1 00:00:00,060 --> 00:00:05,400 ‫Hello and welcome, and in this lecture, we are going to switch to protected mode, so the first thing 2 00:00:05,400 --> 00:00:10,590 ‫I'm going to do is I'm going to go to Google just to show you guys an article on OS Steve. 3 00:00:16,850 --> 00:00:22,130 ‫So I keep showing you this resource because it's incredibly valuable if you ever run into any problems, 4 00:00:22,370 --> 00:00:27,270 ‫obviously you can ask me a question, but this resource is very helpful indeed. 5 00:00:27,800 --> 00:00:30,490 ‫So this is how you enter to mode, right? 6 00:00:31,850 --> 00:00:35,720 ‫So there is an instruction called LGBT, right. 7 00:00:36,260 --> 00:00:38,720 ‫Which loads that the global descriptor table. 8 00:00:38,720 --> 00:00:44,600 ‫And then you set the protection table bit in the C zero register Kontorovich to zero. 9 00:00:44,750 --> 00:00:45,100 ‫Right. 10 00:00:46,010 --> 00:00:54,080 ‫And then you jump with the selector and the offset being the absolute address to where you want to load. 11 00:00:54,290 --> 00:00:57,670 ‫And then at this point in time, you're now in protected mode. 12 00:00:58,040 --> 00:01:00,230 ‫So that's how protective mode works. 13 00:01:00,230 --> 00:01:00,500 ‫Right. 14 00:01:00,680 --> 00:01:07,430 ‫So if we take a look at the global scripter table, you can see that the global descript a table contains 15 00:01:07,430 --> 00:01:10,670 ‫entries telling the CPU about memory segments. 16 00:01:11,360 --> 00:01:15,950 ‫The T is loaded using the LGBT assembly instruction. 17 00:01:16,430 --> 00:01:20,690 ‫It expects the location of a T description structure. 18 00:01:21,800 --> 00:01:22,970 ‫And this is how it looks. 19 00:01:23,000 --> 00:01:30,370 ‫You have a size and an offset, so this is essentially a pointer to the actual descriptor entries themselves. 20 00:01:31,130 --> 00:01:39,890 ‫And then you have a bunch of entries, right, that describe the memory and the access rights to that 21 00:01:39,890 --> 00:01:40,350 ‫memory. 22 00:01:40,670 --> 00:01:44,870 ‫So you don't have to worry about all of this stuff because we're just going to set it to some default 23 00:01:44,870 --> 00:01:45,680 ‫values. 24 00:01:45,950 --> 00:01:49,670 ‫And the reason being is we're going to create a kernel that uses paging. 25 00:01:49,670 --> 00:01:54,400 ‫We won't really care as much about this global descriptive table. 26 00:01:54,410 --> 00:02:00,920 ‫We'll just put in some default values and we'll work with that, because if you go back to Visual Studio 27 00:02:00,920 --> 00:02:05,840 ‫Code and the first thing we're going to do is we're going to get rid of what we did before when we were 28 00:02:05,840 --> 00:02:09,730 ‫loading sectors into memory, because we don't really need that right now. 29 00:02:09,740 --> 00:02:12,680 ‫So it's best to keep the code nice and clean. 30 00:02:12,680 --> 00:02:12,980 ‫Right. 31 00:02:13,430 --> 00:02:16,770 ‫If you don't know if you don't already have this and don't worry about it. 32 00:02:17,600 --> 00:02:22,690 ‫I just want to get rid of all of this stuff because it's just keeps it cleaner, right. 33 00:02:24,920 --> 00:02:30,930 ‫OK, so that's what we did last time, so let's now create the global descriptive table. 34 00:02:30,980 --> 00:02:36,180 ‫OK, so what we're basically making now is we're making these entries, OK? 35 00:02:36,440 --> 00:02:38,750 ‫These this represents bits, by the way. 36 00:02:39,620 --> 00:02:42,440 ‫So, you know, bets 16 to 31. 37 00:02:42,650 --> 00:02:42,990 ‫Right. 38 00:02:43,070 --> 00:02:46,610 ‫Bid zero to 15, you know, just to make that clear. 