Overview

Overview EE 4504 Computer Organization Section 3 Computer Memory Historically, the limiting factor in a computer’s performance has been memory access time – Memory speed has been slow compared to the speed of the processor – A process could be bottlenecked by the memory system’s inability to “keep up” with the processor Our goal in this section is to study the development of an effective memory organization that supports the processing power of the CPU – General memory organization and performance – “Internal” memory components and their use – “External” memory components and their use Reading: Text, chapters 4 and 5 EE 4504 Section 3 1 EE 4504 Section 3 2 1 Terminology Capacity: the amount of information that can be contained in a memory unit --usually in terms of words or bytes Word: the natural unit of organization in the memory, typically the number of bits used to represent a number Addressable unit: the fundamental data element size that can be addressed in the memory -- typically either the word size or individual bytes Unit of transfer: The number of data elements transferred at a time -- usually bits in main memory and blocks in secondary memory Transfer rate: Rate at which data is transferred to/from the memory device Access time: – For RAM, the time to address the unit and perform the transfer – For non-random access memory, the time to position the R/W head over the desired location Memory cycle time: Access time plus any other time required before a second access can be started Access technique: how are memory contents accessed – Random access: » Each location has a unique physical address » Locations can be accessed in any order and all access times are the same » What we term “RAM” is more aptly called read/write memory since this access technique also applies to ROMs as well » Example: main memory EE 4504 Section 3 3 EE 4504 Section 3 4 2 – Sequential access: » Data does not have a unique address » Must read all data items in sequence until the desired item is found » Access times are highly variable » Example: tape drive units – Direct access: » Data items have unique addresses » Access is done using a combination of moving to a general memory “area” followed by a sequential access to reach the desired data item » Example: disk drives – Associative access: » A variation of random access memory » Data items are accessed based on their contents rather than their actual location » Search all data items in parallel for a match to a given search pattern » All memory locations searched in parallel without regard to the size of the memory Extremely fast for large memory sizes » Cost per bit is 5-10 times that of a “normal” RAM cell » Example: some cache memory units EE 4504 Section 3 5 EE 4504 Section 3 6 3 Memory Hierarchy Major design objective of any memory system – To provide adequate storage capacity at – An acceptable level of performance – At a reasonable cost Four interrelated ways to meet this goal – Use a hierarchy of storage devices – Develop automatic space allocation methods for efficient use of the memory – Through the use of virtual memory techniques, free the user from memory management tasks – Design the memory and its related interconnection structure so that the processor can operate at or near its maximum rate Basis of the memory hierarchy – Registers internal to the CPU for temporary data storage (small in number but very fast) – External storage for data and programs (relatively large and fast) – External permanent storage (much larger and much slower) Characteristics of the memory hierarchy – Consists of distinct “levels” of memory components – Each level characterized by its size, access time, and cost per bit – Each increasing level in the hierarchy consists of modules of larger capacity, slower access time, and lower cost/bit Goal of the memory hierarchy – Try to match the processor speed with the rate of information transfer from the lowest element in the hierarchy EE 4504 Section 3 7 EE 4504 Section 3 8 4 Memory Technology Size Access Type Time Registers in the CPU Cache Main memory Disk cache Magnetic disk Cache Main Memory Magnetic Disk Optical Disk Semiconductor RAM Semiconductor RAM Hard Disk CD-ROM 128-512 KB 4-128 MB Gigabyte Gigabyte 10 ns 50 ns 10 ms, 10 MB/sec 300 ms, 600 KB/sec Magnetic Tape Optical disk Magnetic tape Tape 100s MB Sec-min., 10MB/min Typical memory Parameters The memory hierarchy EE 4504 Section 3 9 EE 4504 Section 3 10 5 ... - --nqh--