7-1 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Computer Architecture and Organization Miles Murdocca and Vincent Heuring Chapter 7 – Memory

7-2 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Chapter Contents 7.1 The Memory Hierarchy 7.2 Random-Access Memory 7.3 Memory Chip Organization 7.4 Case Study: Rambus Memory 7.5 Cache Memory 7.6 Virtual Memory 7.7 Advanced Topics 7.8 Case Study: Associative Memory in Routers 7.9 Case Study: The Intel Pentium 4 Memory System

7-3 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring The Memory Hierarchy

7-4 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Functional Behavior of a RAM Cell Static RAM cell (a) and dynamic RAM cell (b).

7-5 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Simplified RAM Chip Pinout

7-6 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring A Four-Word Memory with Four Bits per Word in a 2D Organization

7-7 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring A Simplified Representation of the Four-Word by Four-Bit RAM

7-8 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring 2-1/2D Organization of a 64-Word by One-Bit RAM

7-9 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Two Four-Word by Four-Bit RAMs are Used in Creating a Four-Word by Eight-Bit RAM

7-10 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Two Four-Word by Four-Bit RAMs Make up an Eight-Word by Four-Bit RAM

7-11 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Single-In-Line Memory Module 256 MB dual in-line memory module organized for a 64-bit word with 16 16M × 8-bit RAM chips (eight chips on each side of the DIMM).

7-12 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Single-In- Line Memory Module Schematic diagram of 256 MB dual in-line memory module. (Source: adapted from s.ti.com/sc/ds/tm4en64 kpu.pdf.)

7-13 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring A ROM Stores Four Four-Bit Words

7-14 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring A Lookup Table (LUT) Implements an Eight-Bit ALU

7-15 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Flash Memory (a) External view of flash memory module and (b) flash module internals. (Source: adapted from HowStuffWorks.com.)

7-16 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Cell Structure for Flash Memory Current flows from source to drain when a sufficient negative charge is placed on the dielectric material, preventing current flow through the word line. This is the logical 0 state. When the dielectric material is not charged, current flows between the bit and word lines, which is the logical 1 state.

7-17 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Rambus Memory Comparison of DRAM and RDRAM configurations.

7-18 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Rambus Memory Rambus technology on the Nintendo 64 motherboard (left) enables cost savings over the conventional Sega Saturn motherboard design (right). Nintendo 64 game console:

7-19 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Placement of Cache Memory in a Computer System The locality principle: a recently referenced memory location is likely to be referenced again (temporal locality); a neighbor of a recently referenced memory location is likely to be referenced (spatial locality).

7-20 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring An Associative Mapping Scheme for a Cache Memory

7-21 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Associative Mapping Example Consider how an access to memory location (A035F014) 16 is mapped to the cache for a 2 32 word memory. The memory is divided into 2 27 blocks of 2 5 = 32 words per block, and the cache consists of 2 14 slots: If the addressed word is in the cache, it will be found in word (14) 16 of a slot that has tag (501AF80) 16, which is made up of the 27 most significant bits of the address. If the addressed word is not in the cache, then the block corresponding to tag field (501AF80) 16 is brought into an available slot in the cache from the main memory, and the memory reference is then satisfied from the cache.

7-22 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Associative Mapping Area Allocation Area allocation for associative mapping scheme based on bits stored:

7-23 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Replacement Policies When there are no available slots in which to place a block, a replacement policy is implemented. The replacement policy governs the choice of which slot is freed up for the new block. Replacement policies are used for associative and set-associative mapping schemes, and also for virtual memory. Least recently used (LRU) First-in/first-out (FIFO) Least frequently used (LFU) Random Optimal (used for analysis only – look backward in time and reverse- engineer the best possible strategy for a particular sequence of memory references.)

7-24 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring A Direct Mapping Scheme for Cache Memory

7-25 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Direct Mapping Example For a direct mapped cache, each main memory block can be mapped to only one slot, but each slot can receive more than one block. Consider how an access to memory location (A035F014) 16 is mapped to the cache for a 2 32 word memory. The memory is divided into 2 27 blocks of 2 5 = 32 words per block, and the cache consists of 2 14 slots: If the addressed word is in the cache, it will be found in word (14) 16 of slot (2F80) 16, which will have a tag of (1406) 16.

