WEBVTT 1 00:01.420 --> 00:03.460 Hello and welcome back. 2 00:03.460 --> 00:07.600 In this lesson, I'm going to show you how to create functions. 3 00:07.600 --> 00:13.690 And creating functions is also the same as creating what we call stack frames. 4 00:13.690 --> 00:15.520 So what are stack frames? 5 00:15.850 --> 00:17.170 Let me explain. 6 00:17.470 --> 00:20.530 For example, you see here push -. 7 00:21.480 --> 00:31.560 Move - - and then at the end here you will have move - - and pop -. 8 00:31.830 --> 00:40.350 So a stack frame is starting from these two statements and then ending with two statements here. 9 00:40.620 --> 00:42.240 So that's a stack frame. 10 00:42.240 --> 00:45.150 There is one more line here which we have removed. 11 00:45.150 --> 00:53.190 It is supposed to read move - - over here is move - -. 12 00:53.340 --> 01:02.010 So over here there should be one more move - - the reverse of this, okay. 13 01:02.010 --> 01:04.140 So that is a stack frame. 14 01:04.140 --> 01:10.650 So this itself is a function. A function starts from a stack frame, starts from this opening statements, 15 01:10.650 --> 01:12.060 and ends here. 16 01:12.300 --> 01:14.220 And then there is a return. 17 01:14.580 --> 01:17.940 Now we are going to create a function within this big function. 18 01:17.940 --> 01:20.040 So how do we create a function? 19 01:20.190 --> 01:24.720 So to create a function, you must first find out where you want to put your function. 20 01:24.720 --> 01:29.640 Let's say we want to put our function over here, over here. 21 01:29.640 --> 01:31.680 So we need to copy this address. 22 01:32.250 --> 01:33.840 Copy this address. 23 01:35.160 --> 01:38.880 And then over here we make a call to the function. 24 01:41.310 --> 01:43.050 Then paste the address that we have copied. 25 01:44.970 --> 01:47.730 So this here is a call to the function. 26 01:47.730 --> 01:55.980 So when the execution comes to this line, it will call this function at this address 401569, which is 27 01:55.980 --> 01:57.690 401569. 28 01:57.690 --> 02:00.210 Over here it is supposed to jump over there. 29 02:00.210 --> 02:00.600 Right. 30 02:01.650 --> 02:03.840 And then, uh, maybe it's not so obvious. 31 02:03.840 --> 02:05.430 Let's choose an address. 32 02:05.430 --> 02:06.300 Scroll down here. 33 02:07.170 --> 02:09.090 Copy this address. 34 02:09.090 --> 02:09.300 All right. 35 02:09.300 --> 02:11.520 Let's change it so that it's more obvious. 36 02:15.960 --> 02:16.290 Okay. 37 02:16.290 --> 02:21.030 So now it will call 40156F, which is over here. 38 02:21.030 --> 02:22.200 40156F. 39 02:22.350 --> 02:25.350 So at this address here we need to create a function. 40 02:25.350 --> 02:29.070 So to create a function we must create a stack frame. 41 02:29.430 --> 02:34.200 So the first statement in the stack frame is always push -. 42 02:35.670 --> 02:42.120 Then it will be followed by move - -. 43 02:44.860 --> 02:46.870 So this is the start of your function. 44 02:47.110 --> 02:50.320 Then the function always ends with the opposite of this. 45 02:50.320 --> 02:56.230 That means somewhere down here, after you have done whatever you want to do within the function, you 46 02:56.230 --> 02:58.270 should reverse these two, this and this. 47 02:58.270 --> 03:01.300 So down here let's say we want to reverse that. 48 03:01.480 --> 03:13.270 So we come back here. We will do the opposite: move - - and then followed by pop -. 49 03:13.510 --> 03:16.150 And finally a return, ret. 50 03:17.320 --> 03:20.470 So the ret keyword is the opposite of a call. 51 03:20.770 --> 03:24.220 So when you run it will call this function. 52 03:24.220 --> 03:25.210 That means it will jump to 53 03:25.210 --> 03:29.620 here, perform these two opening statements of the stack frame. 54 03:29.740 --> 03:38.110 Then perform whatever you want to do inside here, and then do the reverse to, uh, unwind the stack 55 03:38.110 --> 03:43.480 frame, and then finally return back to the line after the call. 56 03:43.660 --> 03:45.790 So let's try this now and see how it works. 57 03:45.790 --> 03:47.350 So I put a breakpoint here. 58 03:47.350 --> 03:49.090 So we run to the breakpoint. 59 03:49.450 --> 03:51.940 Then we step over until we come to our call. 60 03:52.270 --> 03:59.650 So now in our call, if we were to continue to step over, what will happen is it will execute this line 61 03:59.650 --> 04:04.720 and then immediately come here. You won't see that the call going into the function. 62 04:04.720 --> 04:10.990 If you want to see the call going into the function, you need to use step into over here, step into. 63 04:10.990 --> 04:15.940 So when you click step into, you can see that it steps into the function. 64 04:16.600 --> 04:19.600 Now you're in the first line of the stack frame. 65 04:19.600 --> 04:21.520 So this is the start of the function. 66 04:21.520 --> 04:27.880 And then you can continue to step over over here, step over, step over, and perform whatever instructions, 67 04:27.880 --> 04:34.420 if any, until you come to the end of the stack frame where it will then unwind the stack frame to 68 04:34.420 --> 04:36.820 revert back to the original. 69 04:36.910 --> 04:39.850 So let's revert everything back to original. 70 04:40.790 --> 04:45.590 And then finally, when it finishes with the function, it will return to the line after this call. 71 04:45.590 --> 04:49.340 So from here it's going to jump back up and land over here. 72 04:49.340 --> 04:50.240 Let's take a look. 73 04:50.390 --> 04:52.700 Step over and you see it goes back here. 74 04:53.660 --> 04:58.100 So this is how you can create, uh, functions using stack frames. 75 04:58.100 --> 05:07.100 So every function usually has these opening statements to create the stack frame and the closing statements 76 05:07.100 --> 05:09.920 to destroy or unwind the stack frame. 77 05:09.920 --> 05:15.050 So this opening two lines are also known as function prologue. 78 05:15.080 --> 05:19.100 Function prologue meaning the start of the function. 79 05:19.460 --> 05:25.340 And these two lines are function epilogue, also known as the end of the function. 80 05:26.570 --> 05:28.580 So that's all for this lesson. 81 05:28.580 --> 05:29.960 Thank you for watching.