Expanding a bit on Will’s answer:
There are several mechanisms to get an immediate into an operation or onto the belt. The reader-phase flow-side con operation can supply any non-NaR scalar value of any width (including 16-byte on Mill family members supporting native quad), but not vector values. The reader-phase exu-side operation rd can supply any of a hardware-defined preselected set of values known as popCons (popular constants), both scalar and vector. Both con and rd were used in the little demo program. Lastly, many op-phase exu-side operations such as add, sub, and the relational ops, both scalar and vector, have immediate forms in which the second argument is a small literal encoded in the operation.
Of these, the exu-side ops, both popCons and immediates, are used as entropy-soaks; popCons for entropy in the readerBlock decode block and immediates for the exuBlock decode block. There is also an entropy-soaker for the exu-side writerBlock decode block, but it is NYF. There is no flow-side entropy-soaker, although one can think of the conBlock block and extBlock block as being entropy-soakers for the ops in the flowBlock block. There is nothing to prevent an entropy-soaker operation in flowBlock itself, but we’ve never found an operation that is naturally flow-side that can use a (varying) few bits of immediate.
The take-away on this is that Mill encoding is not perfect maximal entropy, but it is pretty close and much denser than any other CPU encoding.