[CS61C FA20] Lecture 09.2 - RISC-V Decisions II: A Bit About Machine Program — Transcript

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1. 0:00[Music]
2. 0:09welcome back
3. 0:10we're still talking about 365 assembly
4. 0:13language but we're going to take
5. 0:14a bit of a detour to see what is
6. 0:16happening with that assembly program
7. 0:18once it is written so say that we have
8. 0:22two pieces of assembly code food.s and
9. 0:24bar.tests which are our assembler source
10. 0:27files which are written in plain text
11. 0:30we're going to pass them through the
12. 0:32assembler which will produce machine
13. 0:35code object files
14. 0:37food.o and bar.toe
15. 0:41those are you know those er ex
16. 0:44extensions dot o stand for the object
17. 0:48files
18. 0:49we may have uh pre-built object file
19. 0:51libraries like lib.o
20. 0:53that may contain some commonly used
21. 0:55functions like for example a
22. 0:57division linker will take our object
23. 1:00files
24. 1:01and the library files and link them
25. 1:04together
26. 1:05now what is linking them together will
27. 1:08place the addresses in appropriate
28. 1:09spaces
29. 1:10and put everything together such that it
30. 1:12looks like one
31. 1:13piece of a program the output is a dot
32. 1:16out
33. 1:16which is machine code executable file so
34. 1:19where does that machine code executable
35. 1:21file
36. 1:22live it is not small enough
37. 1:25to fit in the register so therefore it
38. 1:27has to live in the memory
39. 1:29remember our memory is big and cons
40. 1:32is byte addressable however these bytes
41. 1:35are put together to form
42. 1:3732-bit words we'll have a section of
43. 1:40memory where the data will live
44. 1:43and then there will be a section of a
45. 1:45memory where the program will
46. 1:47all reside those are typically
47. 1:52kept separate from each other with an
48. 1:54exception in 61c
49. 1:56exams where data tends to run
50. 2:00on top of a program and causes
51. 2:02interesting effects over there
52. 2:04but in practice that usually doesn't
53. 2:06happen so
54. 2:08we got a pretty good idea of how does
55. 2:10the program look like now
56. 2:12it is essentially a sequence of bricks
57. 2:14five instructions
58. 2:15where each of these five instruction
59. 2:17instructions is
60. 2:1832 bits wide and we will be executing
61. 2:22them
62. 2:23typically in order until we run into
63. 2:26one of those that corresponds to a
64. 2:27branch or a jump
65. 2:29and then we are going to perhaps jump to
66. 2:32another
67. 2:32place in memory to a different address
68. 2:35in memory
69. 2:36what does the processor do with this
70. 2:38program
71. 2:40well this is a picture that we hopefully
72. 2:43have
73. 2:44have had tattooed already
74. 2:48so our processor to the left has
75. 2:51a control unit and a data path and then
76. 2:54memory is to the right
77. 2:57inside the processor inside the data
78. 3:00path there is one very special register
79. 3:03which is called the program counter
80. 3:05product counter
81. 3:06is internal to the processor and it
82. 3:09holds the byte address of the next
83. 3:11instruction that is going to be executed
84. 3:15so it is something that is really
85. 3:17important
86. 3:18a control unit is going to use the
87. 3:21program counter
88. 3:22to producing the to fetch the
89. 3:25instruction
90. 3:26from the memory take a look at what is
91. 3:28that instruction
92. 3:29and execute it by using the data path
93. 3:32and the memory system and then
94. 3:36we'll update the program counter to
95. 3:39point to the next instruction that is
96. 3:40going to be executed
97. 3:42most commonly that will be the next
98. 3:44instruction in sequence
99. 3:46but sometimes it may be a branch the
100. 3:48next instruction in sequence
101. 3:50is four bytes away you have to keep that
102. 3:52in mind
103. 3:53it is one word away but that is four
104. 3:56bytes
105. 3:57so in order to point to the next address
106. 4:00we need to increment the program counter
107. 4:03by
108. 4:03four bytes or to load
109. 4:06into a program counter a new address
110. 4:09if it is a branch
111. 4:13hopefully you get now a reasonable
112. 4:15picture
113. 4:16of how does the processor execute the
114. 4:18program it may be
115. 4:19helpful understanding what comes next
116. 4:24in the meantime a few other interesting
117. 4:27things that will help us or helpful
118. 4:28things that will
119. 4:29help us understand how to um
120. 4:32work through the functions later on um
121. 4:36first there are there is something there
122. 4:39called
123. 4:39symbolic register names there are 32
124. 4:43registers in risk 5 architecture
125. 4:45the first one is reserved to to be a
126. 4:47zero but the
127. 4:49other 31 are assignable but some of them
128. 4:52are going to have very special purposes
129. 4:54and it will be kind of hard to remember
130. 4:56which one does which out of 31 of them
131. 4:59so we give them
132. 5:00symbolic names for example
133. 5:03physical registers x10 to x17
134. 5:08are have symbolic names a to a zero to a
135. 5:11seven
136. 5:12for argument registers for function
137. 5:15calls
138. 5:16or zero is spelled out as a
139. 5:20as a symbolic name for a register x
140. 5:23naught
141. 5:26there is another useful thing which are
142. 5:29pseudo instructions
143. 5:30remember risk 5 does not have
144. 5:35many instructions and some of them you
145. 5:38know look
146. 5:39fairly cryptic although you know we can
147. 5:41do things in just a simple
148. 5:42one instruction it looks cryptic for
149. 5:44example if we want to copy a value of
150. 5:46one
151. 5:47register to another we have to do
152. 5:50add immediate this of
153. 5:54a source to a destination register with
154. 5:57an immediate value of zero other
155. 6:00processors may have an instruction
156. 6:02move so in this case
157. 6:05move instruction is a symbolic
158. 6:07instruction that corresponds to add
159. 6:09immediate with a zero
160. 6:11so move the destination register source
161. 6:14register
162. 6:15is the same as add immediate destination
163. 6:18register source register zero
164. 6:19it copies the the contents of a source
165. 6:22register
166. 6:23to the destination register the other
167. 6:25one that is
168. 6:26also that will find user useful is
169. 6:29load immediate although it does not work
170. 6:31with the memory it simply
171. 6:33loads an immediate value into a register
172. 6:37if it is a short immediate that can fit
173. 6:40as
174. 6:43as an argument of an ad immediate and
175. 6:48finally
176. 6:48a common instruction that we will run
177. 6:52frequently in in the code is no
178. 6:54operation or it stands as not
179. 6:56that's when processor needs to wait for
180. 6:58something for example for a datum to
181. 6:59come from the memory
182. 7:01it is spinning around and doing nothing
183. 7:02that's a knob well
184. 7:04knob is anything that uses x0 as the
185. 7:07destination register
186. 7:08so x0x0 add the mediatek 0x00
187. 7:13is a good knob
188. 7:17these instructions don't change anything
189. 7:19in the way how they're being
190. 7:21executed they just make the code
191. 7:24more readable so instead of trying to
192. 7:26figure out what
193. 7:27does add immediate our destination our
194. 7:29source zero do
195. 7:31we just write a move that's it for now
196. 7:34see you in a bit