7-26 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Direct Mapping Area Allocation Area allocation for direct mapping scheme based on bits stored:

7-27 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring A Set Associative Mapping Scheme for a Cache Memory

7-28 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Set-Associative Mapping Example Consider how an access to memory location (A035F014) 16 is mapped to the cache for a 2 32 word memory. The memory is divided into 2 27 blocks of 2 5 = 32 words per block, there are two blocks per set, and the cache consists of 2 14 slots: The leftmost 14 bits form the tag field, followed by 13 bits for the set field, followed by five bits for the word field:

7-29 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Set Associative Mapping Area Allocation Area allocation for set associative mapping scheme based on bits stored:

7-30 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Cache Read and Write Policies

7-31 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Hit Ratios and Effective Access Times Hit ratio and effective access time for single level cache: Hit ratios and effective access time for multi-level cache:

7-32 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Direct Mapped Cache Example Compute hit ratio and effective access time for a program that executes from memory locations 48 to 95, and then loops 10 times from 15 to 31. The direct mapped cache has four 16-word slots, a hit time of 80 ns, and a miss time of 2500 ns. Load-through is used. The cache is initially empty.

7-33 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Table of Events for Example Program

7-34 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Calculation of Hit Ratio and Effective Access Time for Example Program

7-35 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Multi-level Cache Memory As an example, consider a two-level cache in which the L1 hit time is 5 ns, the L2 hit time is 20 ns, and the L2 miss time is 100 ns. There are 10,000 memory references of which 10 cause L2 misses and 90 cause L1 misses. Compute the hit ratios of the L1 and L2 caches and the overall effective access time. H 1 is the ratio of the number of times the accessed word is in the L1 cache to the total number of memory accesses. There are a total of 85 (L1) and 15 (L2) misses, and so: (Continued on next slide.)

7-36 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Multi-level Cache Memory (Cont’) H 2 is the ratio of the number of times the accessed word is in the L2 cache to the number of times the L2 cache is accessed, and so: The effective access time is then: = 5.23 ns per access

7-37 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Neat Little LRU Algorithm A sequence is shown for the Neat Little LRU Algorithm for a cache with four slots. Main memory blocks are accessed in the sequence: 0, 2, 3, 1, 5, 4.

7-38 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Cache Coherency The goal of cache coherence is to ensure that every cache sees the same value for a referenced location, which means making sure that any shared operand that is changed is updated throughout the system. This brings us to the issue of false sharing, which reduces cache performance when two operands that are not shared between processes share the same cache line. The situation is shown below. The problem is that each process will invalidate the other’s cache line when writing data without a real need, unless the compiler prevents this.

7-39 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Overlays A partition graph for a program with a main routine and three subroutines:

7-40 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Virtual Memory Virtual memory is stored in a hard disk image. The physical memory holds a small number of virtual pages in physical page frames. A mapping between a virtual and a physical memory:

7-41 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Page Table The page table maps between virtual memory and physical memory.

7-42 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Using the Page Table A virtual address is translated into a physical address: Typical page table entry

7-43 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Using the Page Table (cont’) The configuration of a page table changes as a program executes. Initially, the page table is empty. In the final configuration, four pages are in physical memory.

7-44 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Segmentation A segmented memory allows two users to share the same word processor code, with different data spaces:

7-45 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Fragmentation (a) Free area of memory after initial- ization; (b) after fragment- ation; (c) after coalescing.

7-46 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Translation Lookaside Buffer An example TLB holds 8 entries for a system with 32 virtual pages and 16 page frames.

7-47 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Putting it All Together An example TLB holds 8 entries for a system with 32 virtual pages and 16 page frames.

7-48 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Content Addressable Memory – Addressing Relationships between random access memory and content addressable memory:

7-49 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Overview of CAM Source: (Foster, C. C., Content Addressable Parallel Processors, Van Nostrand Reinhold Company, 1976.)

7-50 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Addressing Subtrees for a CAM

7-51 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Associative Memory in Routers A simple network with three routers. The use of associative memories in high-end routers reduces the lookup time by allowing a search to be performed in a single operation. The search is based on the destination address, rather than the physical memory address. Access methods for this memory have been standardized into an interface interoperability agreement by the Network Processing Forum.

7-52 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring Block Diagram of Dual-Read RAM A dual-read or dual-port RAM allows any two words to be simultaneously read from the same memory.

7-53 Chapter 7 - Memory Computer Architecture and Organization by M. Murdocca and V. Heuring © 2007 M. Murdocca and V. Heuring The Intel 4 Pentium Memory System