1 23:59:59,500 --> 00:00:05,374 [MUSIC]. 2 00:00:05,374 --> 00:00:07,682 Hello again. Now that, that we know how to implement 3 00:00:07,682 --> 00:00:11,200 conditionals and loops in assembly. Let's see how we implement a little more 4 00:00:11,200 --> 00:00:15,352 complex control structure. we talk, we're going to talk about switch 5 00:00:15,352 --> 00:00:18,488 statements. And the way switch statements work in C 6 00:00:18,488 --> 00:00:20,860 is as follows. You have an example here. 7 00:00:20,860 --> 00:00:24,655 If I say switch x, what I'm saying is now there's going to be a bunch of options 8 00:00:24,655 --> 00:00:27,900 that depending on the value of x, I'm going to execute a different piece of 9 00:00:27,900 --> 00:00:31,090 code. Okay? 10 00:00:31,090 --> 00:00:34,933 So here in our example if x happens to be set to 1, we execute this piece of code 11 00:00:34,933 --> 00:00:38,047 here. And then this break statement just says 12 00:00:38,047 --> 00:00:40,790 that this piece of code is going to be executed. 13 00:00:40,790 --> 00:00:44,820 And then we're going to stop and continue executing past the end of the switch 14 00:00:44,820 --> 00:00:49,450 statements. There are some other cases called fall 15 00:00:49,450 --> 00:00:52,570 through cases that do not have a break statement. 16 00:00:52,570 --> 00:00:57,380 For example, if x is happened to be set to 2. 17 00:00:57,380 --> 00:01:00,130 This piece of code is going to be executed and is going to continue 18 00:01:00,130 --> 00:01:04,550 executing through the next block, until it see's the next break statement. 19 00:01:04,550 --> 00:01:09,774 Then [INAUDIBLE] jumps it out of the switch and continues executing. 20 00:01:09,774 --> 00:01:12,743 Okay? And now so if you have any missing values 21 00:01:12,743 --> 00:01:17,216 in a set cases, for example if x happens to be set to 4, since there's no value, 22 00:01:17,216 --> 00:01:23,710 no case for 4 here. What gets executed is the default value. 23 00:01:23,710 --> 00:01:28,169 So the default block of codes. So, the way to think about the default 24 00:01:28,169 --> 00:01:33,270 set is there, if the value that you're switching on is not in any of the case. 25 00:01:33,270 --> 00:01:40,070 You have to view the default case. And why could implement this with branch 26 00:01:40,070 --> 00:01:46,005 instructions that we say earlier. It gets really complicated quickly, so 27 00:01:46,005 --> 00:01:50,190 another way of oftentimes and this is using what we call a jump table. 28 00:01:50,190 --> 00:01:53,752 And a jump table works as follows. Say here we have our example we have an 29 00:01:53,752 --> 00:01:58,915 example of a switch statement. Now we going to have a table that has 30 00:01:58,915 --> 00:02:06,930 roughly as many entries as the number of cases in our, in our switch statement. 31 00:02:06,930 --> 00:02:08,590 The number of cases now in the switch statement. 32 00:02:09,790 --> 00:02:13,246 It's, now in each one of these entries the table points to the address of a code 33 00:02:13,246 --> 00:02:17,298 block. Which is course, corresponds to the code 34 00:02:17,298 --> 00:02:20,850 block that has to be executed, based on the case value. 35 00:02:20,850 --> 00:02:26,760 So, for example, if our x happens to be val 1. 36 00:02:26,760 --> 00:02:30,750 we're going to jump, we're going to look at the jump table, get the corresponding 37 00:02:30,750 --> 00:02:33,740 address. And select the tag1 and that's the code 38 00:02:33,740 --> 00:02:37,247 that we're going to execute. We're going to execute this piece of code 39 00:02:37,247 --> 00:02:40,206 here. now what we are going to think about this 40 00:02:40,206 --> 00:02:44,213 is the following. You could have a variable called target 41 00:02:44,213 --> 00:02:48,490 which holds the target address for a given option. 