BSV by Example - Computation Structures Group

Data Type Conversion Functions
Function Type Class Description
pack Bits Converts (packs) from types in the Bits class, including
Bool, Int, and UInt to Bit.
unpack Bits Converts (unpacks) from Bit to other types in Bits class,
including Bool, Int and UInt.
extend BitExtend Performs either a zeroExtend or a signExtend as appropriate,
depending on the data type of the argument.
zeroExtend BitExtend Increases the bit width of an expression by padding the
most significant end with zeros. Normally used for unsigned
values.
signExtend BitExtend Increases the bit width of an expression by replicating the
most significant bit. Normally used for signed values.
truncate BitExtend Shortens the bit width to a particular width.
fromInteger Literal Converts from an Integer type to any type defined in the
Literal type class. Integers are most often used during
static elaboration.
fromReal RealLiteral Converts from a Real type to any type defined in the
valueOf Any numeric
type
3.6 Some common scalar types
RealLiteral type class.
Converts from a numeric type to an Integer. Numeric
types are the n used in types such as Bit#(n) and
Vector#(n, t).
The following pre-defined scalar types are all in the Bits typeclass. All hardware types (values
carried on wires, stored in registers, FIFOs or memories, etc.) must be in the Bits typeclass (but
note, arbitrary types can be used during static elaboration).
3.6.1 Bit#(n)
The most basic type is Bit#(n), a polymorphic data type defining a type consisting of n bits.
Verilog programmers will naturally want to only ever use Bit#(n) as this gives them the complete
bit selecting and operating freedom they are used to with Verilog. But Verilog also allows a lot of
dubious operations on data, for example, automatic truncation. And more importantly, a Bit#(n)
doesn’t really indicate anything about the semantics of the data.
In BSV, we want to take advantage of strong typing by using meaningful data types. This decreases
bugs and helps find bugs earlier in the design cycle, at compile time, rather than at debug or tapeout
time.
The familiar Verilog type bit is a synonym for Bit#(1).
3.6.2 Bool
We use Bool as opposed to bit or Bit#(1) to define true/false logical operations. In BSV, all
“conditions” (in rules, methods, if statements, while statements, etc.) require Bool types.
typedef enum {False, True} Bool;
The implementation of a Bool is one bit. But, be aware, Bool is not considered type-equivalent to
Bit#(1). Use Bool for True or False values and Bit#(1) for variables with a value of 0 or 1. You
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