CSCI 320 — Chapter 4 comments

The PowerPoint

SystemVerilog — some odd parts

Default declarations

If you don’t declare a variable, the variable is a wire.

Legal programs may not compile

Few, if any Verilog/SystemVerilog/VHDL systems, will compile all programs. Even fewer will be synthesizable on any device. Even fewer will be synthesizable on the device you plan to use.

Wires and registers

Verilog supported two bit-level types wire and reg which caused great anguish for Verilog programmers. For the most part, the logic type of SystemVerilog, can be used in place of both wire and reg.

If you are worried about the differences, read the UC-Berkeley CS150 handout Verilog: wire vs. reg or a useful stackoverflow posting which explains the difference.

Packed vs unpacked

SystemVerilog (and Verilog) have packed and unpacked arrays. (These are very different than the packed structures of Pascal: Not that any of you have heard of that.) This results in declarations such as the following: 

logic [11:0] addr ;           // packed
logic data [15:0] ;           // unpacked
logic [15:0] mem [1023:0] ;   // unpacked collection of packed

Packed arrays are often called vectors. A vector must be implemented as a contiguous set of bits and, for that reason, must be constructed from bit-like types, such as logic. Often bit slices, such as data[15:8] are used to refer to part of a vector.

PS: It’s really much more complicated than this.

Equality and matching operators

In SystemVerilog, both 0 == x and 0 != x yield the value x; however 0 === x yields 1 and 0 !== x yields 0. (This is very different from the === operator of JavaScript and PHP.)

There is also wildcard equality which can be used to match logic values. With wildcard matching x and z will match anything. The means that both 2'b00 ==? 2'b0x and 2'b10 !=? 2'b0x yield 1. Wildcards only work in the right operand: Both 2'b0x ==? 2'b00 and 2'b0x !=? 2'b10 yield x. (At least, I think that’s the case.)

Being logical

SystemVerilog supports the logical implication, ->, and logical equivalence, <->, beloved by mathematicians everywhere. (Or, at least by those mathematicians who consider x and z to be Boolean values.)

Truth tables

Truth tables, with don’t cares, can be implemened with a user defined primitive.

Named arguments to modules

Python allows values to be passed to function as either position arguments, such as saturate(A, lo, hi), or keyword arguments, such as saturate(vector=A, lowerbound=lo, upperbound=hi).

SystemVerilog does something similar with named and positional associations. In the introductory SystemVerilog lab the following example of positional association was used to connect the testbench to the device-under-test: 

  fulladder dut(vin[2], vin[1], vin[0], vout[1], vout[0]) ;

However, this could also be done with name association:

  fulladder dut(
    .An    (vin[2]),
    .Bn    (vin[1]),
    .n     (vin[0]),
    .Cnp1  (vout[1]),
    .Sn    (vout[0]) ) ;

Name association is certainly a lot more verbose. However, computer engineers tend to refer to ports with names, such as data_in, rather than numbers, such as 5 for the 5’th connection.

Escaped identifiers

A variable started with a backslash can contain any printable characters. These must be terminated by whitespace. Here are some examples:

This allows the SystemVerilog developer to use the exact same names used by the hardware people.

Building a computer

You can implement your own computer with an FPGA as a System on a chip. There are many open cores that allow this, or you can license an ARM Cortex-M1.

The Spring 2014 homework

Sometimes you need to think like a programmer.