The reg register is a 1-bit wide data type. If more than one bit is required then range declaration should be used (see Vectors for more explanations). Negative values assigned to reg data type variables are treated as unsigned values (see Arithmetic expressions with integers and registers for more explanations). Reg data type variables can be declared as memory.

The integer register is a 32-bit wide data type. Integer declarations cannot contain range specification. Integer variables can be declared as memory. Integers store signed values.

The time register is a 64-bit wide data type that is used to store simulation time and to check timing dependence. Time type registers store values as unsigned numbers. Time declaration cannot contain range specification. Time data type variables can be declared as memories.

The real register is a 64-bit wide data type that stores floating-point values. Real registers cannot be used with event control, concatenations ({}), modulus operator (%), case equality (===, !==), bit-wise operators (~, &, |, ^, ^~, ~^), reduction operators (^, ~^, ^~, &, &~, |, |~) and shift operators (<<, >>). Bit-selects and part-selects on real type variables are not allowed.

The realtime registers are treated in the same way as real registers.


reg scal;
reg [7:0] vect;
reg [7:0] mem [31:0];
integer i;
integer i_mem [7:0];
time t;
time t_mem [3:0];
real r;
realtime rt1, rt2;

'scal' is a 1-bit wide reg type register.

'vect' is an 8-bit wide reg type register.

'mem' is a reg type memory of 32 8-bit words.

'i' is an integer type register.

'i_mem' is an integer type memory.

't' is a time type register.

't_mem' is a time type memory.

'r' is a real type register.

'rt1' and 'rt2' are realtime type registers.

Important Notes

· New values can be assigned to registers only by using procedural statements.

· Some operators, bit-selects, part-selects, range and memory declaration arecannot be used with real and realtime type registers.

· Range can be specified only for reg data type registers.

· Real and realtime registers are initialized to zero value. Reg, integer and time registers are initialized to unknown value (x).

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