programs-variables.md

Specifying Program Variables

Like the IO ports, program state is declared at the top of the program file, before we specify any program behaviour.

Grammar:

<program_variable> :=  <const_var>
                    |  <reg_var>
                    |  <comb_var>

<const_var>        := "const" <var_name> [<var_width>] "=" <expression>

<reg_var>          := "reg" <var_name> [<var_width>]

<comb_var>         := "comb" <var_name> [<var_width>] "=" <expression>

<var_name>         := (a-zA-Z)[a-zA-Z0-9_]*

<var_width>        := "[" <number> ":" <number> "]"

<number>           := (0-9)+

Examples:

const low_halfword   = 32'h0000FFFF
reg  program_counter [31:0]
comb status_bits     [5:0] = {overflow, 3'b000, error}
reg overflow

As with ports, omitting the explicit range declaration (as with the overflow signal above) will make it a 1-bit variable.

Program Variable Properties

name

The name of the variable used to refer to it in programs. Variable names can contain any alpha-numeric characters and underscores.

range

How wide is the variable? Specified as a list where the first element is the highest bit, while the second element is the low bit.

type

Variables can be declared as registers using reg, constants using const or wires using comb.

Register variables are updated only when they are written to by an instruction.

Wire ports are updated continuously whenever a variable in their assignment expression is changed.

Constant ports are assigned to on declaration and take only one value.