A specific type of bit-level manipulation consists in setting or clearing one single bit in a multi-bit value, given its index and its new value. This operation can be implemented in hardware by a BitSet circuit with the following interface:
- Input x is a 4-bit value representing the original value.
- Output y is a 4-bit value representing the modified value, after the bit-set
operation. - Input index is a 2-bit value, ranging from 0 to 3, indicating the index of the bit to modify.
- Input value is a 1-bit value set to 0 or 1, indicating the value that bit index should take in output y. Every other bit in y should match the corresponding bit in x.
Here's the code I have which I took from examples in class:
module BitSet(input [3:0]x,
input [1:0]index,
input value,
output [3:0]y);
always@(x,index,value);
begin
if (index = 2'b00)
y[0] = value;
if(index = 2'b01)
y[1]=value;
if(index = 2'b10)
y[2]=value;
if(index=2'b11)
y[3]=value;
end
endmodule
and here's the testbench:
module BitSet_tb();
reg [3:0]x;
reg [1:0]index;
reg value;
wire [3:0]y;
BitSet uut(
.x(x),
.index(index),
.value(value),
.y(y)
);
initial begin
$monitor ("%d %b %b %b %b", $time, x, index, value, y);
x=4'b0000;
index=2'b00;
value=1'b0;
#10 x=4'b0001;
index=2'b01;
value=1'b0;
#10 x=4'b1111;
index=2'b10;
value=1'b0;
#10 x=4'b1111;
index=2'b11;
value=1'b0;
#10 $finish;
end
endmodule
When compiling, I get the following errors:
bitset.v:10: syntax error
bitset.v:12: error: invalid module item.
bitset.v:13: syntax error
bitset.v:14: error: invalid module item.
bitset.v:15: syntax error
bitset.v:16: error: invalid module item.
bitset.v:17: syntax error
bitset.v:18: error: invalid module item.
bitset.v:19: syntax error
Best Answer
You can simplify your code greatly by not trying to shoe-horn everything into procedural blocks.
Think about how you would implement this with actual logic components. You'd have a decoder and 4 multiplexers.
You can represent that in Verilog with just 4 assignments along the lines of
As for the errors in your code, I suspect they come from assigning to
y
in a procedural block wheny
was not declared as areg
.Also, your code has a functionality error, because it doesn't implement the requirement "Every other bit in y should match the corresponding bit in x."