An 85-year-old man presents with a sudden onset of severe occipital headache and has vomited on two occasions since the headache began. He also complains of double vision. He has a history of hypeension. His examination findings are : - Glasgow Coma Scale 15/15 - Mild neck stiffness noted - Power 5/5 in both upper and lower limbs. Where is the likely site of pathology?