-of bits specified by (RB) 58:63.
-**Please check if this is correct!!! This condition is taken
-from PowerISA spec page 253, definition of MASK128(x,y)!!!**
-A mask is generated having 0-bits from bit (64-n) through
-bit 63 and 1-bits elsewhere.
-
-The rotated data is ANDed with the generated mask, and ORed
-with contents of RC ANDed with inverted mask.
-The result is placed into register RT.
+of bits specified by (RB) 58:63. The same number of
+shifted bits are taken from register RC and placed into
+the LSBs of the result, RT.
+Additionally, the MSB bits of register RA that would normally
+be discarded by a 64-bit left shift are placed into the
+LSBs of RS.