42 00:02:48,490 --> 00:02:52,210 We're going to look up our jump table, which is just an array of instruction 43 00:02:52,210 --> 00:02:55,550 addresses. We're going to index that array with x 44 00:02:55,550 --> 00:03:00,807 and we're going to get the value storing target and we're going to jump on it. 45 00:03:00,807 --> 00:03:04,284 We're going to do a Go To and use that as [UNKNOWN] Go To and to jump to that 46 00:03:04,284 --> 00:03:09,618 address, go to that address. So, these are the way of looking at this 47 00:03:09,618 --> 00:03:13,086 more visually. I have another switch statements in 48 00:03:13,086 --> 00:03:18,181 fourteen other cases I have the corresponding vision of memory. 49 00:03:18,181 --> 00:03:22,789 Just in the color matching there and then our jump table is going to map back to 50 00:03:22,789 --> 00:03:27,762 the corresponding To the corresponding memory region that 51 00:03:27,762 --> 00:03:30,571 implements that block of code corresponding to the, to the case 52 00:03:30,571 --> 00:03:33,470 statement. So, now 1 is green, so that means that 53 00:03:33,470 --> 00:03:36,070 the [INAUDIBLE] 1 here has to point to green. 54 00:03:36,070 --> 00:03:39,282 That's this one. And same, same thing for orange and so 55 00:03:39,282 --> 00:03:41,710 on. Okay? 56 00:03:41,710 --> 00:03:45,420 Now notice an interesting case here, which is the default case. 57 00:03:45,420 --> 00:03:49,450 The default case, as I said before, gets executed if none of the value if, if the 58 00:03:49,450 --> 00:03:53,604 value of x doesn't happen to, happens to be none of the values explicitly defined 59 00:03:53,604 --> 00:03:59,790 by cases, okay? So, here it happens to be 0, right? 60 00:03:59,790 --> 00:04:03,982 We're going to jump to the default case, and also happens to be four. 61 00:04:03,982 --> 00:04:10,730 We're going to jump to the default case. Okay? 62 00:04:10,730 --> 00:04:15,050 And we can use a jump table as long as the value is lower than six. 63 00:04:15,050 --> 00:04:22,760 For any other value that's greater than six, we go to execute the default code. 64 00:04:22,760 --> 00:04:27,163 The default block. for this jump table, just a list of 65 00:04:27,163 --> 00:04:29,815 addresses. And he's how we get setup. 66 00:04:29,815 --> 00:04:35,275 in assembly, you would see a declaration of long words that point, that happen to 67 00:04:35,275 --> 00:04:42,429 be the same value as the label that maps to the address of the code block. 68 00:04:43,750 --> 00:04:49,490 So now this, for example, if we take .L61, that's where the default code block 69 00:04:49,490 --> 00:04:54,413 starts. So, the corresponding structured address 70 00:04:54,413 --> 00:05:00,840 is going to be stored here, and same thing for the other case values. 71 00:05:00,840 --> 00:05:03,380 So, let's see how we use this table in assembly now. 72 00:05:05,160 --> 00:05:11,403 so here we have the jump table. And what we're going to do, is we're 73 00:05:11,403 --> 00:05:17,200 going to, depending on the value of x, which is loaded in edx here. 74 00:05:17,200 --> 00:05:20,542 We're going to find the corresponding entry. 75 00:05:20,542 --> 00:05:26,330 And the way this is done, this is done using a indirect jump. 76 00:05:26,330 --> 00:05:31,221 The indirect jump says that, instead of jumping to specific address, it jumps to 77 00:05:31,221 --> 00:05:37,284 an address containing an address. So, the way this works is this .L62 here, 78 00:05:37,284 --> 00:05:43,608 is the base address of our, jump table. Now the value of edx gets multiplied by 79 00:05:43,608 --> 00:05:46,630 four and [UNKNOWN] because a long word is four bites. 80 00:05:46,630 --> 00:05:53,749 So, if the x is say four, we're going to add zero, we're going to add four, one, 81 00:05:53,749 --> 00:06:01,824 two, three, four. And that's the value that's going to be, 82 00:06:01,824 --> 00:06:04,780 jumped to. 'Kay? 83 00:06:04,780 --> 00:06:09,202 So, another thing to note, here, is that we are comparing whether x happens to be 84 00:06:09,202 --> 00:06:15,467 greater than six. And if it greater than six, it jumps 85 00:06:15,467 --> 00:06:21,620 straight to default. So, now lets see how the code blocks are 86 00:06:21,620 --> 00:06:25,705 implemented. So here, suppose that we have we're 87 00:06:25,705 --> 00:06:32,041 interested in case two here. And in case two happens to be mapped to 88 00:06:32,041 --> 00:06:36,946 label .L57. It means that this is where, the assembly 89 00:06:36,946 --> 00:06:42,590 codes that implements the block for case two is located. 