39 00:02:46,820 --> 00:02:49,080 ‫So, yeah, we're going to we're going to start now. 40 00:02:49,550 --> 00:02:51,620 ‫So we'll put our GDP by here. 41 00:02:51,620 --> 00:02:51,920 ‫Right. 42 00:02:52,850 --> 00:02:57,630 ‫And the very first thing we need is another segment GDP. 43 00:02:57,710 --> 00:02:58,040 ‫No. 44 00:03:04,030 --> 00:03:10,690 ‫And this is just going to have, you know, 64 bits of zeros, essentially just a null descriptor. 45 00:03:11,170 --> 00:03:15,940 ‫I also want to put a good start label here, because we're going to reference that later. 46 00:03:16,540 --> 00:03:19,860 ‫OK, so we're just going to go off Section eight. 47 00:03:20,340 --> 00:03:21,820 ‫This is the offset in the table. 48 00:03:21,820 --> 00:03:31,870 ‫We're about to make the code descriptor and now do GDP code, OK, and we're going to go W zero X FF, 49 00:03:32,500 --> 00:03:38,380 ‫OK, and this is the segment limit first zero to 15 bits. 50 00:03:38,800 --> 00:03:47,590 ‫And just by here, I'm just going to put Secesh point to this just to make things clearer, because 51 00:03:47,590 --> 00:03:49,390 ‫this is this is the code segment now. 52 00:03:49,390 --> 00:03:49,660 ‫Right. 53 00:03:50,080 --> 00:03:52,480 ‫So now we're going to go d w zero. 54 00:03:52,950 --> 00:04:01,060 ‫OK, so this is the base zero to 15 bits. 55 00:04:01,330 --> 00:04:10,090 ‫And I we're just going to go DB zero and this is the base 16 to 23 bits. 56 00:04:10,090 --> 00:04:14,260 ‫And you don't have to worry too much by this because like I said, we're just going to use the defaults 57 00:04:14,620 --> 00:04:18,400 ‫because we don't really want to work with these with these descriptors. 58 00:04:18,400 --> 00:04:24,000 ‫Very often we care more about the paging memory model, which we'll get to in time. 59 00:04:24,520 --> 00:04:27,860 ‫So here we're going to go DB 069 A. 60 00:04:29,110 --> 00:04:30,920 ‫This is the access byte. 61 00:04:31,640 --> 00:04:41,200 ‫OK, so if we go back to the Firefox OS stuff, we can see that the access byte is a bunch of bit mask's. 62 00:04:41,450 --> 00:04:43,710 ‫OK, it's a bit messy basically. 63 00:04:43,720 --> 00:04:45,630 ‫And we've set a bunch of flags in that. 64 00:04:45,700 --> 00:04:48,220 ‫These flags are represented here, you see. 65 00:04:48,220 --> 00:04:54,250 ‫So present bits must be one for all valid selectors privilege. 66 00:04:54,250 --> 00:04:55,960 ‫But, you know, this is the ring level. 67 00:04:56,260 --> 00:05:00,070 ‫Remember I was saying about the rings, so this is the ring level. 68 00:05:01,510 --> 00:05:04,030 ‫And obviously we want it all to be col space. 69 00:05:04,030 --> 00:05:04,330 ‫Right. 70 00:05:04,990 --> 00:05:06,160 ‫These bit fields. 71 00:05:06,160 --> 00:05:07,540 ‫Explain what we're doing here. 72 00:05:07,540 --> 00:05:07,820 ‫Right. 73 00:05:08,170 --> 00:05:13,180 ‫So the executable bid, if that's one and this is in the access byte, remember, if that's one and 74 00:05:13,180 --> 00:05:16,020 ‫the second could be executed and so on, so on. 75 00:05:16,420 --> 00:05:18,030 ‫Now let's carry on. 76 00:05:19,330 --> 00:05:29,650 ‫So we're going to go debe one one zero zero one one one one B, OK, and this is the high four bit flags 77 00:05:29,680 --> 00:05:35,890 ‫and the low forward flags and we'll just going to go DB zero and you have to like I so you don't have 78 00:05:35,890 --> 00:05:37,630 ‫to worry about this stuff very much. 79 00:05:37,630 --> 00:05:37,910 ‫Right. 80 00:05:38,560 --> 00:05:41,680 ‫Twenty four to 31, one a base, 241 bits. 81 00:05:42,040 --> 00:05:45,880 ‫You don't have to worry too much about this stuff because these are just default values. 