WEBVTT 00:00.480 --> 00:06.240 Once the pre-processing phase concludes, the source code is prepared for compilation. 00:06.270 --> 00:12.600 During the compilation phase, the pre-processing code undergoes translation into assembly language. 00:12.600 --> 00:18.540 So it's worth noting that compilers often incorporate significant optimization capabilities in this 00:18.540 --> 00:31.770 phase, so these optimizations can be adjusted using the GCC uppercase or zero through GCC uppercase 00:31.800 --> 00:33.120 or three. 00:33.450 --> 00:36.750 So from 0 to 3 options in GCC. 00:36.870 --> 00:44.190 So the level of optimization choosing can greatly impact the resulting disassembly as you will explore 00:44.190 --> 00:47.070 in detail in next sections. 00:47.070 --> 00:54.360 So now you might wonder why the compilation phase produces assembly language instead of directly generating 00:54.360 --> 00:55.110 machine code. 00:55.110 --> 01:01.810 So this design decisions becomes clearer when considering the multitude of programming languages in 01:01.810 --> 01:02.470 existence. 01:02.470 --> 01:09.460 So aside from C, there are numerous popular compiled languages like C Plus plus objective C, Common 01:09.460 --> 01:13.210 Lisp, Delphi, Go and Haskell to just name a few. 01:13.210 --> 01:21.100 So developing a compiler that directly emits machine code from each of these languages would be an incredibly 01:21.100 --> 01:23.780 daunting and time consuming task. 01:23.800 --> 01:30.640 Instead, it is more practical to emit assembly code, which is already quite challenging for developers, 01:30.640 --> 01:37.840 and utilize a dedicated assembler that handles the final translation from assembly to machine code for 01:37.840 --> 01:39.280 all supported languages. 01:39.310 --> 01:46.540 Therefore, the output of the compilation phase is assembly code represented in a reasonably human readable 01:46.540 --> 01:50.080 format with preserved symbolic information. 01:50.080 --> 01:57.400 So in the case of GCC, which automates all compilation phases by default, you need to instruct it 01:57.400 --> 02:02.710 to halt after the compilation stage and save the assembly files to disk. 02:02.740 --> 02:08.410 To examine the emitted assembly as we did for in previous lecture for pre-processing phase. 02:08.410 --> 02:15.880 So to achieve this you can use the s flag where where's is conventional extension for assembly files? 02:15.910 --> 02:25.090 Additionally, the maxim here m a s m Intel option is passed to GCC instructing it to generate assembly 02:25.090 --> 02:29.140 Intel syntax instead of the default AT&T syntax. 02:29.140 --> 02:36.040 So to provide you with a visual representation, we will use visual and practical representation. 02:36.040 --> 02:39.030 We will create an example code. 02:39.040 --> 02:44.860 So here now let's create this code here. 02:46.570 --> 02:55.390 GCC again, as we said, we will use the uppercase S and m a s m Intel here. 02:55.390 --> 03:05.890 And after that we will input our C file, which is my app dot C here, and that's it. 03:05.890 --> 03:06.970 It's compiled. 03:06.970 --> 03:15.010 And what we're going to do is we will read the compilation example with Mousepad with some text editor 03:15.010 --> 03:15.520 here. 03:15.520 --> 03:17.050 Mouse pad. 03:19.240 --> 03:21.400 The my app dot here. 03:21.400 --> 03:28.540 As you can see, we have C and S, we will open the S, which just generated and here this is assembly 03:28.540 --> 03:33.370 generated by the compilation phase for the Hello World Program. 03:33.370 --> 03:39.100 And for now I won't go into details about the assembly code just for now. 03:39.100 --> 03:46.120 But but what's interesting here is that the assembly code is relatively easy to read because the symbols 03:46.120 --> 03:49.630 and functions have been preserved. 03:49.630 --> 03:57.970 So for instance, constant and variable variables have symbolic names rather than like just addresses, 03:57.970 --> 04:06.040 even if it's just an automatically generated name such as SC zero for the Nameless Hello World String, 04:06.040 --> 04:09.640 and there's an explicit label for the main function. 04:09.970 --> 04:12.790 The only function in this case here. 04:13.510 --> 04:22.180 And the main function is going to be right here above LSB zero and. 04:22.920 --> 04:28.080 And there's an explicit label for the um, also hello world String. 04:28.080 --> 04:28.890 So. 04:30.400 --> 04:36.550 And any reference code to the data are also symbolic, such as reference to this. 04:36.580 --> 04:37.840 Hello world. 04:38.110 --> 04:38.860 Here. 04:40.320 --> 04:40.620 This. 04:40.620 --> 04:41.760 Hello, world here. 04:42.750 --> 04:43.650 And. 04:44.700 --> 04:50.230 And you will have no such luxury when dealing with a stripped binaries. 04:50.250 --> 04:52.380 Later in this course. 04:52.500 --> 04:55.800 Here, for example, we have all the variable names and so on. 04:55.950 --> 04:59.700 And in the next lecture we will go with the assembly phase. 04:59.700 --> 05:01.710 So I'm waiting you in the next lecture.