In register window concept total number of regiters are divided in to windows.Each window is used to store a procedure's local variables and input parameters and and output parameters when it is called.
Caller procedures output parameter registers are callee procedures input parameters.
When program moves from one procedure to other procedure (Procedure1 calls procedure 2) then new window of registers are allocated to new procedure. This nested ness will be supported for maximum length 16. If it excedes 16 then 1st register window will be used and before that the content of the 1st window will be moved to memory.
Advantage of this model is lesser no of data movements from memory to registers.
One more important detail about this model is input registers of callee is shared with output registers of caller. Why ? (Easy think about it)
So now total no of registers = Global registers + no of windows ( local registers per each window + input/output parameter registers)
208 = Global registers + 16(8 + 4)
208 = Global registers + 192
Global registers = 208-192 = 16