It is defined that UInt is the type of unsigned integer. But in such case it seems like the MSB is still a sign. e.g., the most relative QA is Chisel UInt negative value error which works out a workaround but no why. Could you enlight me about the 'why'?
The UInt seems to be defined in chisel3/chiselFrontend/src/main/scala/chisel3/core/Bits.scala but I cannot understand the details. Is the UInt is derived from Bits and Bits is derived from Int of scala?
The simple answer is that this is due to how Scala evaluates things. Consider an example like
val x = 0xFFFFFFFF.U
This statement causes an error. UInt literal are represented internally by BigInts, but the 0xFFFFFFFF is an specifying an Int value. 0xFFFFFFFF is equivalent to the Int value -1. The -1 Int value is converted to BigInt -1 and -1.U is illegal because the .U literal creation method will not accept negative values. Adding the L fixes this because 0xFFFFFFFL is a positive Long value.