You should see a bunch of info, of which are four of the above mentioned registers. Now, type in "RAX {enter}". Then type in a number like 8FABh. Enter "R" again and noticed how the accumulator (AX) has change its number.
These general purpose registers can also be split in half into its higher and lower order components. Instead of having one register AX, you can have two registers, AH and AL. Note however that while you have a range of 0 to FFFFh for AX, you will now have a range of 0 to FF for AH and AL. You cannot change these directly in debug, but be aware that programs will use it. If AX contains 0A4Ch, then AH will contain 0Ah and AL will contain 4Ch.