加法可以优化啊

typedef enum logic [1:0] {S0, S1, S2} statetype; The enumerated encodings default to numerical order: S0 = 00, S1 = 01, and S2 = 10.

The most common type of net is called a wire or tri. These two types are synonymous, but wire is conventionally used when a single driver is present and tri is used when multiple drivers are present. Thus, wire is obsolete in SystemVerilog because logic is preferred for signals with a single driver.

#(parameter width = 8)
mux2 #(12) lowmux(d0, d1, s[0], low);

SystemVerilog allows a #(parameter . . .) statement before the inputs and outputs to define parameters. Do not confuse the use of the # sign indicating delays with the use of #(...) in defining and overriding parameters.

A testbench is an HDL module that is used to test another module, called the device under test (DUT). Some tools also call the module to be tested the unit under test (UUT).

assert (y === 1) else $error("000 failed.");

The SystemVerilog assert statement checks whether a specified condition is true. If not, it executes the else statement. The $error system task in the else statement prints an error message describing the assertion failure. assert is ignored during synthesis. The === and !== operators work correctly with operands that could be x or z.

When X appears in a truth table, it indicates that the value of the variable in the truth table is unimportant (can be either 0 or 1). When X appears in a circuit, it means that the circuit node has an unknown or illegal value.

The fundamental reason that large ripple-carry adders are slow is that the carry signals must propagate through every bit in the adder. A carry-lookahead adder (CLA) is another type of carry propagate adder that solves this problem by dividing the adder into blocks and providing circuitry to quickly determine the carry out of a block as soon as the carry in is known. Thus, it is said to look ahead across the blocks rather than waiting to ripple through all the full adders inside a block. For example, a 32-bit adder may be divided into eight 4-bit blocks.

Prefix adders extend the generate and propagate logic of the carry-lookahead adder to perform addition even faster.

Hardware description languages provide the + operation to specify a CPA. Modern synthesis tools select among many possible implementations, choosing the cheapest (smallest) design that meets the speed requirements.

If ALUControl = 00, the output multiplexer chooses A + B. If ALUControl = 01, the ALU computes A − B...

Some ALUs produce extra outputs, called flags, that indicate information about the ALU output.

Multiplication of 1-bit binary numbers is equivalent to the AND operation. As with adders, many different multiplier designs with different speed/cost trade-offs exist. Synthesis tools may pick the most appropriate design given the timing constraints.

A multiply accumulate operation multiplies two numbers and adds them to a third number—typically, the accumulated value. These operations, also called MACs, are often used in digital signal processing (DSP) algorithms such as the Fourier transform, which requires a summation of products.

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