The answer for the following program is {6,7,8} but I don't understand why, please explain a bit:
module q ();
typedef byte byteq[$];
initial begin
byte ans[$];
ans = byteq'({>>byte{24'h060708}});
$display("A:expect '{6,7,8} get %p", ans);
end
endmodule
The >> operator is not logical shift in this context, but it is called stream operator.
Stream operators determine the order in which blocks of data are streamed: >> causes blocks of data to be streamed in left-to-right order, while << causes blocks of data to be streamed in right-to-left order.
Consider the following lines for example:
$display ("%h", {>>byte{24'h060708}} );
$display ("%h", {<<byte{24'h060708}} );
In both, the number 24'h060708 should be first sliced into bytes (called slice_size). The first one prints the bytes from left to right, whereas the second one prints them from right to left. Therefor, the output is:
060708
080706
Now, in line ans = byteq'({>>byte{24'h060708}}); you are using bit-stream casting, which casts 24'h060708 number sliced in bytes represented from left to right into byteq, which is a queue of bytes.
For a full explanation, refer to IEEE Std 1800-2012 § 11.4.14 Streaming operators (pack/unpack)
The slice_size determines the size of each block, measured in bits. If a slice_size is not specified, the default is 1. If specified, it may be a constant integral expression or a simple type. If a type is used, the block size shall be the number of bits in that type. If a constant integral expression is used, it shall be an error for the value of the expression to be zero or negative.
The stream_operator<<or>>determines the order in which blocks of data are streamed:>>causes blocks of data to be streamed in left-to-right order, while<<causes blocks of data to be streamed in right-to-left order. Left-to-right streaming using>>shall cause the slice_size to be ignored and no re-ordering performed. Right-to-left streaming using<<shall reverse the order of blocks in the stream, preserving the order of bits within each block. For right-to-left streaming using<<, the stream is sliced into blocks with the specified number of bits, starting with the right-most bit. If as a result of slicing the last (left-most) block has fewer bits than the block size, the last block has the size of the remaining bits; there is no padding or truncation.
In your case {>>byte{24'h060708} creates the stream {8'h06, 8'h07, 8'h08}. byteq'() is casting the stream into bytes, but it is unnecessary since the stream is already matches the correct size. ans = {>>byte{24'h060708}}; will have the same result. If you change the stream_operator to << the stream would be {8'h08, 8'h07, 8'h06}.