82 00:05:46,300 --> 00:05:51,220 ‫Obviously you can play with these values if you want, but I don't recommend it because there's no benefit 83 00:05:51,220 --> 00:05:52,710 ‫to sticking with this memory model. 84 00:05:52,960 --> 00:05:57,130 ‫We are basically going to just make it so we can access all of the memory. 85 00:05:57,130 --> 00:05:58,120 ‫That's what we're doing here. 86 00:05:58,390 --> 00:06:00,850 ‫This is offset zero zero x ten. 87 00:06:01,000 --> 00:06:06,460 ‫OK, so this is for our data segment and so on the stack segment and all that. 88 00:06:06,850 --> 00:06:14,320 ‫So we're going to go get data wco x, f, f, f. 89 00:06:15,370 --> 00:06:21,460 ‫OK, and again, this is the segment limit, you know what, less copy and paste this, actually, because 90 00:06:21,460 --> 00:06:22,530 ‫it's just easier, right? 91 00:06:23,020 --> 00:06:26,140 ‫Copy and paste this down here, because the structure is exactly the same. 92 00:06:26,150 --> 00:06:36,400 ‫So so the only things we need to change are the access by to nine to OK, which change the flags a little 93 00:06:36,400 --> 00:06:36,690 ‫bit. 94 00:06:38,640 --> 00:06:46,370 ‫And that is that is actually it, and obviously GDP data is wrong, it should be data by my bed, OK? 95 00:06:46,590 --> 00:06:51,720 ‫And I'm just going to put a comment here on what should be what they should be linked to. 96 00:06:52,560 --> 00:06:59,370 ‫So this should be linked to these SS, ESFS, G.S.. 97 00:06:59,610 --> 00:07:01,790 ‫OK, so that's all we need to worry about. 98 00:07:01,950 --> 00:07:07,470 ‫Now down here we go, G.T. and because remember, these are just labels guys, you know, assembly works 99 00:07:07,890 --> 00:07:18,630 ‫and then we go GDP to Scripter, OK, and that'll be W G.T. and minus TGT start minus one. 100 00:07:18,870 --> 00:07:22,390 ‫OK, and that'll give us the size of the descriptor. 101 00:07:23,310 --> 00:07:24,420 ‫And now here. 102 00:07:25,400 --> 00:07:28,730 ‫We go get Star, and that is the offset. 103 00:07:29,030 --> 00:07:34,330 ‫Now an important note, our origin is zero. 104 00:07:34,550 --> 00:07:40,040 ‫OK, so we need to take this. 105 00:07:42,020 --> 00:07:45,030 ‫Into account when making our descriptor. 106 00:07:45,230 --> 00:07:46,920 ‫So this is pretty easy to do. 107 00:07:46,970 --> 00:07:52,330 ‫We just need to scroll up, change our origin back to zero seven zero zero, OK? 108 00:07:53,180 --> 00:07:58,010 ‫And now after our jump shots start, instead of having our segment here, because remember, this is 109 00:07:58,010 --> 00:08:05,450 ‫16 bytecode of the moment, instead of having our segment here as the seven C0, we change it to zero. 110 00:08:05,900 --> 00:08:12,590 ‫OK, so then obviously one, when it does this jump, the code segment will change to zero. 111 00:08:13,100 --> 00:08:19,040 ‫And then in that way then our offsets at zero seven zero work absolutely fine. 112 00:08:19,400 --> 00:08:26,720 ‫Now obviously our data segment segment and all that sort of stuff also needs to now become zero for 113 00:08:26,720 --> 00:08:28,760 ‫that to work as expected. 114 00:08:28,760 --> 00:08:30,560 ‫And we can also get rid of this here. 115 00:08:31,920 --> 00:08:35,670 ‫OK, that should work absolutely fine. 116 00:08:35,780 --> 00:08:43,460 ‫So what we're going to do now is we're going to create a load protected label. 117 00:08:43,850 --> 00:08:45,590 ‫We're going to get rid of this jump as well. 118 00:08:47,310 --> 00:08:51,500 ‫OK, and now we're going to clear the interrupts. 