WEBVTT 1 00:01.700 --> 00:03.050 Hello and welcome. 2 00:03.050 --> 00:07.670 In this lesson, we are going to take a look at the TEST instruction. 3 00:08.720 --> 00:16.670 The TEST instruction takes the format TEST register, immediate or TEST register, register. 4 00:16.670 --> 00:22.040 These are the two most common formats being used by the TEST instruction. 5 00:22.880 --> 00:27.170 The purpose of the TEST instruction is to test the bits. 6 00:27.350 --> 00:33.050 What it essentially does is perform an AND logical operation between two values. 7 00:33.050 --> 00:40.760 For example, if you have the first value of this bit binary number, and if we wanted to test whether 8 00:40.760 --> 00:50.120 the fourth bit and the second bit is one, then we will use a test in this way: 1010. 9 00:50.930 --> 01:01.940 So if any of these bits here are non-zero, then it will result in the zero flag being cleared. 10 01:03.110 --> 01:04.880 The zero flag here being cleared. 11 01:05.390 --> 01:13.970 However, if any of these tests, any of the bits here results in a zero, let's say both are zeros, 12 01:13.970 --> 01:18.080 then the zero flag will be set to one. 13 01:19.220 --> 01:22.370 So let's take a look at a practical example. 14 01:22.790 --> 01:30.200 So I've opened the template2.exe file in x64dbg and put the breakpoint here. 15 01:30.410 --> 01:34.370 So now let us move this value to the - register. 16 01:46.160 --> 01:47.960 And then we will do a test. 17 02:03.570 --> 02:12.360 So what we are doing is we are moving this value into the - register, and then we are doing a test 18 02:15.840 --> 02:18.270 for the second and the fourth bit. 19 02:25.330 --> 02:27.190 Let us now run to our breakpoint. 20 02:27.820 --> 02:29.080 And then step over it. 21 02:30.610 --> 02:36.280 Now we have this in the - register, hex 88. 22 02:36.280 --> 02:40.990 So let us check what is hex 88 using our programmer's calculator. 23 02:45.430 --> 02:51.280 Let's look for the calculator and make sure it is set to programmer. 24 02:51.910 --> 02:54.250 And here we set it to byte. 25 02:55.330 --> 02:58.630 Now we can select hex and then key in 88. 26 02:58.630 --> 03:05.080 And we check to make sure that the binary is the same as the one we input. 27 03:06.220 --> 03:07.540 So it is the same. 28 03:08.200 --> 03:12.730 Next, we are going to test our - with A. 29 03:12.760 --> 03:14.860 So A should be 1010. 30 03:15.580 --> 03:18.430 So we clear this and we check what is A. 31 03:18.580 --> 03:20.710 And indeed it is 1010. 32 03:22.270 --> 03:27.940 Now we are going to step over both of these and check the zero flag. 33 03:30.070 --> 03:31.180 The zero flag is cleared. 34 03:31.960 --> 03:33.370 So it is correct. 35 03:33.970 --> 03:42.250 The zero flag is cleared because the result of this binary AND here and AND binary AND over here will yield 36 03:42.670 --> 03:45.640 a one, a non-zero. 37 03:45.790 --> 03:48.100 That's why the flag is cleared. 38 03:50.100 --> 03:54.420 If both of these yield zero, then the zero flag will be set. 39 03:54.900 --> 03:58.470 So let's try another test now. 40 03:59.520 --> 04:02.370 We are going to test our -. 41 04:04.830 --> 04:06.300 And see what we get. 42 04:07.560 --> 04:13.080 So this test is to test whether or not - is zero. 43 04:14.250 --> 04:15.930 So we can do that now. 44 04:16.050 --> 04:20.280 We test - with -. 45 04:23.950 --> 04:28.180 So we step over this and see what happens to the zero flag. 46 04:28.960 --> 04:33.250 Now the - has got the value 88 like this. 47 04:34.480 --> 04:36.220 So when we step over, 48 04:38.410 --> 04:39.850 it still remains the same. 49 04:39.850 --> 04:47.320 And the zero flag is still clear because the value in - is a non-zero. 50 04:47.320 --> 04:49.450 That is why the zero flag is clear. 51 04:49.870 --> 04:50.860 It is not set. 52 04:52.000 --> 04:57.640 However, let us now modify the - value to make it zero. 53 05:01.340 --> 05:02.330 Now it is zero. 54 05:02.330 --> 05:04.310 And now we perform another test. 55 05:05.210 --> 05:06.260 - -. 56 05:12.000 --> 05:14.730 So go with it and watch what happens to the zero flag. 57 05:15.870 --> 05:19.770 This time the zero flag is set to one because the value in - is zero. 58 05:20.820 --> 05:28.770 So this is a very common way to use the test in order to check for the value, whether or not a register 59 05:28.770 --> 05:29.520 is zero. 60 05:30.450 --> 05:39.210 So whenever you perform a test, if the value in - is zero, then the zero flag will be set. 61 05:40.440 --> 05:46.800 However, if you perform the test and you find that - is a non-zero, the zero flag will be cleared. 62 05:47.970 --> 05:54.390 Now, this is very commonly used when testing for the return value from a function. 63 05:55.050 --> 06:02.370 Whenever we call a function, the function normally returns the result in the - register. 64 06:03.030 --> 06:09.060 The register value will either be zero, one, negative one, or some other non-zero value. 65 06:09.510 --> 06:15.570 So by using TEST -, we can quickly determine the result of the function call. 66 06:16.140 --> 06:23.820 So normally if a function call returns zero, it would mean that the function successfully 67 06:23.820 --> 06:24.480 ran. 68 06:25.050 --> 06:27.030 And if it returns negative one, 69 06:27.960 --> 06:31.350 it means it could mean that the function somehow failed. 70 06:31.380 --> 06:38.100 The test failed in its function. And in programming in assembly, 71 06:38.130 --> 06:42.540 this is a normal way for returning values from a function. 72 06:42.780 --> 06:46.200 So whenever a function returns a value, it will return it in -. 73 06:46.200 --> 06:52.110 And we use TEST - to check for the value of the function 74 06:52.110 --> 06:54.450 call, the return value of the function call. 75 06:55.590 --> 06:56.550 So 76 06:56.550 --> 06:58.530 TEST has two uses. 77 06:58.530 --> 07:01.200 One is to check for the individual bits. 78 07:01.200 --> 07:08.940 And the second use of the TEST is to test whether a register is zero by using a test on itself. 79 07:08.970 --> 07:09.960 -. 80 07:10.350 --> 07:15.120 If we wanted to test whether the value in - is zero, then you will use TEST 81 07:15.420 --> 07:15.960 - -. 82 07:17.350 --> 07:23.920 Now, one thing to note is that the TEST performs an AND operation, logical AND. 83 07:24.490 --> 07:30.970 And the good thing about this TEST is that it does not affect the result of the register. 84 07:31.330 --> 07:36.490 The register's original value is always preserved, unlike an AND operation. 85 07:36.790 --> 07:45.760 If you do an AND operation, it will alter the result in the - register or - register. 86 07:46.270 --> 07:52.660 But if you use a TEST, it will not affect the original values in those registers. 87 07:53.200 --> 07:57.160 So that's why TEST is the preferred method 88 07:57.160 --> 08:03.730 if we wanted to test for the register value, whether the value is zero or non-zero. 89 08:04.570 --> 08:06.580 So that's all for this lesson. 90 08:06.610 --> 08:08.290 Thank you for watching.