Virtual Memory Review Goal: give illusion of a large memory Allow many processes to share single memory Strategy Break physical memory up into blocks (pages) Page might be in physical memory or on disk. Addresses: Generated by lw/sw: virtual Actual memory locations: physical

Memory access Load/store/PC computes a virtual address Need address translation Convert virtual addr to physical addr Use page table for lookup Check virtual address: If page is in memory, access memory with physical address May also need to check access permissions If page is in not in memory, access disk Page fault Slow – so run another program while it’s doing that Do translation in hardware Software translation would be too slow!

Handling a page fault Occurs during memory access clock cycle Handler must: Find disk address from page table entry Choose physical page to replace if page dirty, write to disk first Read referenced page from disk into physical page

TLB: Translation Lookaside Buffer Address translation has a high degree of locality If page accessed once, highly likely to be accessed again soon. So, cache a few frequently used page table entries TLB = hardware cache for the page table Make translation faster Small, usually fully-associative TLB entries contain Valid bit = TLB hit Other housekeeping bits Tag = virtual page number Data = Physical page number Misses handled in hardware (dedicated FSM) or software (OS code reads page table)

TLB Misses TLB miss means one of two things Page is present in memory, need to create the missing mapping in the TLB Page is not present in memory (page fault), need to transfer control to OS to deal with it. Need to generate an exception Copy page table entry to TLB – use appropriate replacement algorithm if you need to evict an entry from TLB.

Optimizations Make the common case fast Speed up TLB hit + L1 Cache hit Do TLB lookup and cache lookup in parallel Possible if cache index is independent of virtual address translation Have cache indexed by virtual addresses

TLB Example , 7.33 Given: 40-bit virtual addr 16KB pages 36-bit physical byte address 2-way set associative TLB with 256 total entries Total page table size? Memory organization?

Page table/address parameters 16KB=2^14, so 16K pages need 14 bits for an offset inside a page. The rest of the virtual address is the virtual page index, and it's = 26 bits long, for 2^26 page table entries. Each entry contains 4 bits for valid/protection/dirty information, and the physical frame number, which is = 22 bits long, for a total of 26 bits. The total page table size is then 26 bits * 2^26 entries = 208 MB

Memory organization tagindexpage offset 26-bit virtual page number valid/etc. Tagphysical page # valid/etc. tagphysical page # ………… == 2-1 MUX 22 physical page #page offset Physical Address 2214 TLB: 128 sets, each w/2 entries

Fun stuff: bootstrapping on x86 You power up your PC, what happens? Hardware reset Initialize all bits of cache, regs, buffers, etc. to a known value; go to real mode EIP (PC) initialized to 0x0000FFF0, base address (CS) initialized to 0xFFFF0000 So execution begins at 0xFFFFFFF0, 16 bytes from top of physical memory, in EPROM. EPROM remapped to this high address by system chipset First things to execute in EPROM set up IDT = interrupt descriptor table Jump to the BIOS

Bootstrapping 2 BIOS: basic input/output system Built-in software that determines what a computer can do without accessing programs from a disk Placed in ROM chip that comes with the PC BIOS and booting BIOS will search devices in specified order for bootable ones Take the current boot disk Load first sector (usually 512 bytes) at 0000:7C00. Look for a special signature (0xAA55) at the end of it (the last two bytes) Signature not there: “Invalid / Non System Disk” error If signature ok, start executing code in this sector Code limited to 512 bytes! Probably jump to secondary boot program, which will load OS

Bootstrapping 3 For hard disks, first sector is the MBR = Master Boot Record MBR has three parts Code to load a bootsector from one of four primary partitions Top level partition table for hard drive, at offset 446. Has four 16-byte records. Magic values at offsets : 0xAA55 Sector 0 of HD partition contains actual bootloader. Either finds/loads OS, or loads secondary bootloader

Inside a bootloader Get disk geometry for floppies Find second-stage loader if need >512 bytes for code. Load it Get/check system info Check for 32-bit CPU Check for fast enough CPU Detect drives and their geometries Find/load config file Present a boot menu to the user Store magic values – which bootloader was used, drive we booted from, etc. Find/validate/copy/load kernel Floppy: when done loading, turn off floppy drive motor :) mov dx,3F2h mov al,0 out dx,al