Hardware-oriented types

The intbv class

Hardware design involves dealing with bits and bit-oriented operations. The standard Python type int has most of the desired features, but lacks support for indexing and slicing. For this reason, MyHDL provides the intbv class. The name was chosen to suggest an integer with bit vector flavor.

intbv works transparently with other integer-like types. Like class int, it provides access to the underlying two’s complement representation for bitwise operations. However, unlike int, it is a mutable type. This means that its value can be changed after object creation, through methods and operators such as slice assignment.

intbv supports the same operators as int for arithmetic. In addition, it provides a number of features to make it suitable for hardware design. First, the range of allowed values can be constrained. This makes it possible to check the value at run time during simulation. Moreover, back end tools can determine the smallest possible bit width for representing the object. Secondly, it supports bit level operations by providing an indexing and slicing interface.

intbv objects are constructed in general as follows:

intbv([val=None] [, min=None]  [, max=None])

val is the initial value. min and max can be used to constrain the value. Following the Python conventions, min is inclusive, and max is exclusive. Therefore, the allowed value range is min .. max-1.

Let’s us look at some examples. First, an unconstrained intbv object is created as follows:

>>> a = intbv(24)

After object creation, min and max are available as attributes for inspection. Also, the standard Python function len() can be used to determine the bit width. If we inspect the previously created object, we get:

>>> print a.min
>>> print a.max
>>> print len(a)

As the instantiation was unconstrained, the min and max attributes are undefined. Likewise, the bit width is undefined, which is indicated by a return value 0.

A constrained intbv object is created as follows:

>>> a = intbv(24, min=0, max=25)

Inspecting the object now gives:

>>> a.min
>>> a.max
>>> len(a)

We see that the allowed value range is 0 .. 24, and that 5 bits are required to represent the object.

Sometimes hardware engineers prefer to constrain an object by defining its bit width directly, instead of the range of allowed values. The following example shows how to do that:

>>> a = intbv(24)[5:]

What actually happens here is that first an unconstrained intbv is created, which is then sliced. Slicing an intbv returns a new intbv with the constraints set up appropriately. Inspecting the object now shows:

>>> a.min
>>> a.max
>>> len(a)

Note that the max attribute is 32, as with 5 bits it is possible to represent the range 0 .. 31. Creating an intbv this way has the disadvantage that only positive value ranges can be specified. Slicing is described in more detail in Bit slicing.

To summarize, there are two ways to constrain an intbv object: by defining its bit width, or by defining its value range. The bit width method is more traditional in hardware design. However, there are two reasons to use the range method instead: to represent negative values as observed above, and for fine-grained control over the value range.

Fine-grained control over the value range permits better error checking, as there is no need for the min and max bounds to be symmetric or powers of 2. In all cases, the bit width is set appropriately to represent all values in the range. For example:

>>> a = intbv(6, min=0, max=7)
>>> len(a)
>>> a = intbv(6, min=-3, max=7)
>>> len(a)
>>> a = intbv(6, min=-13, max=7)
>>> len(a)

Bit indexing

As an example, we will consider the design of a Gray encoder. The following code is a Gray encoder modeled in MyHDL:

from myhdl import Signal, delay, Simulation, always_comb, instance, intbv, bin

def bin2gray(B, G, width):
    """ Gray encoder.

    B -- input intbv signal, binary encoded
    G -- output intbv signal, gray encoded
    width -- bit width

    def logic():
        for i in range(width):
            G.next[i] = B[i+1] ^ B[i]

    return logic

This code introduces a few new concepts. The string in triple quotes at the start of the function is a doc string. This is standard Python practice for structured documentation of code.

Furthermore, we introduce a third decorator: always_comb(). It is used with a classic function and specifies that the resulting generator should wait for a value change on any input signal. This is typically used to describe combinatorial logic. The always_comb() decorator automatically infers which signals are used as inputs.

Finally, the code contains bit indexing operations and an exclusive-or operator as required for a Gray encoder. By convention, the lsb of an intbv object has index 0.

To verify the Gray encoder, we write a test bench that prints input and output for all possible input values:

def testBench(width):

    B = Signal(intbv(0))
    G = Signal(intbv(0))

    dut = bin2gray(B, G, width)

    def stimulus():
        for i in range(2**width):
            B.next = intbv(i)
            yield delay(10)
            print "B: " + bin(B, width) + "| G: " + bin(G, width)

    return dut, stimulus

We use the conversion function bin() to get a binary string representation of the signal values. This function is exported by the myhdl package and supplements the standard Python hex() and oct() conversion functions.

As a demonstration, we set up a simulation for a small width:

sim = Simulation(testBench(width=3))

The simulation produces the following output:

% python bin2gray.py
B: 000 | G: 000
B: 001 | G: 001
B: 010 | G: 011
B: 011 | G: 010
B: 100 | G: 110
B: 101 | G: 111
B: 110 | G: 101
B: 111 | G: 100
StopSimulation: No more events

Bit slicing

For a change, we will use a traditional function as an example to illustrate slicing. The following function calculates the HEC byte of an ATM header.

from myhdl import intbv, concat

COSET = 0x55

def calculateHec(header):
    """ Return hec for an ATM header, represented as an intbv.

