[CS61C FA20] Lecture 13.5 - Compilation, Assembly, Linking, Loading: Loader — Transcript

Full transcript

1. 0:00and welcome back we're at the last stage
2. 0:03the loader this is it we've got an adot
3. 0:05out it was produced
4. 0:06what do we do so let's take a look at
5. 0:08the picture where are we now the
6. 0:09compilers worked now i got a dot s
7. 0:11the assembly did its work i got a dot oh
8. 0:14the linker is working got an a dot
9. 0:15out or an executable hopefully rename
10. 0:17don't have your function be called a dot
11. 0:19out have it go
12. 0:19give it give it a good name but i've
13. 0:21linked it all those libraries i've done
14. 0:22it statically linked we talked about
15. 0:23other options but i've done it
16. 0:24statically
17. 0:25now i have a standalone working
18. 0:27executable how do i run this darn thing
19. 0:29it's in disk
20. 0:30how do i get this thing running that's
21. 0:32at the job of the loader so let's talk
22. 0:33about that okay load into memory and
23. 0:34then run this whole thing
24. 0:36so the input is executable code out or
25. 0:39some well better name for it
26. 0:40the output is the programmers are
27. 0:41actually running executable executables
28. 0:44on disk we know that okay the loader's
29. 0:46job
30. 0:47is to load it into memory and set up all
31. 0:49that needs to set up and start it
32. 0:50running and in reality the load is
33. 0:51actually the os that's we've seen that
34. 0:52before and that's actually one of those
35. 0:54tasks
36. 0:54if you take 162 you'll learn all about
37. 0:56what loaders do you say oh and i learned
38. 0:58that
39. 0:59there was a reference to this they said
40. 1:01the lotus job is to run the program
41. 1:02how do you actually do that they'll
42. 1:03teach you explicit lecture of what this
43. 1:05what happens
44. 1:06so this is one slide it's a lot on the
45. 1:08slide but this is one slide it's all the
46. 1:09pieces let's actually go through one by
47. 1:11one
48. 1:11what it does okay i've got a header file
49. 1:15i know from the header how big things
50. 1:17are how big the text is
51. 1:19how big the data is okay i create a new
52. 1:22address space we'll see this a little
53. 1:24later we haven't done this virtual
54. 1:25memory at all
55. 1:26you think you have 2 to the 32 space
56. 1:29well you know really but
57. 1:30for now you do it creates an address
58. 1:32space for the program large enough so i
59. 1:34give it some space
60. 1:35i don't really give a 2-32 but pretend
61. 1:37you do it for now
62. 1:38to be able to store all the text and all
63. 1:41the data
64. 1:41okay along with a little bit of stack
65. 1:44it's a little bit of room so i'll give
66. 1:45it a little stack i give it so i
67. 1:46basically make some
68. 1:48room remember you just keep thinking
69. 1:50this i get the whole 2 to the 32 but i'm
70. 1:52going to
71. 1:53i don't really but for now you do but
72. 1:55okay so you get
73. 1:56enough for the text the data and stack
74. 1:58for now because i'm using any more and
75. 2:00as you need more you'll get more but
76. 2:01for now you have it all and it just puts
77. 2:03room for that okay so this is a little
78. 2:05weird to understand that line
79. 2:06because i haven't talked to you about
80. 2:07virtual memory yet i now copy
81. 2:09i gotta read from disk write it into
82. 2:11memory i copy that stuff the
83. 2:13instructions the text
84. 2:14the data whatever data you had from the
85. 2:16executable into that address space
86. 2:20now this now this memory can it contains
87. 2:22a copy of what was in on the disk
88. 2:23got that what arguments were there to
89. 2:27the program
90. 2:27you might have command line arguments i
91. 2:30got to copy those to the stack
92. 2:32why in the stack i don't know how big
93. 2:33they'll be i don't just put them into
94. 2:34the into registers
95. 2:35they might be really big i might have a
96. 2:36huge long file name it might be much
97. 2:38bigger than my registers can handle so i
98. 2:39copy all of them to the stack
99. 2:41whatever command arguments on the stack
100. 2:43ready to go
101. 2:44i can initialize my machine registers
102. 2:47mostly we treat our machine registers as
103. 2:49um
104. 2:50as being cleared even though well come
105. 2:52on did you say that
106. 2:54we don't ever clear things and see yeah
107. 2:56but when you start a new program
108. 2:58sometimes you clean these guys so let's
109. 2:59clear the
110. 3:00set the registers up um most of them are
111. 3:02cleared stack pointer is obviously
112. 3:04assigned the address of the first free
113. 3:05stack location you got to set up sp
114. 3:07for the right thing and assume you clear
115. 3:09the rest of the registers
116. 3:10and now finally this is it last point
117. 3:12bullet on the light on this slide for
118. 3:13the loader
119. 3:14i jumped to a startup routine kind of
120. 3:16complicated but you'll see this in 162
121. 3:18that copies the program arguments
122. 3:21from the stack to the registers what
123. 3:22might they be what's the argument to
124. 3:24main
125. 3:25arg c and arc v so that sounds like
126. 3:27that's a zero and a one
127. 3:29a zero is arg c a one is arc v a pointer
128. 3:33to that place where that where they live
129. 3:34on the stack
130. 3:35but all of that the value of r c is here
131. 3:38and a zero
132. 3:39arc v pointer lives in a1 that's it
133. 3:42and now set the pc to the right place
134. 3:45yoink
135. 3:46go if the main return routine returns
136. 3:50it terminates us with us with a system
137. 3:52call whatever the return value is pass
138. 3:54that back
139. 3:54up to the so this is important if you
140. 3:56exit zero that exit zero has to somehow
141. 3:58bubble its way up so that that is what
142. 3:59the program returns is a zero
143. 4:01that's it that's what the loader does
144. 4:03the loader is basically os
145. 4:04which does all those things essentially
146. 4:06copies things to memory
147. 4:07sets up the registers sets up the stack
148. 4:10points you know passes an rxcrv
149. 4:13says a pc your line is the first line of
150. 4:15main ready
151. 4:16go and just whatever whatever magic
152. 4:18happens happens as soon as it does that
153. 4:20pretty cool right that's it that's the
154. 4:23compiler assembler linker and loader
155. 4:26let's see an example in the next lecture
156. 4:28we'll see you there