90 00:06:42,590 --> 00:06:46,970 So, we have a division here, so that's why we must have a division instruction. 91 00:06:46,970 --> 00:06:50,270 And note that this a fall through case, so it's going to be in case x is 2, is 92 00:06:50,270 --> 00:06:54,911 going to be executing. And it's going to continue executing 93 00:06:54,911 --> 00:07:00,110 through the next case block, until the break statement. 94 00:07:00,110 --> 00:07:04,898 And the break statement here is implemented by essentially just finishing 95 00:07:04,898 --> 00:07:09,044 the execution. Returning from the function in which the 96 00:07:09,044 --> 00:07:15,246 switch block is inserted. And note here that the default case also, 97 00:07:15,246 --> 00:07:20,809 after this there is a return. So, if you look at here in label .L61 98 00:07:20,809 --> 00:07:26,610 you're going to see here. That after, right after being done with 99 00:07:26,610 --> 00:07:31,170 the corresponding code block, we also leave and return from the function. 100 00:07:33,090 --> 00:07:36,935 So, here's the, the rest of the code block for our example. 101 00:07:36,935 --> 00:07:42,647 again you see, the important thing to notice here is that, you have returns 102 00:07:42,647 --> 00:07:50,112 after every single block that had breaks. And for both case five and, and case six, 103 00:07:50,112 --> 00:07:55,904 they're both mapped to case L60, which is the label right here. 104 00:07:55,904 --> 00:08:00,940 Now let's look at how these labels are defined now. 105 00:08:00,940 --> 00:08:05,227 If you look at the object code, they, you see that they become an instruction 106 00:08:05,227 --> 00:08:09,425 [INAUDIBLE] before as chosen by the linker, okay? 107 00:08:09,425 --> 00:08:13,455 And there'll be one of those for each one of the the case values in our switch 108 00:08:13,455 --> 00:08:17,160 statement. So, if you go look at entire table you 109 00:08:17,160 --> 00:08:21,970 see there will be one address for case statement. 110 00:08:21,970 --> 00:08:27,940 And those have two as we saw before put points at a code block. 111 00:08:27,940 --> 00:08:32,158 So now we can look at each one of those you see mapped to a block of codes 112 00:08:32,158 --> 00:08:39,475 corresponding to the code inserted in the, in the case block for that value. 113 00:08:39,475 --> 00:08:44,659 so here's our, our, our list of instruction addresses for the jump table, 114 00:08:44,659 --> 00:08:49,600 and each one of them points to a block of code. 115 00:08:49,600 --> 00:08:51,170 And notice that we have some repeated ones. 116 00:08:51,170 --> 00:08:54,866 Remember that some, some of the cases has, for example, follow through or cases 117 00:08:54,866 --> 00:08:58,109 that are not defined. So, in this case here it's, it's the 118 00:08:58,109 --> 00:09:01,550 default value. For value four, if you remember it has to 119 00:09:01,550 --> 00:09:05,910 go through the default case, so it points there. 120 00:09:05,910 --> 00:09:10,860 So the same thing for value zero. Now let me ask you a question. 121 00:09:10,860 --> 00:09:14,330 Would you implement this one with a jump table? 122 00:09:15,560 --> 00:09:17,060 Well, look at the size of this value here. 123 00:09:17,060 --> 00:09:19,210 Probably not. This table would be very, very, very 124 00:09:19,210 --> 00:09:21,420 large. Okay? 125 00:09:21,420 --> 00:09:26,740 So, jump jump tables are, are, are used normally when you have few case values 126 00:09:26,740 --> 00:09:33,458 and you can build a small table for it. In this case here, it would be much more 127 00:09:33,458 --> 00:09:36,979 efficient to just implement this with branch instructions. 128 00:09:38,160 --> 00:09:42,480 So, now we conclude our exodus six assembly programming section. 129 00:09:42,480 --> 00:09:44,020 And I'll see you next time.