WEBVTT 00:00.470 --> 00:07.040 Hello, my name is Stephan and welcome to this in-depth exploration of an assembly code example designed 00:07.040 --> 00:11.570 to calculate the sum of numbers from zero to a given value. 00:11.600 --> 00:18.320 This tutorial aims to guide you through the intricate workings of the code, breaking down each instruction 00:18.320 --> 00:25.530 to unveil the underlying mechanisms of register manipulation, loop constructs and interfacing with 00:25.530 --> 00:30.500 the external functions such as values printf here. 00:30.500 --> 00:37.040 So by the end of this lecture, you will have gained a profound understanding of these components, 00:37.160 --> 00:39.950 come together to create a functional program. 00:39.950 --> 00:43.850 And now let's delve into the specifics of the code. 00:43.940 --> 00:48.830 We will first declare the external function declaration. 00:48.830 --> 00:55.550 The External Directive announces the existence of the printf function, which resides externally, implying 00:56.270 --> 01:00.090 that its implementation will be linked later. 01:00.090 --> 01:03.930 So here we will write extern printf. 01:04.490 --> 01:10.550 And after that we will create a section data and this is the data section. 01:10.550 --> 01:15.140 And here we will define a 64 bit quad word. 01:15.170 --> 01:21.080 Here we will number number equal five. 01:21.110 --> 01:25.730 Here we are defining the 64 bit quad word, which is eight bytes. 01:26.990 --> 01:28.870 Uh, let's actually write it down here. 01:28.880 --> 01:30.320 Eight bytes. 01:33.620 --> 01:34.520 Quadword. 01:35.330 --> 01:45.020 We are defining the eight bytes Quadword variable named number with an initial value of five and we 01:45.020 --> 01:45.650 are here. 01:45.740 --> 01:47.380 Fmt db. 01:48.500 --> 01:50.030 We will add a string here. 01:50.030 --> 01:53.840 The sum from zero to. 01:55.650 --> 01:57.090 L d. 01:58.100 --> 01:58.940 Is. 02:01.130 --> 02:03.040 Is old. 02:03.410 --> 02:05.240 And here we will pass ten. 02:06.320 --> 02:06.790 Atlas. 02:07.870 --> 02:10.270 Here, ten and zero. 02:11.350 --> 02:20.130 Here we are defining a null terminated string format suitable for printf, and we have also developed 02:20.130 --> 02:23.230 the section BSS. 02:23.260 --> 02:27.610 This is the section section uninitialized data. 02:27.910 --> 02:34.990 Here currently empty and this section is used to reverse space for variables and this that will be initialized 02:34.990 --> 02:36.580 during program execution. 02:36.850 --> 02:39.970 We will also define the section text. 02:40.420 --> 02:42.460 Section Text. 02:43.090 --> 02:49.870 We will create a global main here and you can define main as the global entry point of the program. 02:50.260 --> 02:52.000 We will create the main. 02:52.000 --> 02:55.270 Now this is a function prologue here. 02:55.960 --> 02:57.760 Posh RVP. 02:59.110 --> 03:01.170 Here and move RVP. 03:02.900 --> 03:05.210 RWP rsp. 03:07.800 --> 03:08.610 Rtsp. 03:09.930 --> 03:20.070 And here in this line of code, we are preserving the value of base pointer rbpp by pushing it on to 03:20.070 --> 03:21.460 the stack. 03:21.480 --> 03:29.010 And here we are setting the base pointer rbpp to the current stack pointer RSP. 03:29.400 --> 03:33.660 And now we will go to initialization path phase here. 03:33.690 --> 03:37.050 So we will use move rc. 03:39.760 --> 03:40.600 Number. 03:41.290 --> 03:42.530 The number we defined here? 03:42.530 --> 03:43.100 Five. 03:43.780 --> 03:44.430 Here. 03:44.470 --> 03:45.280 Here we are. 03:45.390 --> 03:46.200 Load the. 03:46.480 --> 03:54.610 We are telling the assembly to load the value stored in the memory location number into the loop contour 03:54.640 --> 04:01.930 here, which in this case is rc x and we will move racks. 