WEBVTT 1 00:01.530 --> 00:01.890 Hello. 2 00:01.890 --> 00:02.610 Welcome back. 3 00:02.610 --> 00:06.120 In this lesson we are going to do some practicals on what we have studied. 4 00:06.120 --> 00:11.250 In the last lesson, we are going to create a simple function to add numbers. 5 00:11.250 --> 00:17.700 So let's say you pass eight arguments to the function, and the function will add all those arguments and 6 00:17.700 --> 00:19.020 save it to the register -. 7 00:19.590 --> 00:24.270 So - is traditionally used to return the result of a function call. 8 00:24.450 --> 00:25.860 So let's get started. 9 00:26.010 --> 00:32.430 So the first thing we need to do is, uh, we have to move those arguments to a register. 10 00:32.910 --> 00:35.400 So here we move the arguments. 11 00:35.400 --> 00:40.530 So the first example, we are going to have only four numbers to add. 12 00:40.980 --> 00:43.590 So the first number, we'll move one to -. 13 00:47.200 --> 00:50.830 Second number, we move to -. 14 00:54.640 --> 00:56.620 Third number, we move to -. 15 00:56.650 --> 01:00.010 This is we are following the Microsoft convention. 16 01:01.600 --> 01:07.210 And then we move to - fourth number. 17 01:07.980 --> 01:13.590 So since we don't use the stack, we don't have to allocate the arguments for the stack, but we still 18 01:13.590 --> 01:15.660 have to create the shadow space. 19 01:15.660 --> 01:19.470 So to create shadow space, we need to sub - 32. 20 01:19.470 --> 01:25.920 Because Microsoft convention requires us to create 32 bytes shadow space. 21 01:26.940 --> 01:31.440 Next, we are going to make a call to our function, assuming our function is over here. 22 01:31.440 --> 01:36.210 So we copy the address first, and then we put it here. 23 01:37.320 --> 01:40.590 We do a call, paste the address I will copy. 24 01:40.590 --> 01:41.850 So this is our function. 25 01:45.320 --> 01:45.680 All right. 26 01:45.680 --> 01:51.290 So when a call returns, it's going to save. You can see the result in the - register. 27 01:51.410 --> 01:51.920 All right. 28 01:51.920 --> 01:54.440 So you can do whatever you want with it. 29 01:54.620 --> 01:56.870 But when the call returns, you need to clean the stack. 30 01:56.870 --> 02:01.700 So since you sub - 20, you need to add 31 02:02.940 --> 02:05.010 - 20. 32 02:05.280 --> 02:07.290 So 20 here is actually hex here. 33 02:07.530 --> 02:11.340 So in the decimal will be 32. 34 02:13.130 --> 02:17.840 So now we can implement our function call, which is here. 35 02:18.290 --> 02:24.200 So in the function call, the first thing you do is to, uh, push -. 36 02:24.800 --> 02:27.710 Note that I'm just referring to the notes here. 37 02:28.840 --> 02:29.620 The notes here. 38 02:30.040 --> 02:31.000 Like this. 39 02:32.850 --> 02:34.770 In the function call, you need to push -. 40 02:34.800 --> 02:37.710 Remember we studied before in the earlier lessons. 41 02:37.980 --> 02:41.910 The first thing you do in the function call is push - and move - -. 42 02:43.580 --> 02:45.350 So that's what we're doing here. 43 02:47.570 --> 02:50.240 Next is to move - -. 44 02:53.990 --> 02:57.830 Thereafter, we can access those numbers. 45 02:58.190 --> 03:05.660 But first we need to zero out - so that - maybe - with itself. 46 03:05.750 --> 03:06.890 We are zeroing in. 47 03:06.890 --> 03:08.300 Zeroing it out. 48 03:08.990 --> 03:09.410 Why? 49 03:09.440 --> 03:13.100 Because we are going to use - to be the accumulator 50 03:13.190 --> 03:14.870 to sum up all the numbers. 51 03:15.380 --> 03:19.040 So the first number, we are going to do is to add 52 03:23.090 --> 03:25.220 - -. 53 03:27.430 --> 03:28.630 - 54 03:32.710 --> 03:35.680 That is the first argument. 55 03:37.450 --> 03:39.250 Then we add - -. 56 03:39.460 --> 03:40.660 - 57 03:43.090 --> 03:44.080 Then we add 58 03:44.970 --> 03:45.960 - -. 