WEBVTT 1 00:01.100 --> 00:01.700 Welcome back. 2 00:01.700 --> 00:05.870 Now we are going to do the same thing, but this time we are going to create four local variables. 3 00:05.960 --> 00:10.430 So same way we will move our 4 00:12.950 --> 00:14.210 first parameter. 5 00:15.020 --> 00:20.000 Move the second parameter to -, which is two. 6 00:21.020 --> 00:30.110 Move the third parameter to -, three, and then move the fourth parameter to -. 7 00:32.480 --> 00:33.530 Which is four. 8 00:33.590 --> 00:34.100 All right. 9 00:35.550 --> 00:40.560 Okay, so next thing we need to do is to create the shadow space. 10 00:40.710 --> 00:42.930 So we sub - 11 00:45.780 --> 00:46.530 32. 12 00:47.820 --> 00:48.930 Then we make a call. 13 00:50.580 --> 00:52.290 So the call here is same. 14 00:52.290 --> 00:55.080 We want to use the same address for the function. 15 00:55.380 --> 01:01.440 So we copy this address and we make a call to that address. 16 01:04.490 --> 01:06.200 So we will call here. 17 01:06.500 --> 01:08.150 And here we do the same thing. 18 01:08.150 --> 01:10.490 We push -. 19 01:12.020 --> 01:13.490 Push -. 20 01:15.630 --> 01:21.090 Move - -, and now we are going to create four local variables. 21 01:21.090 --> 01:29.400 So we need to do sub - 32, because each variable takes up eight bytes or quadword. 22 01:29.820 --> 01:32.160 Since we have four, four times eight is 32. 23 01:33.690 --> 01:36.120 How do we make use of those local variables? 24 01:36.180 --> 01:46.860 For example, if we want to store, um, your first parameter to the first address 25 01:48.500 --> 01:49.880 of the local variable. 26 01:50.420 --> 01:51.590 What do you do? 27 01:51.620 --> 01:52.640 Take a look at this. 28 01:53.150 --> 01:57.320 So if you want access to those local variables, you need to use offsets. 29 01:57.350 --> 01:58.370 Take a look at this. 30 01:58.760 --> 02:01.640 - minus eight is local variable number one. 31 02:01.640 --> 02:05.390 Minus 16 is the second one, and third one, fourth one, and so on. 32 02:05.960 --> 02:11.600 So we're going to use this as our guide to access those local variables. 33 02:12.110 --> 02:13.640 Okay, so the first one. 34 02:13.730 --> 02:15.140 - minus eight. 35 02:15.860 --> 02:20.480 So let's say we want to save the first argument to that local variable. 36 02:20.510 --> 02:21.290 - minus eight. 37 02:22.100 --> 02:23.210 This is how we do it. 38 02:24.020 --> 02:24.770 We move 39 02:28.080 --> 02:31.560 -, you have to dereference it, - minus eight. 40 02:32.220 --> 02:34.230 This is the first local variable. 41 02:35.430 --> 02:35.790 -. 42 02:38.290 --> 02:38.680 All right. 43 02:38.680 --> 02:40.000 Next one will be 44 02:42.590 --> 02:48.740 second local variable will be - minus 16. 45 02:50.670 --> 02:51.660 And then this one, 46 02:51.660 --> 02:52.650 - will go there. 47 02:54.780 --> 02:58.290 Do the same thing for the third local variable. 48 03:00.360 --> 03:03.210 Minus 24, and - will go there. 49 03:05.560 --> 03:07.600 And the last local variable. 50 03:10.770 --> 03:16.560 - minus 32, and - will go to 51 03:17.780 --> 03:18.260 there. 52 03:21.080 --> 03:21.410 All right. 53 03:21.410 --> 03:21.800 So. 54 03:22.370 --> 03:27.290 So this is just to demonstrate, uh, how to create local variables and use them. 55 03:27.770 --> 03:28.160 Right. 56 03:28.160 --> 03:30.290 It is not meant to be optimized code. 57 03:30.290 --> 03:30.620 Right. 58 03:30.620 --> 03:32.270 So just to demonstrate. 