Commit 6c613e1

@@ -142,6 +142,23 @@ Instructions: Please answer the following questions in complete sentences. Your
     kernel mode.
 
 1. Define cache, and explain cache coherency.
+
+    The cache acts as a faster storage system that can store items on a temporary basis.
+    When a piece of data is needed, the cache can be used to fetch the data once
+    and stored in the cache. Subsequent attempts to access the same data can be
+    fetched from the cache at a faster rate of access. Cache is meant to be temporary
+    for fetching frequently used data quickly. However, the cache can become stale
+    and out of sync with the backing storage from where the cache initially retrieved
+    the data. Cache expiration is one of the hard problems in computer science.
+
+    In a multitasking environment if several processes wish to access a piece of data
+    then each process must access the current value of that piece of data. If that
+    piece of data is stored in different caches it is possible for the same piece
+    of data to have a different value if there isn't a mechanism to synchronize each
+    of the caches with any mutations made to the piece of data. Cache coherency
+    ensures that the data stored in different layers of cache are synchronized and
+    this is typically handled in the hardware layer.
+
 1. Describe why direct memory access (DMA) is considered an efficient mechanism for performing I/O.
 1. Describe why multicore processing is more efficient than placing each processor on its own chip.
 1. Describe the relationship between an API, the system-call interface, and the operating system.