https://homeostasis.scs.carleton.ca/wiki/index.php?action=history&feed=atom&title=Operating_Systems_2015F_Lecture_13 2026-05-01T05:38:15Z Revision history for this page on the wiki MediaWiki 1.42.1 https://homeostasis.scs.carleton.ca/wiki/index.php?title=Operating_Systems_2015F_Lecture_13&diff=20349&oldid=prev 2015-10-21T19:04:35Z

<p>Created page with &quot;==Video== The video for the lecture given on October 21, 2015 [http://homeostasis.scs.carleton.ca/~soma/os-2015f/lectures/comp3000-2015f-lec13-21Oct2015.mp4 is now available]...&quot;</p> <p><b>New page</b></p><div>==Video==<br /> <br /> The video for the lecture given on October 21, 2015 [http://homeostasis.scs.carleton.ca/~soma/os-2015f/lectures/comp3000-2015f-lec13-21Oct2015.mp4 is now available].<br /> <br /> <br /> ==Notes==<br /> <br /> &lt;pre&gt;<br /> ----------<br /> <br /> Project<br /> - know what you are curious about<br /> - digging into the kernel<br /> <br /> Interview<br /> - purpose: are you hacking the kernel<br /> - test of how bloody are your knuckles<br /> - don&#039;t talk to me until you&#039;ve spent at least 10 hours on your own looking at code (outside of tutorial)<br /> - for example, what code runs when I press a key?<br /> - pass/fail (100 or 0) - and you can come back<br /> - you can come in starting NOW<br /> - I am available Wednesday and Friday during the break<br /> - email to make an appointment, giving at least 3 times you can come in for 15-30 minutes.<br /> - after, Mondays and Tuesdays are best<br /> <br /> <br /> Virtual Memory<br /> - TLB stores a cache of virtual to physical address mappings<br /> - cache of WHAT<br /> - need a data structure that maps an entire address space (2^32 or 2^64) memory locations.<br /> - virtual and physical address space may be different<br /> - generally you have less physical memory than virtual memory,<br /> so can use fewer bits in physical addresses<br /> - for 64 bit systems, physical addresses are like 48 bits<br /> or a few more<br /> - but sometimes you get physical addresses that are larger that virtual addresses<br /> - apple 2&#039;s had an 8 bit processor with 16 bit addresses<br /> - to address more than 64K, you did &quot;bank switching&quot;,<br /> one set of 16K addresses were mapped onto arbitrary banks of upper memory, using a &quot;selector&quot; address.<br /> - for 32 bit addresses, that&#039;s 4G of RAM<br /> - intel makes processors with extensions to access more ram (PAE)<br /> - each process is limited to 4G, but kernel deals with mess of extra RAM<br /> <br /> <br /> - mapping memory is expensive, do it in chunks<br /> - should the chunks be variable or fixed sized?<br /> - variable sized chunks are too hard to pack<br /> (segments, with a base address and a bound address)<br /> segments are still used in executable file formats<br /> and for setting permissions on ranges of memory<br /> (sometimes)<br /> - fixed sized chunks of memory are pages<br /> 4K or 8K<br /> sometimes really big (2M), for mapping video memory<br /> - pages are loaded into memory frames in RAM<br /> <br /> - map pages to frames (equal size)<br /> - mapping data structure needs to be sparse<br /> - not use memory for unallocated areas<br /> - must store data structure in a set of pages<br /> - this is a page table<br /> - but it is really a tree<br /> <br /> - leaves are data pages<br /> - nodes are pages<br /> - each internal node is an array of page table entries (PTEs)<br /> - evenly fit into page of course<br /> <br /> - with 32 bit addresses and a 4K page<br /> - [ virt page # ][ offset #] = virtual address<br /> - offset is used to index into actual page<br /> - virt page # is used to walk page table<br /> - how many bits? 12 bits for offset, 20 for virt page #<br /> <br /> - page table has to map 2^20 =&gt; 2^20<br /> - physical address = [ frame # ][ offset #]<br /> - to translate, map page to frame number and copy offset<br /> - PTE = [ frame # ][ metadata ]<br /> - frame # is 20 bits, 12 bits of metadata<br /> - takes up 32 bits<br /> - how many PTEs in a page? 4096 bytes per page/<br /> 4 bytes per PTE<br /> = 1024 PTEs in a page<br /> - tree has 2 levels. Top node and middle nodes<br /> - top node has 1024 &quot;pointers&quot; (PTEs)<br /> - middle nodes each have 1024 PTEs, each referring to data<br /> or code pages<br /> - note any PTE may be marked as &quot;not present&quot;<br /> - So for a 4K of address space, you need 3 pages (12K)<br /> - 1 top, 1 middle, and 1 data<br /> <br /> - why care?<br /> - page tables are managed by the kernel<br /> - specifically, every process has its own page table<br /> (virtual memory mapping)<br /> <br /> - what metadata is stored in a PTE<br /> - permissions (read, write, exec)<br /> - valid? (allocated memory?)<br /> - present? (in RAM?)<br /> - dirty? (has it been modified since loaded)<br /> &lt;/pre&gt;</div>

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