59 03:32.270 --> 03:39.860 So now you can do your addition and do whatever you want using, uh, this address of 60 03:39.860 --> 03:41.480 all these four variables, okay. 61 03:41.480 --> 03:42.890 Let's say now you want to do an addition. 62 03:42.890 --> 03:44.720 Let's say, let's demonstrate. 63 03:44.840 --> 03:49.070 So you XOR - with itself. 64 03:49.910 --> 03:58.310 And then you're going to add - with the first local variable, which is keyword - minus eight. 65 04:04.080 --> 04:06.900 Repeat the second one for the second one. 66 04:08.650 --> 04:11.350 And I need to remind you, this is not optimized code. 67 04:12.970 --> 04:17.260 This is just to demonstrate how to access local variables. 68 04:26.690 --> 04:27.170 Okay. 69 04:27.170 --> 04:32.270 Next one should be 24, which is 24. 70 04:34.840 --> 04:36.520 The next one will be 71 04:44.400 --> 04:45.120 32. 72 04:47.780 --> 04:50.570 So, uh, that's it. 73 04:53.210 --> 04:59.450 So at this point, your - will have the result of the additions. 74 04:59.750 --> 05:13.250 So at this point we need to move - -, and then we do a pop -, and then a return. 75 05:14.460 --> 05:15.060 Like this. 76 05:16.350 --> 05:16.980 Next, 77 05:16.980 --> 05:21.480 we are going to come back here after this return and we clean the stack. 78 05:22.380 --> 05:31.170 So we clean the stack the same way we use add - 32, because we allocate 32 bytes for shadow space 79 05:31.170 --> 05:31.560 over here. 80 05:31.560 --> 05:32.490 So we need to clean it. 81 05:34.740 --> 05:40.320 Note that if you over here, if you want to access the result of - and do something with it, you can 82 05:40.320 --> 05:40.830 do it this way. 83 05:41.130 --> 05:47.340 For example, if you want to move the result of the addition to register -, you could do something 84 05:47.340 --> 05:48.090 like this. 85 05:50.000 --> 05:55.250 Because when the function returns, it will have the result of the addition in -, and you can move 86 05:55.250 --> 05:58.280 it, save it to - or whatever you want to do with it. 87 05:58.910 --> 06:02.450 So we put a breakpoint here and run to the breakpoint and step over it. 88 06:03.380 --> 06:06.350 Move all the four parameters to the registers. 89 06:06.410 --> 06:09.410 So we have 1, 2, 3, 4 here. 90 06:09.410 --> 06:12.110 Then you create a shadow space. 91 06:12.500 --> 06:15.590 Then you step into the function call. 92 06:16.980 --> 06:18.780 So now you're inside. 93 06:20.090 --> 06:24.410 So now you created your new base pointer by doing these two lines. 94 06:24.500 --> 06:28.670 So your base pointer is now 654. 95 06:28.670 --> 06:33.560 So at this point in time, you are already done this. 96 06:34.340 --> 06:35.780 You created the base pointer. 97 06:35.780 --> 06:42.680 So now you're going to create these four local variables by doing this, by - minus 32. 98 06:42.710 --> 06:45.230 So - was previously here. 99 06:45.230 --> 06:48.680 So minus 1, 2, 3, 4, 4 times 8, 32. 100 06:48.710 --> 06:51.740 So - minus 32, you create four local variables. 101 06:52.430 --> 06:56.990 That's why we're doing this - minus, sub - 32. 102 06:57.170 --> 07:00.410 This is hex 20, which is 32 in decimal. 103 07:01.130 --> 07:06.620 So next, once you created the four local variables, then we are going to assign the arguments to it. 104 07:06.620 --> 07:10.760 So the first argument is - minus eight. 105 07:10.760 --> 07:13.130 Here, - minus eight. 106 07:13.370 --> 07:16.040 So we're going to assign - to that. 107 07:16.370 --> 07:18.260 So let's do that now. 108 07:18.650 --> 07:20.810 So let's step over. 109 07:22.480 --> 07:23.530 -. 