Dec1999 CS202 : Computer Architecture and Data Communications (Question 2)Suggested Solutions

(a)	What is the purpose of computer memory? It is the portion of a computer systems that stores data and instructions (1).	[1]
(b)	The various types of memorymaybe described using a number of key characteristics .List two of these. Any two from the following,or similar: Location (internal/external)(1)Capacity (word size/number of words)(1) Unit of transfer (word/block)(1)	[2]
(c)	Brieflydescribe the following memoryaccess methods.For each one give an example of a memorydevice which uses that method. (i)sequential Access is made in a specific linear sequence.Stored addressing information is used to separate records and assist in the retrieval process (1).e.g.magnetic tape (1) (ii)direct Individual blocks or records have a unique address based on physical location. Access is accomplished by direct access to reach a general area,plus sequential seeking to find a specific record (1).e.g.hard disk (1) (iii)random Individual blocks or records have a unique addressable mechanism.The time to access a given location is independent of the sequence of prior accesses and is constant (1). e.g.main memory (1) (iv)associative Enables one to do a comparison of desired bit locations within the word for a specified match.A word in the memory is retrieved based on the portion of its contents (1). e.g.cache memory (1) In each case,give one mark for a reasonable explanation,and another mark for a orrect example.	[8]
(d)	Why are cache replacement algorithms needed? How maytheyhave a significant impact on performance? Award marks for these or other valid points;two marks for ‘why ’and two for ‘how ’: When a new block is brought into the cache,one of the existing blocks must be replaced (1). An algorithm must be used to determine which block should be replaced (1).The block to be replaced should be the least likely to be needed (1),otherwise blocks will regularly be being fetched and overwritten,giving poor performance (1).	[4]