L17-4up - Virtual Memory You heard me right, kid. GIGABYTES...

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L17 – Virtual Memory 1 6.004 – Fal 2010 11/4/10 Virtual Memory You heard me right, kid. GIGABYTES of main memory! Quiz #3 tomorrow! L17 – Virtual Memory 2 6.004 – Fal 2010 11/4/10 Lessons from History… There is only one mistake that can be made in computer design that is di f cult to recover from—not having enough address bits for memory addressing and memory management. Gordon Bell and Bill Strecker speaking about the PDP-11 in 1976 A partial list of successful machines that eventually starved to death for lack of address bits includes the PDP 8, PDP 10, PDP 11, Intel 8080, Intel 8086, Intel 80186, Intel 80286, Motorola 6800, AMI 6502, Zilog Z80, Cray-1, and Cray X-MP. ± erson Why? Address size determines minimum width of anything that can hold an address: PC, registers, memory words, HW for address arithmetic (BR/JMP, LD/ST). When you run out of address space it’s time for a new ISA! L17 – Virtual Memory 3 6.004 – Fal 2010 11/4/10 Top 10 Reasons for a BIG Address Space 10. Keeping Micron and Rambus in business. 9. Unique addresses within every internet host. 8. Generating good 6.004 quiz problems. 7. Performing 32-bit ADD via table lookup 6. Support for meaningless advertising hype 5. Emulation of a Turing Machine’s tape. 4. Bragging rights at geek parties. 3. Isolating ISA from IMPLEMENTATION details of HW con±guration shouldn’t enter into SW design 2. Usage UNCERTAINTY provide for run-time expansion of stack and heap 1. Programming CONVENIENCE create regions of memory with di f erent semantics: read-only, shared, etc. avoid annoying bookkeeping L17 – Virtual Memory 4 6.004 – Fal 2010 11/4/10 Squandering Address Space Address Space CODE, large monolithic programs (eg, O F ce, Netscape). ... • only small portions might be used • add-ins and plug-ins • shared libraries/DLLs ••• STACK: How much to reserve? (consider RECURSION!) HEAP: N variable-size data records. .. Bound N? Bound Size? OBSERVATIONS: • Can’t BOUND each usage. .. without compromising use. • Actual use is SPARSE • Working set even MORE sparse
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L17 – Virtual Memory 5 6.004 – Fal 2010 11/4/10 Extending the Memory Hierarchy So, we’ve used SMALL fast memory + BIG slow memory to fake BIG FAST memory. Can we combine RAM and DISK to fake DISK size at RAM speeds? VIRTUAL MEMORY • use of RAM as cache to much larger storage pool, on slower devices • TRANSPARENCY - VM locations "look" the same to program whether on DISK or in RAM. • ISOLATION of RAM size from software. CPU FAST STATIC "CACHE" DYNAMIC RAM "MAIN MEMORY" 3x-20x "Secondary Storage" DISK 10 4 x-10 5 x L17 – Virtual Memory 6 6.004 – Fal 2010 11/4/10 Virtual Memory ILLUSION: Huge memory (2 32 bytes? 2 64 bytes?) ACTIVE USAGE: small fraction (2 24 bytes?) HARDWARE: • 2 30 (1 G) bytes of RAM • 2 37 (128 G) bytes of DISK. .. ... maybe more, maybe less!
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L17-4up - Virtual Memory You heard me right, kid. GIGABYTES...

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