110 07:24.160 --> 07:25.990 Now look, - minus eight. 111 07:26.230 --> 07:33.760 - is 65 4065 FD F0, which is here. 112 07:34.360 --> 07:35.650 Minus eight is here. 113 07:35.980 --> 07:38.140 So the first variable is assigned there. 114 07:38.140 --> 07:46.570 So FD F0 is referring to this location -, and then - minus eight above it is referring to this. 115 07:48.940 --> 07:54.160 And then we have just stored the argument - into it, which is one. 116 07:54.160 --> 07:56.200 So now we're going to do the next one. 117 07:56.200 --> 07:59.320 We're going to save - into second variable. 118 07:59.320 --> 07:59.980 Step over. 119 08:00.670 --> 08:05.590 And now you see second variable is stored in - minus 16. 120 08:06.700 --> 08:08.200 Now we step over. 121 08:08.200 --> 08:12.820 Next one, - is going to store - minus 24. 122 08:12.970 --> 08:21.820 So you see - minus 24 is here. - is here, minus 8, minus 16, minus 24. 123 08:21.820 --> 08:23.410 So this is - minus 24. 124 08:23.440 --> 08:25.450 This is where you stored -. 125 08:26.050 --> 08:29.980 Now we're going to store - into the fourth local variable. Step over. 126 08:30.130 --> 08:35.590 And you see the fourth local variable is stored with - minus 32, which is here. 127 08:35.590 --> 08:40.510 So this is how you can create local variables and access them and use them. 128 08:40.960 --> 08:42.700 Next we are going to do the addition. 129 08:42.700 --> 08:45.760 So we XOR -, - becomes zero. 130 08:45.760 --> 08:47.800 Then we start adding them. 131 08:47.800 --> 08:52.390 First we take - minus eight, which is here, add to -. 132 08:54.700 --> 09:00.520 So - becomes one. Next is the - minus 16, which is here. 133 09:02.970 --> 09:06.420 You can also see from here, - minus 16 is two. 134 09:08.070 --> 09:10.470 Add two to -, so - becomes three. 135 09:10.470 --> 09:11.700 One plus two is three. 136 09:11.850 --> 09:13.650 Next is - minus 18. 137 09:13.650 --> 09:15.660 - minus 18 is. 138 09:15.660 --> 09:18.180 Remember, this is hex here. Minus 18 is three. 139 09:18.180 --> 09:19.110 So this is three. 140 09:19.110 --> 09:24.570 Over here, you see the address FD8, same as this FD8 here. 141 09:26.460 --> 09:26.790 Let's add it. 142 09:27.530 --> 09:31.880 So we're gonna take that three there, add to whatever is already in there. 143 09:32.030 --> 09:32.720 So three. 144 09:32.720 --> 09:34.640 So three plus three should give you a six there. 145 09:35.150 --> 09:36.500 And now you got six. 146 09:36.620 --> 09:40.250 Next, we're going to access - minus 32. 147 09:40.760 --> 09:46.730 Minus 32 is four at this address 6540 here at E0. 148 09:46.790 --> 09:51.530 So - is here. - minus 32 is here. 149 09:51.950 --> 09:54.860 So you take this four, add to whatever is in -. 150 09:56.400 --> 09:58.680 So - becomes ten. Then 151 09:58.680 --> 09:59.790 now we're going to clean the stack. 152 09:59.790 --> 10:02.130 So we move - into -. 153 10:03.420 --> 10:07.260 Pop -, and we return to the line after this. 154 10:07.290 --> 10:09.750 Now we clean the stack in the main function. 155 10:10.950 --> 10:14.430 Then we now take the result in - and do whatever you want. 156 10:14.430 --> 10:16.830 In this case, we just move it to -. 157 10:17.370 --> 10:19.890 And now we see - has got the result ten. 158 10:20.340 --> 10:27.630 So this is how we can create functions with four arguments and four local variables. 159 10:27.750 --> 10:29.100 That's all for this video. 160 10:29.100 --> 10:30.480 Thank you for watching.