Đang tải... (xem toàn văn)
PowerPoint Presentation COMPUTER MEMORY (DTV340) I INTRODUCTION – MEMORY HIERARCHY University of Science Faculty of Electronics and Telecommunication Memory Description Capacity of a memory is describ[.]
University of Science Faculty of Electronics and Telecommunication COMPUTER MEMORY (DTV340) I INTRODUCTION – MEMORY HIERARCHY Memory Description Capacity of a memory is described as # addresses x Word size Memory # of addr # of data lines # of addr lines # of total bytes 1M x 1,048,576 20 MB 2M x 2,097,152 21 MB 1K x 1024 10 512 B 4M x 32 4,194,304 32 22 16 MB 16K x 64 16,384 64 14 128 KB Endianess Big Endian Little Endian Least significant byte of a multi-byte word is stored at the lowest memory address e.g Intel x86 Some embedded & DSP processors would support both for interoperability Most significant byte of a multi-byte word is stored at the lowest memory address e.g Sun Sparc, PowerPC Endianess Most modern computers are byte addressable Word address 0 3 4 7 -3 -4 Store -4 • • • • • • 0x00010203 k Byte address Byte address k -4 k -3 k - k - (a) Big-endian assignment k -4 k - k - k k (a) Little-endian assignment Memory Hierarchy The memory unit is an essential component in any digital computer since it is needed for storing programs and data Not all accumulated information is needed by the CPU at the same time Therefore, it is more economical to use lowcost storage devices to serve as a backup for storing the information that is not currently used by CPU Memory Hierarchy Since 1980, CPU has outpaced DRAM Gap grew 50% per year Memory Hierarchy Q How architects address this gap? A Put smaller, faster “cache” memories between CPU and DRAM Create a “memory hierarchy” Memory Hierarchy The memory unit that directly communicate with CPU is called the main memory Devices that provide backup storage are called auxiliary memory The memory hierarchy system consists of all storage devices employed in a computer system from the slow by high-capacity auxiliary memory to a relatively faster main memory, to an even smaller and faster cache memory Memory Hierarchy The main memory occupies a central position by being able to communicate directly with the CPU and with auxiliary memory devices through an I/O processor A special very-high-speed memory called cache is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate Memory Hierarchy CPU Main Memory Cache Magnetic Disks I/O Processor ROM ROM is used for storing programs that are PERMENTLY resident in the computer and for tables of constants that not change in value once the production of the computer is completed The ROM portion of main memory is needed for storing an initial program called bootstrap loader, witch is to start the computer software operating when power is turned off ROM Memory Model Example 32-bit address space can address up to 4GB (232) different memory locations 0x00000000 0x0A 0x00000001 0xB6 0x00000002 0x41 0x00000003 0xFC 0xFFFFFFFF 0x0D Lower Memory Address Higher Memory Address Flat Memory Model Memory Address Map Memory Address Map is a pictorial representation of assigned address space for each chip in the system To demonstrate an example, assume that a computer system needs 128 bytes of RAM and 512 bytes of ROM The RAM have 128 byte and need seven address lines, where the ROM have 512 bytes and need address lines Memory Address Map ... addressable Word address 0 3 4 7 -3 -4 Store -4 • • • • • • 0x00010203 k Byte address Byte address k -4 k -3 k - k - (a) Big-endian assignment k -4 k - k - k k (a) Little-endian assignment Memory Hierarchy... Read-only memory Flash memory Electrically, byte-level Not possible Write Mechanism Electrically Read-mostly memory Electrically, byte-level Electrically, block-level Volatility Volatile Masks UV light,... main memory is about to 7~10 Auxiliary memory access time is usually 1000 times that of main memory Types of Internal memories Memory Type RAM ROM PROM Category Read-write memory Read-only memory