There are 24 registers available in the Retroputer CPU numbered r0 through r23. These registers are either sixteen bits wide (even registers) or eight bytes wide (odd registers). The low even register bytes are the same as the odd register bytes meaning that it is possible to access just the low bytes or the high bytes of a sixteen bit register.
In machine language, the access to an even register is always assumed to be a sixteen bit request. As such, it is not possible to directly access the high level bytes of an even register as if it were an eight bit register.
The odd registers are always assumed to be eight bit requests.