119 00:08:53,100 --> 00:09:02,670 ‫And we're going to go our GDP lowed global descriptive table and we're going to go GDP descriptor. 120 00:09:04,170 --> 00:09:11,100 ‫So obviously what this will do is this a look down here, it'll S.A.T. descriptor, it will find the 121 00:09:11,100 --> 00:09:14,670 ‫size here and it'll finally offset here, OK? 122 00:09:14,910 --> 00:09:20,180 ‫And then it will look into there and load all of this because GDP starts. 123 00:09:20,880 --> 00:09:25,860 ‫That is the address to our label here, which describes the table. 124 00:09:26,040 --> 00:09:34,590 ‫Do you understand by doing the end minus GDP start, we get the sites and then it also expects us to 125 00:09:34,590 --> 00:09:36,950 ‫minus one on that on that size as well. 126 00:09:39,120 --> 00:09:43,590 ‫Now, if we go move IEX, c0, OK. 127 00:09:44,660 --> 00:09:48,260 ‫Or or X, CLX one. 128 00:09:50,940 --> 00:09:58,380 ‫Move Cosio X, so a reset in that register now, now that we've set up it and then we need to scroll 129 00:09:58,380 --> 00:10:03,350 ‫down ways to get rid of our little print routine here, we're not going to use it anymore. 130 00:10:05,740 --> 00:10:11,920 ‫We need to now go Bitz 32 so that all code underneath here is seen as 32 bit code. 131 00:10:14,510 --> 00:10:19,460 ‫And I will go low 32, OK, and we'll just do a Infonet jump there. 132 00:10:20,270 --> 00:10:22,010 ‫That's that's all I want to do at the moment. 133 00:10:22,280 --> 00:10:30,230 ‫So we go back up to low protected and at the very top of the file, we're just going to go code seg 134 00:10:30,590 --> 00:10:31,460 ‫equals. 135 00:10:33,330 --> 00:10:35,730 ‫Geeta Teko minus T starts. 136 00:10:38,250 --> 00:10:44,880 ‫Data sig equals GDP data minus GDP start. 137 00:10:48,220 --> 00:10:56,230 ‫OK, so what that'll do, that'll give us these offsets, xox eight zero 10, et cetera, et cetera. 138 00:10:56,240 --> 00:10:56,540 ‫Right. 139 00:10:57,070 --> 00:11:07,250 ‫So now what we can do is we can go and just do a jump jump cosac low 32 and there we are. 140 00:11:07,660 --> 00:11:11,520 ‫Code SEC gets replaced zero six eight six zero eight. 141 00:11:11,560 --> 00:11:16,540 ‫So what this what this code does, it switches to the code say to the code selector. 142 00:11:16,540 --> 00:11:16,900 ‫Right. 143 00:11:17,290 --> 00:11:24,400 ‫And then it jumps to the low 30s to absolute address which is here, OK, and then we just jump forever 144 00:11:24,400 --> 00:11:25,390 ‫and do nothing else. 145 00:11:25,910 --> 00:11:26,840 ‫That's what's happening. 146 00:11:27,430 --> 00:11:29,410 ‫So we now need to obviously test this. 147 00:11:29,470 --> 00:11:37,000 ‫If we go to the terminal and we type make OK, if we now do QM, you can see nothing crashes. 148 00:11:37,300 --> 00:11:42,360 ‫We're now going to have to have to attach GDB to ensure this is worked successfully. 149 00:11:42,820 --> 00:11:45,820 ‫So we're going to go pseudo and install gdb. 150 00:11:46,720 --> 00:11:47,220 ‫OK. 151 00:11:48,600 --> 00:11:55,890 ‫I already have it cool, so we're now going to go gdb target remote. 152 00:11:58,580 --> 00:12:09,710 ‫Kumu das systemin dash zae six and sixty four, that's a tough food dobbyn dash capital s. 153 00:12:11,190 --> 00:12:15,330 ‫Dash, gdb, WDIA and just press enter. 154 00:12:16,320 --> 00:12:21,450 ‫OK, and now type C for continue press enter. 155 00:12:22,080 --> 00:12:23,050 ‫OK, good. 156 00:12:23,100 --> 00:12:29,610 ‫And now the programs run, the operating systems running, go back to the terminal and tie and hold 157 00:12:29,610 --> 00:12:31,380 ‫control and then see. 158 00:12:32,790 --> 00:12:42,480 ‫OK, and we can now see where the program is currently executing seven C six a now go layout ASRM and 159 00:12:42,480 --> 00:12:43,200 ‫look at that. 160 00:12:43,200 --> 00:12:48,240 ‫We can see that we are now on our infinite jump that we wrote there. 