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Chapter 13: Direct Memory Access

Chapter 13: Direct Memory Access. “…DMA…provides direct access to the memory while the microprocessor is temporarily disabled.” Typical uses of DMA Video displays for refreshing the screen Hard disk reads and writes High-speed memory-to-memory transfers Timing behavior Shown in Fig. 13-1

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Chapter 13: Direct Memory Access

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  1. Chapter 13: Direct Memory Access • “…DMA…provides direct access to the memory while the microprocessor is temporarily disabled.” • Typical uses of DMA • Video displays for refreshing the screen • Hard disk reads and writes • High-speed memory-to-memory transfers • Timing behavior • Shown in Fig. 13-1 • HOLD  HLDA • Microprocessor suspends execution of its program and places its address, data, and control bus into high-impendence (Z) states

  2. Basic DMA Definitions • DMA normally occurs between an I/O device and memory without the use of the CPU • DMA read • Transfers data from the memory to the I/O device • DMA write • Transfers data from an I/O device to memory • DMAC controls both memory and I/O device simultaneously

  3. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip (initialization) 2. DMA request from I/O 3. Bus request from DMAC 4. Bus grant from CPU 5. DMA grant from DMAC 6. Data transfer 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  4. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O (via DREQs, e.g., 4 channels in 8237) 3. Bus request from DMAC 4. Bus grant from CPU 5. DMA grant from DMAC 6. Data transfer 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  5. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O 3. Bus request from DMAC (HRQ, hold request in 8237) 4. Bus grant from CPU 5. DMA grant from DMAC 6. Data transfer 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  6. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O 3. Bus request from DMAC 4. Bus grant from CPU (setting all the bus outputs of processor to Z) 5. DMA grant from DMAC 6. Data transfer 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  7. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O 3. Bus request from DMAC 4. Bus grant from CPU 5. DMA grant from DMAC (via DACKs) 6. Data transfer 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  8. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O 3. Bus request from DMAC 4. Bus grant from CPU 5. DMA grant from DMAC 6. Data transfer (if DRAM read) // MRDC & IOWC signals are controlled 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  9. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O 3. Bus request from DMAC 4. Bus grant from CPU 5. DMA grant from DMAC 6. Data transfer (if DMA write) // MWTC & IORC signals are controlled 7. DMA sends INTR to CPU to inform completion of DMA. IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  10. ADDR DATA MWTC MRDC CPU IOWC 1. CPU sends information for data transfer to DMAC chip 2. DMA request from I/O 3. Bus request from DMAC 4. Bus grant from CPU 5. DMA grant from DMAC 6. Data transfer 7. DMA sends INTR to CPU to inform completion of DMA IORC (Bus Request) (DMA Request) Hold DMAC I/O Memory (Bus Grant) (DMA Grant) HoldA

  11. DMA Operation Initiation • CPU sends information about required data transfer operation to the DMAC chip • Source device/address, destination device/address, data block size, type of data transfer (demand, single, block), etc. • Uses OUT assembly instructions to send DMAC chip this information • DMAC chip requests a DMA to the CPU by asserting the HOLD line (via its HRQ) • CPU acknowledges request by asserting HLDA • Request priority in the microprocessor • Reset > Hold > Interrupt

  12. Three Types of DMA Mode • Demand mode • transfers data until DREQ becomes inactive • Single mode • releases HOLD after each byte of data is transferred • If DREQ is active, DMAC requests a DMA transfer to microprocessor • Block mode • automatically transfers the number of bytes indicated by the count register for the channel

  13. Advanced Topics • Lecture • Cache (5/31) • DRAM (already touched in Chapter 10) • Flash memory-based storage (6/14) • Practice • Introduction to RTL design in Verilog (6/2, LG105) • Two practices (6/7 and 6/9, LG114) • Note: the two practices are run in the same manner as the normal practices. 1st and 2nd sessions (3:20pm~4:00pm, and 4:00pm~4:40pm)

  14. [Source: K. Asanovic, 2008] Processor-DRAM Gap (latency) µProc 60%/year 1000 CPU “Moore’s Law” Processor-Memory Performance Gap:(grows 50% / year) 100 Performance 10 DRAM 7%/year DRAM 1 1988 1986 1987 1989 1990 1991 1992 1993 1994 1995 1996 1980 1981 1982 1983 1984 1985 1997 1998 1999 2000 Time Four-issue 2GHz superscalar accessing 100ns DRAM could execute 800 instructions during time for one memory access!

  15. [Source: J. Kubiatowicz, 2000] What is a cache? • Small, fast storage used to improve average access time to slow memory. • Exploits spatial and temporal locality • In computer architecture, almost everything is a cache! • Registers a cache on variables • First-level cache a cache on second-level cache • Second-level cache a cache on memory • Memory a cache on disk (virtual memory) • TLB a cache on page table • Branch-prediction a cache on prediction information? Proc/Regs L1-Cache Bigger Faster L2-Cache Memory Disk, Tape, etc.