    The hec polynomial is 1 + x + x**2 + x**8.
    hec = intbv(0)
    for bit in header[32:]:
        hec[8:] = concat(hec[7:2],
                         bit ^ hec[1] ^ hec[7],
                         bit ^ hec[0] ^ hec[7],
                         bit ^ hec[7]
    return hec ^ COSET

The code shows how slicing access and assignment is supported on the intbv data type. In accordance with the most common hardware convention, and unlike standard Python, slicing ranges are downward. The code also demonstrates concatenation of intbv objects.

As in standard Python, the slicing range is half-open: the highest index bit is not included. Unlike standard Python however, this index corresponds to the leftmost item. Both indices can be omitted from the slice. If the leftmost index is omitted, the meaning is to access “all” higher order bits. If the rightmost index is omitted, it is 0 by default.

The half-openness of a slice may seem awkward at first, but it helps to avoid one-off count issues in practice. For example, the slice hex[8:] has exactly 8 bits. Likewise, the slice hex[7:2] has 7-2=5 bits. You can think about it as follows: for a slice [i:j], only bits below index i are included, and the bit with index j is the last bit included.

When an intbv object is sliced, a new intbv object is returned. This new intbv object is always positive, and the value bounds are set up in accordance with the bit width specified by the slice. For example:

>>> a = intbv(6, min=-3, max=7)
>>> len(a)
>>> b = a[4:]
>>> b
>>> len(b)
>>> b.min
>>> b.max

In the example, the original object is sliced with a slice equal to its bit width. The returned object has the same value and bit width, but its value range consists of all positive values that can be represented by the bit width.

The object returned by a slice is positive, even when the original object is negative:

>>> a = intbv(-3)
>>> bin(a, width=5)
>>> b = a[5:]
>>> b
>>> bin(b)

The bit pattern of the two objects is identical within the bit width, but their values have opposite sign.

The modbv class

In hardware modeling, there is often a need for the elegant modeling of wrap-around behavior. intbv instances do not support this automatically, as they assert that any assigned value is within the bound constraints. However, wrap-around modeling can be straightforward. For example, the wrap-around condition for a counter is often decoded explicitly, as it is needed for other purposes. Also, the modulo operator provides an elegant one-liner in many scenarios:

count.next = (count + 1) % 2**8

However, some interesting cases are not supported by the intbv type. For example, we would like to describe a free running counter using a variable and augmented assignment as follows:

count_var += 1

This is not possible with the intbv type, as we cannot add the modulo behavior to this description. A similar problem exist for an augmented left shift as follows:

shifter <<= 4

To support these operations directly, MyHDL provides the modbv type. modbv is implemented as a subclass of intbv. The two classes have an identical interface and work together in a straightforward way for arithmetic operations. The only difference is how the bounds are handled: out-of-bound values result in an error with intbv, and in wrap-around with modbv. For example, the modulo counter above can be modeled as follows:

count = Signal(modbv(0, min=0, max=2**8))
count.next = count + 1

The wrap-around behavior is defined in general as follows:

val = (val - min) % (max - min) + min

In a typical case when min==0, this reduces to:

val = val % max

Unsigned and signed representation

intbv is designed to be as high level as possible. The underlying value of an intbv object is a Python int, which is represented as a two’s complement number with “indefinite” bit width. The range bounds are only used for error checking, and to calculate the minimum required bit width for representation. As a result, arithmetic can be performed like with normal integers.

In contrast, HDLs such as Verilog and VHDL typically require designers to deal with representational issues, especially for synthesizable code. They provide low-level types like signed and unsigned for arithmetic. The rules for arithmetic with such types are much more complicated than with plain integers.

In some cases it can be useful to interpret intbv objects in terms of “signed” and “unsigned”. Basically, it depends on attribute min. if min < 0, then the object is “signed”, otherwise it is “unsigned”. In particular, the bit width of a “signed” object will account for a sign bit, but that of an “unsigned” will not, because that would be redundant. From earlier sections, we have learned that the return value from a slicing operation is always “unsigned”.

In some applications, it is desirable to convert an “unsigned” intbv to a “signed”, in other words, to interpret the msb bit as a sign bit. The msb bit is the highest order bit within the object’s bit width. For this purpose, intbv provides the intbv.signed() method. For example:

>>> a = intbv(12, min=0, max=16)
>>> bin(a)
>>> b = a.signed()
>>> b
>>> bin(b, width=4)

intbv.signed() extends the msb bit into the higher-order bits of the underlying object value, and returns the result as an integer. Naturally, for a “signed” the return value will always be identical to the original value, as it has the sign bit already.

As an example let’s take a 8 bit wide data bus that would be modeled as follows:

data_bus = intbv(0)[8:]

Now consider that a complex number is transferred over this data bus. The upper 4 bits of the data bus are used for the real value and the lower 4 bits for the imaginary value. As real and imaginary values have a positive and negative value range, we can slice them off from the data bus and convert them as follows:

real.next = data_bus[8:4].signed()
imag.next = data_bus[4:].signed()