161 00:12:48,240 --> 00:12:51,720 ‫So we should be in protective mode if we now go in four registers. 162 00:12:53,080 --> 00:13:00,700 ‫And just press enter and yes, we are now in protective mode, you can see the code segment is now said 163 00:13:00,700 --> 00:13:01,300 ‫to eight. 164 00:13:02,440 --> 00:13:08,110 ‫OK, but we haven't set up our data segment registers and all that stuff, so we're going to do that 165 00:13:08,110 --> 00:13:11,140 ‫now so we can exit out of the emulator now. 166 00:13:13,420 --> 00:13:23,950 ‫So to set the data registers is easy enough inside low 30 to here we just go move X data sec move, 167 00:13:23,950 --> 00:13:34,600 ‫dcx move, esx move FCX Move, G.S. X, move ss x. 168 00:13:34,600 --> 00:13:37,960 ‫And now just for good measure, we're going to set the base pointer. 169 00:13:39,060 --> 00:13:47,710 ‫It's a point to and we use SBP now, by the way, to point to zero zero zero to one, two, three, 170 00:13:47,710 --> 00:13:52,910 ‫four, five in memory, and then we're going to set the stat points of the base pointer as well. 171 00:13:53,320 --> 00:14:00,130 ‫So obviously, we're just setting the standpoint of further in memory now that because now we now we 172 00:14:00,130 --> 00:14:01,180 ‫can access more memory. 173 00:14:01,180 --> 00:14:01,400 ‫Right. 174 00:14:02,290 --> 00:14:03,730 ‫Let's just test it one more time. 175 00:14:03,730 --> 00:14:04,600 ‫We're going to go make. 176 00:14:05,570 --> 00:14:14,810 ‫And now we're going to go jadi gdb target remote kumu dash system, desex a six hundred sixty four Dash 177 00:14:14,890 --> 00:14:17,270 ‫HDA Doc Ford, Sasebo Dobbyn. 178 00:14:19,100 --> 00:14:28,160 ‫Dash capital capitalis dash gdb Stelio, we're going to proceed to continue control so we can see that's 179 00:14:28,160 --> 00:14:36,140 ‫why we're running low assim and yes, we can see our Infonet jump there if we now go in four registers. 180 00:14:38,860 --> 00:14:47,110 ‫We can see that the coat that the coat segments, eight stacked segments, 10 data segments, 10 flags 181 00:14:47,110 --> 00:14:49,540 ‫and all that, also zero x 10. 182 00:14:49,840 --> 00:14:52,180 ‫So, yeah, we are now in protected mode. 183 00:14:53,410 --> 00:14:59,560 ‫The is now running in a 32 bit mode that we can no longer access the BIOS if we attempt to access the 184 00:14:59,560 --> 00:15:03,700 ‫bios in our thirty two big code here, really bad things will happen. 185 00:15:04,600 --> 00:15:10,510 ‫Unfortunately this means that we come from the desk as we did in the in a few lectures ago. 186 00:15:10,870 --> 00:15:12,110 ‫We can't do that anymore. 187 00:15:12,280 --> 00:15:17,220 ‫We now have to write a disk driver of our own if we want to read from the disk. 188 00:15:17,560 --> 00:15:19,480 ‫So that's another important thing to note. 189 00:15:20,020 --> 00:15:21,670 ‫So yeah, congratulations. 190 00:15:21,670 --> 00:15:23,440 ‫You are now in protective mode. 191 00:15:23,700 --> 00:15:29,320 ‫The next few lectures will obviously have to write a disk driver of our own so that we can load the 192 00:15:29,320 --> 00:15:34,570 ‫rest of the kernel because remember, the bios only loads the first sector into memory for us. 193 00:15:34,960 --> 00:15:40,840 ‫So as soon as our program gets bigger than 512 bytes, we're going to certainly run into problems. 194 00:15:41,710 --> 00:15:42,850 ‫Congratulations.