59 03:46.380 --> 03:47.340 - 60 03:50.480 --> 03:53.660 Then we add - -. 61 03:59.300 --> 04:01.280 Then we have to return now. 62 04:01.700 --> 04:05.780 So we're not creating any local variables, so we can just return. 63 04:06.260 --> 04:14.090 So before we return, we need to move, need to clean the stack, although we don't use it, but we 64 04:14.090 --> 04:18.470 always do this - -. 65 04:20.490 --> 04:22.230 Maybe pop -. 66 04:24.090 --> 04:25.380 Then we return. 67 04:28.000 --> 04:28.390 Let's see. 68 04:30.300 --> 04:30.570 Okay. 69 04:30.570 --> 04:32.970 So we put a breakpoint here. 70 04:33.090 --> 04:34.560 We run to our breakpoint. 71 04:34.560 --> 04:35.580 We step over. 72 04:36.300 --> 04:43.020 So we see we move all this into the registers -, -, -, -. 73 04:43.770 --> 04:46.590 Next, we are going to create shadow space. 74 04:46.920 --> 04:47.850 Then we will call. 75 04:48.450 --> 04:50.550 So we want to follow and trace the code. 76 04:50.550 --> 04:51.780 We need to step into it. 77 04:51.780 --> 04:53.100 So step into the call. 78 04:53.100 --> 04:56.640 We jump down here, then we push -. 79 04:57.360 --> 04:59.040 So - is now on the stack. 80 05:00.000 --> 05:01.230 This is the old -. 81 05:02.460 --> 05:08.910 - 65420, which is here, 65420, which is here. 82 05:09.960 --> 05:10.620 Is -. 83 05:11.310 --> 05:13.320 Then we move - to -. 84 05:14.560 --> 05:17.230 So now - goes to 85 05:17.230 --> 05:23.290 656540, 6550, 86 05:23.290 --> 05:25.270 and the old - is pushed to the stack. 87 05:25.510 --> 05:26.860 Here, you can see here. 88 05:27.280 --> 05:31.720 So when the call finishes, it will unwind the stack and go back to this old -. 89 05:32.170 --> 05:40.360 But now that you have moved - -, - is pointing to the same location as the -. 90 05:40.390 --> 05:46.810 So this now, from here onwards, will be used as a base pointer to reference the arguments if necessary. 91 05:47.200 --> 05:51.820 So now we are going to add all these registers. 92 05:51.970 --> 05:54.340 First, - becomes zero. 93 05:54.370 --> 06:03.340 Then you add one to -, - to -, and - to -, and - to -, -. 94 06:03.820 --> 06:09.070 So by now - will have the result of the addition one plus two plus three plus four. 95 06:09.790 --> 06:11.440 One plus two is three. 96 06:11.440 --> 06:14.680 Three plus three is six, six plus four is ten. 97 06:14.680 --> 06:15.700 So result should be ten. 98 06:15.700 --> 06:16.510 So we got ten. 99 06:16.510 --> 06:17.500 So it's correct. 100 06:17.980 --> 06:19.660 Then we now clean the stack. 101 06:20.480 --> 06:21.080 And then pop. 102 06:21.080 --> 06:23.930 The old - value back to it. 103 06:24.290 --> 06:30.290 So the old - at the top of the stack will be the old -. 104 06:30.440 --> 06:31.880 So now we pop it back. 105 06:33.530 --> 06:34.160 So now 106 06:34.160 --> 06:38.750 - points to the old stack, and then now we return. 107 06:38.750 --> 06:41.840 So when we return, it will go back to the line after the call. 108 06:42.470 --> 06:45.110 Then from here you can clean the stack. 109 06:45.440 --> 06:47.540 So here you need to clean the stack. 110 06:47.540 --> 06:52.430 Here you sub, uh, - 32. 111 06:52.460 --> 06:54.140 So here you need to clean the stack. 112 06:54.140 --> 06:55.400 Sorry, that's a mistake. 113 06:55.400 --> 06:56.810 There it should be add. 114 06:58.910 --> 07:00.440 - 32. 115 07:01.940 --> 07:02.570 Yes. 116 07:02.750 --> 07:03.050 Like that. 117 07:03.380 --> 07:03.920 All right. 118 07:03.920 --> 07:05.210 So now you clean the stack. 119 07:07.670 --> 07:12.050 And now the stack is back to the original state, which it was in before the call. 120 07:12.380 --> 07:18.530 So this is how you can implement a function call with four parameters and no local variables. 121 07:18.590 --> 07:19.820 That's all for this video. 122 07:19.820 --> 07:20.990 Thank you for watching.