WEBVTT 00:00.000 --> 00:10.091 Sv39, Field by Field: the bits of Sv39, RISC-V's paging scheme, behind exercise 33k_paging. 00:10.691 --> 00:17.816 The page is stepped by script, the explainer was built with Claude, and its numbers are examples, not the lecture's. 00:18.320 --> 00:21.413 Watch this before Tuesday's lecture, October 6. 00:22.013 --> 00:30.547 On Friday, October 9, you will write Pte::new and Pte::pa: an address into a page-table entry, and back. 00:31.147 --> 00:34.133 No solution code here: only the machine's state. 00:34.640 --> 00:39.269 A program uses virtual addresses. Memory sits at physical ones. 00:39.869 --> 00:44.733 A page table, a tree of tables three levels deep, turns one into the other. 00:45.240 --> 00:48.568 To the page table, a virtual address is four fields. 00:49.168 --> 00:53.691 The low bits are the offset: which byte inside a 4 KiB page. 00:54.291 --> 00:56.360 It is the same in the physical address. 00:56.880 --> 01:01.701 Above the offset is the virtual page number, VPN, in three parts, 01:02.301 --> 01:04.115 one for each level of the tree. 01:04.715 --> 01:08.213 Each is a slot number: a position in that level's table. 01:08.720 --> 01:13.029 Each slot of a table holds one page-table entry, a PTE. 01:13.629 --> 01:14.931 It carries two things: 01:15.531 --> 01:19.669 a physical page number, and flags: single bits, each with a letter. 01:20.200 --> 01:23.187 The page number fills bits 53 down to 10. 01:23.787 --> 01:28.203 It is the address of the page the entry names, with the offset left off. 01:28.720 --> 01:30.896 This row is that page's physical address. 01:31.496 --> 01:35.891 Its page number sits right above the offset, so it starts at bit 12. 01:36.491 --> 01:38.347 The empty row will be its entry. 01:38.880 --> 01:42.848 An entry names a whole page, so it has no place for an offset. 01:43.848 --> 01:45.768 The low 12 bits are set aside. 01:46.280 --> 01:48.541 Now the page number goes into the entry. 01:50.041 --> 01:52.580 The same 44 bits, in the same order, 01:53.180 --> 01:55.633 but starting at bit 10, not bit 12. 01:56.160 --> 01:58.592 The entry's low ten bits are still empty. 01:59.592 --> 02:02.749 That is where the flags go: here V, R and X. 02:03.349 --> 02:09.451 In hex, the finished entry looks nothing like the address: bit 10 falls inside a hex digit. 02:09.960 --> 02:12.712 Now the other direction, starting from the entry. 02:13.312 --> 02:15.957 Its low ten bits are not part of any address, 02:16.957 --> 02:18.728 so this time they are set aside. 02:20.240 --> 02:24.059 The page number, read from bit 10, is placed at bit 12. 02:25.059 --> 02:26.915 Below it are twelve zero bits: 02:27.515 --> 02:29.840 this is the address of the page's first byte. 02:30.360 --> 02:31.533 Now one mistake. 02:32.133 --> 02:35.589 The entry is right: its page number starts at bit 10. 02:37.089 --> 02:40.439 But this read starts at bit 12, where an address keeps it. 02:41.039 --> 02:44.537 Two bits too many at the high end, two missed at the low end. 02:46.040 --> 02:49.923 The result looks like a page's address, ending in twelve zeros. 02:51.423 --> 02:54.132 But it is not the address the entry was built from. 02:54.732 --> 03:00.876 For this mistake, Friday's test prints: [fail] Pte::pa did not recover the address. 03:01.400 --> 03:04.685 Every level of the tree uses the same entry format. 03:05.285 --> 03:11.195 So an entry must say what its page number names: the next table down, or a page of memory. 03:11.720 --> 03:16.563 R, W and X are a page's permissions: read, write and execute. 03:17.163 --> 03:20.789 With any of them set, the entry is a leaf, and names a page. 03:21.789 --> 03:25.331 With all three clear, it is a branch, and names the next table. 03:25.840 --> 03:27.675 And V, for valid, comes first. 03:28.675 --> 03:33.027 With V clear, the entry is not valid: the slot counts as empty, 03:34.027 --> 03:35.627 whatever its other bits hold. 03:36.160 --> 03:38.336 On Friday you write both directions. 03:38.936 --> 03:44.547 Pte::new: a page's physical address and flags go in, an entry comes out. 03:45.147 --> 03:49.605 Pte::pa: an entry goes in, the page's address comes out. 03:50.120 --> 03:56.051 The test's first check, check 0, builds one entry, then asks for its address and flags back. 03:56.651 --> 04:01.408 This line means the page number sat at the wrong bit, going in or coming out, 04:02.008 --> 04:03.608 or was never placed at all. 04:04.120 --> 04:06.787 The second line means the address did come back, 04:07.387 --> 04:10.416 but the entry's low ten bits did not keep the flags. 04:10.920 --> 04:13.245 One page number, at two different bits: 04:13.845 --> 04:17.195 bit 12 in a physical address, bit 10 in an entry. 04:17.795 --> 04:22.915 Next: Sv39, Level by Level, and walk, the third function you write. 04:23.515 --> 04:26.096 Then Tuesday's lecture, and Friday's session.