04:02.840 --> 04:03.440 Zero. 04:03.620 --> 04:12.170 We are initialize telling the assembly to initialize the accumulator racks to zero. 04:13.280 --> 04:16.610 And now we will create this loop here. 04:16.690 --> 04:17.480 B Loop. 04:17.900 --> 04:20.600 So this is the loop body. 04:20.630 --> 04:23.240 We will add like this. 04:23.360 --> 04:24.890 We will add Rex. 04:24.890 --> 04:25.280 Actually. 04:25.280 --> 04:25.760 Why? 04:26.780 --> 04:28.310 This here is like this. 04:32.000 --> 04:35.150 So bloop ad rex. 04:37.090 --> 04:40.960 Are racks and ask X. 04:41.920 --> 04:49.780 Here we are telling the assembler to add the value of the looping counter rc x to the accumulator rax 04:49.780 --> 04:50.470 rax. 04:50.620 --> 04:52.960 And now we will loop. 04:54.490 --> 04:55.540 Up here. 04:56.600 --> 05:04.520 Now we will decrement the loop counter arcs and jump back to bloop bloop. 05:04.520 --> 05:13.970 If RC X is not zero and here we can write our loop termination and here. 05:18.760 --> 05:20.870 Move the. 05:23.320 --> 05:26.080 Are the I fmtm here. 05:26.080 --> 05:31.330 Load the address of the format string into RDA or RDA. 05:34.390 --> 05:35.620 Or die here. 05:35.620 --> 05:36.010 Yeah. 05:36.040 --> 05:37.720 MOV RSI. 05:40.400 --> 05:41.210 Number. 05:41.990 --> 05:44.810 There are a lot of the value of number. 05:45.290 --> 05:55.390 This is the sum to be displayed into RSI and move X to racks. 05:55.400 --> 06:01.200 So load the computer to sum of value from racks into RDX. 06:01.250 --> 06:03.440 Move Racks. 06:05.000 --> 06:05.630 Zero. 06:05.750 --> 06:11.030 Clear racks in preparation for the printf call. 06:11.090 --> 06:13.810 Now we will also call the printf here. 06:13.820 --> 06:14.840 Call printf. 06:14.870 --> 06:18.860 We are invoking the printf function to display the formatted output. 06:18.860 --> 06:22.730 And now we will write our function epilogue here. 06:22.910 --> 06:24.650 MOV rsp. 06:26.230 --> 06:31.900 Rdrp E.r.v are telling the Assembly to restore the stack pointer rsp to its original value. 06:31.930 --> 06:39.820 Cleaning up the stack and we will pop the rbp restore the original value of the base pointer r p and 06:39.820 --> 06:42.640 after that we will return from the main function. 06:46.650 --> 06:47.190 This. 06:47.280 --> 06:55.590 The assembly code showcased here efficiently calculates the summation of numbers starting from zero 06:55.590 --> 06:57.960 up to a specified value. 06:57.990 --> 07:08.190 The key mechanism is the loop construct in which the RC X register serves as a loop counter with each 07:08.190 --> 07:09.300 iteration. 07:09.330 --> 07:21.330 RC x is automatically decremented and the the loop body adds its value to the accumulator racks or racks. 07:21.330 --> 07:30.570 And once the loop concludes, the accumulated sum is presented through the printf function, and a thought 07:31.200 --> 07:38.410 provoking experiment arises by setting number to a notably larger value such as 1 billion. 07:38.430 --> 07:44.160 By doing so, you can observe the program's execution time using the Linux Time command, which we will 07:44.160 --> 07:45.900 do that in the next lecture. 07:45.930 --> 07:49.990 Leading to valuable insights into its performance characteristics. 07:49.990 --> 07:57.130 And this enlightening tutorial empowers you to grasp the intricate interplay of registers, memory and 07:57.130 --> 07:59.530 control flow in assembly language programming. 07:59.620 --> 08:03.670 Fostering a deeper understanding of low level computer operation.