WEBVTT 1 00:01.850 --> 00:03.320 Hello and welcome back. 2 00:03.320 --> 00:07.040 In this lecture, we are going to learn about push and pop. 3 00:07.400 --> 00:11.120 So let's open our template, second template. 4 00:13.530 --> 00:21.750 So let's go to our code and let's try the examples of how to use push and pop. 5 00:22.980 --> 00:27.630 So push basically means that you push something to the stack. 6 00:27.750 --> 00:28.980 So here is the stack. 7 00:28.980 --> 00:31.530 Here, a stack is just a part of memory. 8 00:32.220 --> 00:38.580 So let's type the command push -. 9 00:39.840 --> 00:45.420 So when you push -, whatever's in - will be pushed to the stack here. 10 00:46.110 --> 00:49.380 So if we run now and step over. 11 00:50.550 --> 00:52.080 And execute this. 12 00:52.560 --> 00:56.790 Watch what happens to the value here. 13 00:56.790 --> 00:59.160 So let's say we set our - to be one. 14 00:59.160 --> 01:00.900 Right-click, modify value. 15 01:01.680 --> 01:03.120 Set it to be one. 16 01:05.690 --> 01:07.400 And then now I step over. 17 01:07.400 --> 01:09.590 This one will be pushed to the stack here. 18 01:10.430 --> 01:11.990 Step over, and you see. 19 01:11.990 --> 01:15.230 Now you push - to the stack. 20 01:15.230 --> 01:17.870 So now, now the stack here has got one. 21 01:18.980 --> 01:20.480 Now let's try another one. 22 01:20.720 --> 01:22.790 We are going to push our - to the stack. 23 01:22.790 --> 01:26.330 So before that, let's modify our - by right-clicking it. 24 01:26.690 --> 01:31.280 Modify it, uh, let's say to two. 25 01:31.970 --> 01:33.650 So now our - has got two. 26 01:33.680 --> 01:36.620 We're going to push this value to the stack. 27 01:36.620 --> 01:38.600 So what command should we use? 28 01:39.800 --> 01:40.220 Push. 29 01:43.250 --> 01:43.520 All right. 30 01:43.520 --> 01:46.940 So now we are going to execute this and watch what happens to the stack. 31 01:47.390 --> 01:48.410 Step over. 32 01:48.980 --> 01:53.180 And now the value in - has been pushed to the top of the stack. 33 01:53.600 --> 01:57.800 So the top of the stack is the latest value being pushed. 34 01:58.010 --> 02:00.290 The bottom of the stack are the older values. 35 02:00.290 --> 02:06.620 So you will notice that one was pushed first, and then two is pushed next. 36 02:06.620 --> 02:10.370 So one came from -, and two came from -. 37 02:11.150 --> 02:13.160 So what's the usefulness of this? 38 02:13.190 --> 02:19.520 The usefulness of this is that you can use this stack as a way to save the state of previous data, 39 02:20.150 --> 02:23.810 so that you can make use of the register for other purposes. 40 02:23.810 --> 02:29.870 And then later, when you want to restore the original values of the registers, you can pop these values 41 02:29.870 --> 02:31.280 back to the registers. 42 02:31.310 --> 02:41.660 So now that we have got a copy of the values - and - inside the stack, we can make use of the - 43 02:41.660 --> 02:43.640 and - to do anything we want. 44 02:43.730 --> 02:46.100 So for example, let's do something now. 45 02:46.100 --> 02:48.230 Let's move, let's move, 46 02:48.230 --> 02:52.820 uh, the, uh, hex code A. 47 02:54.680 --> 02:55.580 Two -. 48 03:00.730 --> 03:01.420 Okay. 49 03:01.420 --> 03:07.450 So now I'm going to destroy the value in - with something else. 50 03:07.480 --> 03:09.610 Overwrite it with a new value A. 51 03:09.610 --> 03:10.660 So I step over that. 52 03:10.660 --> 03:12.610 And now - has got A. 53 03:12.910 --> 03:17.500 Now let's also overwrite - for another value called B. 54 03:17.890 --> 03:20.890 So let's say I did this -, -, B. 55 03:23.450 --> 03:26.510 And B, and I step over. 56 03:28.750 --> 03:29.950 And you will see it's been. 57 03:30.070 --> 03:34.030 Don't worry if it changes to - and -, it is still -. 58 03:35.050 --> 03:39.100 So now B has overwritten the original value in - previously. 59 03:39.130 --> 03:42.460 - and - has got one and two in. 60 03:42.640 --> 03:44.500 Now we overwrite it with A and B. 61 03:45.100 --> 03:51.250 Okay, so now we are going to restore the original values of - and - from the stack. 62 03:51.250 --> 03:53.500 So to do that, we use the pop instruction. 63 03:53.860 --> 03:55.360 So we use the pop instruction. 64 03:55.360 --> 03:56.440 So we type pop. 65 03:57.610 --> 04:04.720 So what pop does is it will take whatever is on top of the stack and restore it to whatever register 66 04:04.720 --> 04:05.560 you specify. 67 04:05.590 --> 04:11.020 So in this case, we want to restore the value two into -. 68 04:11.200 --> 04:13.840 So we have to pop -. 69 04:14.110 --> 04:20.140 So pop means to take whatever's on top of the stack and put it back in this register, 70 04:20.170 --> 04:21.070 -. 71 04:21.850 --> 04:24.010 So now let's run this, step over. 72 04:25.000 --> 04:29.020 And you see now two has got restored back in -. 73 04:29.050 --> 04:33.010 And now the two has been taken off the top of the stack. 74 04:33.010 --> 04:37.240 So now the top of the stack becomes the value one. 75 04:37.270 --> 04:40.900 So the meaning of pop means to take off whatever's on top of the stack. 76 04:41.530 --> 04:49.600 So now next command is to pop -, because we want to restore this one back to -. 77 04:49.630 --> 04:53.050 So we should type pop -. 78 04:54.650 --> 04:56.720 So now let's run this command. 79 04:56.840 --> 04:59.660 And you see now - became one. 80 04:59.930 --> 05:02.210 So, and then you notice the stack. 81 05:02.210 --> 05:04.940 Now the one has been taken off the top of the stack. 82 05:05.270 --> 05:09.680 So this concept of the stack is like a pancake. 83 05:09.860 --> 05:16.070 So in a pancake which is stacked, whatever is last in will be the first out. 84 05:16.490 --> 05:21.410 So that is the concept, is same concept used in the stack, in the memory stack here. 85 05:21.740 --> 05:28.040 So this is how we can make use of the stack to serve as a temporary storage for data. 86 05:28.040 --> 05:33.830 And later on, when you want to restore that the register, you can just pop it back to the register. 87 05:33.920 --> 05:37.730 One thing to note is the order of pop is very important. 88 05:37.760 --> 05:43.580 If you push - first and - second, then when you pop, you have to pop them in reverse order. 89 05:43.580 --> 05:46.820 So you need to pop - first, and then you pop -. 90 05:46.850 --> 05:49.700 If you mess up this order, then what? 91 05:49.850 --> 05:54.890 What in fact you are doing is to modify, the swap, the values. 92 05:54.890 --> 05:57.860 So now in the next exercise, I want you to try this. 93 05:57.980 --> 06:00.500 Write a program to swap values, 94 06:00.680 --> 06:01.850 - and -. 95 06:01.940 --> 06:07.340 So give that a try, and then watch the solution in the next lesson. 96 06:07.430 --> 06:09.020 That's all for this video. 97 06:09.020 --> 06:10.490 Thank you for watching.