  16. [Source: K. Asanovic, 2008] n loop iterations subroutine call subroutine return argument access vector access scalar accesses Typical Memory Reference Patterns Address Instruction fetches Spatial locality Temporal & Spatial locality Stack accesses Temporal locality Spatial locality Data accesses Time Temporal locality

  17. [Source: K. Asanovic, 2008] Temporal Locality Spatial Locality Memory Reference Patterns Memory Address (one dot per access) Time Donald J. Hatfield, Jeanette Gerald: Program Restructuring for Virtual Memory. IBM Systems Journal 10(3): 168-192 (1971)

  18. [Source: K. Asanovic, 2008] A Typical Memory Hierarchy c.2008 Split instruction & data primary caches (on-chip SRAM) Multiple interleaved memory banks (off-chip DRAM) L1 Instruction Cache Unified L2 Cache Memory CPU Memory Memory L1 Data Cache RF Memory Multiported register file (part of CPU) Large unified secondary cache (on-chip SRAM)

  19. [Source: K. Asanovic, 2008] Itanium-2 On-Chip Caches(Intel/HP, 2002) Level 1, 16KB, 4-way s.a., 64B line, quad-port (2 load+2 store), single cycle latency Level 2, 256KB, 4-way s.a, 128B line, quad-port (4 load or 4 store), five cycle latency Level 3, 3MB, 12-way s.a., 128B line, single 32B port, twelve cycle latency L3 and L2 caches occupy more than 2/3 of total area!

  20. [Source: K. Asanovic, 2008] Workstation Memory System(Apple PowerMac G5, 2003) • Dual 2GHz processors, each has: • 64KB I-cache, direct mapped • 32KB D-cache, 2-way • 512KB L2 unified cache, 8-way • All 128B lines 1GHz, 2x32-bit bus, 16GB/s AGP Graphics Card, 533MHz, 32-bit bus, 2.1GB/s North Bridge Chip Up to 8GB DDR SDRAM, 400MHz, 128-bit bus, 6.4GB/s PCI-X Expansion, 133MHz, 64-bit bus, 1 GB/s

  21. Cache Policies • Inclusion • Placement • Replacement

  22. [Source: K. Asanovic, 2008] Inclusion Policy • Inclusive multilevel cache: • Inner cache holds copies of data in outer cache • External access need only check outer cache • Most common case • Exclusive multilevel caches: • Inner cache may hold data not in outercache • Swap lines between inner/outer caches on miss • Used in AMD Athlon with 64KB primary and 256KB secondary cache Why choose one type or the other? • Cache size matters. • In general, if L2 size >> L1 size, then inclusion policy

  23. [Source: Garcia, 2008] Types of Cache Miss • “Three Cs” • 1st C: Compulsory Misses • Happen when warming up the cache • 2nd C: Conflict Misses • E.g., two addresses are mapped to the same cache line • Solution: increase associativity • 3rd C: Capacity Misses • E.g., sequential access of 40KB data via 32KB data cache

  24. [Source: K. Asanovic, 2008] Placement Policy 1 1 1 1 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 2 2 2 2 2 2 2 2 2 2 0 1 2 3 4 5 6 7 8 9 3 3 0 1 Block Number 0 1 2 3 4 5 6 7 8 9 Memory Conflict miss! Set Number 0 1 2 3 4 5 6 7 0 1 2 3 Cache Fully (2-way) Set Direct Associative Associative Mapped anywhere anywhere in only into set 0 block 4 (12 mod 4) (12 mod 8) block 12 can be placed

  25. [Source: K. Asanovic, 2008] Direct-Mapped Cache Block Offset Tag Index t k b V Tag Data Block 2k lines t = HIT Data Word or Byte

  26. [Source: K. Asanovic, 2008] Placement Policy 1 1 1 1 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 2 2 2 2 2 2 2 2 2 2 0 1 2 3 4 5 6 7 8 9 3 3 0 1 Block Number 0 1 2 3 4 5 6 7 8 9 Memory Conflict miss! Set Number 0 1 2 3 4 5 6 7 0 1 2 3 Cache Fully (2-way) Set Direct Associative Associative Mapped anywhere anywhere in only into set 0 block 4 (12 mod 4) (12 mod 8) block 12 can be placed

  27. [Source: K. Asanovic, 2008] 2-Way Set-Associative Cache Block Offset Tag Index b t k V Tag Data Block V Tag Data Block Set t Data Word or Byte = = HIT

  28. [Source: Garcia, 2008] 4-Way Set Associative Cache Circuit tag index Mux is time consuming!

  29. [Source: K. Asanovic, 2008] Fully Associative Cache V Tag Data Block t = Tag t = HIT Block Offset Data Word or Byte = b

  30. [Source: Garcia, 2008] Fully Associative Cache • Benefit of Fully Assoc Cache • No Conflict Misses (since data can go anywhere) • Drawbacks of Fully Assoc Cache • Need hardware comparator for every single entry • If we have a 64KB of data in cache with 4B entries, we need 16K comparators and 16K input MUX • Infeasible for large size caches • However, used for small size (e.g., 128 entry) caches, e.g., TLB

  31. [Source: K. Asanovic, 2008] Replacement Policy • In an associative cache, which block from a set should be evicted when the set becomes full? • Random • used in highly (fully) associative caches, e.g., TLB • Least Recently Used (LRU) • LRU cache state must be updated on every access • true implementation only feasible for small sets (2-way) • pseudo-LRU binary tree often used for 4-8 way • First In, First Out (FIFO) a.k.a. Round-Robin • used in highly associative caches • Other options, e.g., recent frequently used, etc. • This is a second-order effect. Why? Replacement only happens on misses

  32. [Source: J. Kubiatowicz, 2000] 3Cs Absolute Miss Rate (SPEC92) Conflict Compulsory vanishingly small

  33. [Source: J. Kubiatowicz, 2000] 2:1 Cache Rule miss rate 1-way associative cache size X = miss rate 2-way associative cache size X/2 Conflict

  34. [Source: A. Hartstein, 2006] Rule If the workload is large, the cache miss rate is observed to decrease as a power law of the cache size If the cache size is doubled, the miss rate drops by the factor of

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