[CS61C FA20] Lecture 18.1 - Single-Cycle CPU Datapath I: RISC-V Processor Design — Transcript

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1. 0:00[Music]
2. 0:10welcome to a new module in 61c
3. 0:13in this module we'll design a risk 5
4. 0:16processor
5. 0:18that's my favorite part of the entire
6. 0:20course
7. 0:22because it will really help us connect
8. 0:24the software story
9. 0:26with the hardware story that we have
10. 0:27been telling
11. 0:29let's get into it but first let's recap
12. 0:33where do we stand 61c
13. 0:36is centered around the instruction set
14. 0:39architecture
15. 0:40which is the part that separates and
16. 0:43joins together
17. 0:44the software and the hardware
18. 0:48early in the class we have spotlighted
19. 0:52the higher level language which was c
20. 0:54and there is five assembly language
21. 0:58assembly language compiles to the
22. 1:00machine code and
23. 1:02any hardware that implements that same
24. 1:05ris 5 isa
25. 1:07should be able to execute any risk 5
26. 1:10binary machine code so that's great
27. 1:14we are going to see now how do we build
28. 1:16hardware that actually
29. 1:19executes risk 5 code and keep in mind
30. 1:23there may be many different variations
31. 1:26of that hardware
32. 1:27but they all have to execute that code
33. 1:31so we have seen how we build
34. 1:35digital systems out of logic gates
35. 1:39based on boolean equations so what we'll
36. 1:42do now
37. 1:43we'll zoom in on the design of a data
38. 1:46path
39. 1:46and control that put together
40. 1:50when put together comprise a
41. 1:52microprocessor
42. 1:56this picture of machine structures
43. 2:00still applies but we like to look at the
44. 2:02world now in a more
45. 2:0521st century kind of way in the new
46. 2:08school of machine structures we
47. 2:12emphasize parallelism and we have seen
48. 2:14that things can be
49. 2:15paralyzed in many ways in higher levels
50. 2:18but keep in mind that
51. 2:20at a very low level
52. 2:23hardware descriptions are inherently
53. 2:27parallel
54. 2:28all the gates switch together
55. 2:31and operate in in with a very high
56. 2:33degree of parallelism
57. 2:35but we'll find out that that those
58. 2:37functional units
59. 2:38that are built out of these gates also
60. 2:41operate
61. 2:41in parallel and then we can have
62. 2:44multiple functional units
63. 2:47that can execute multiple instructions
64. 2:51in parallel
65. 2:53so we'll be emphasizing this
66. 2:56level of concurrency and parallelism
67. 2:58throughout
68. 2:59the our story of describing the hardware
69. 3:04one more reminder remember our
70. 3:07very important picture about the layers
71. 3:10of abstraction
72. 3:11we have started with the c as an example
73. 3:15of a high level language
74. 3:16then we talked about the assembly
75. 3:17language and
76. 3:20described the machine language or
77. 3:22machine code that risk 5 executes
78. 3:25and i mentioned this already in this
79. 3:27intro
80. 3:28and mentioned this before
81. 3:31there can be many implementations of the
82. 3:34risk 5 isa
83. 3:36but they all have to execute that binary
84. 3:39code
85. 3:40so on the bottom out of the gates that
86. 3:43we have learned so far
87. 3:44we can put different implementations
88. 3:48of risk 5 processors but they should all
89. 3:51be able to
90. 3:52execute the same binary so
91. 3:55we are going to guide you through a
92. 3:58design of one particular
93. 4:00data path and control associated control
94. 4:04but we are going to see that there can
95. 4:06be many variants of that
96. 4:11remember our microprocessor so far we
97. 4:13have only talked about
98. 4:15a single core microprocessor but we can
99. 4:18have multiple cores
100. 4:19that can operate in parallel in our
101. 4:21single core microprocessor model
102. 4:23we had the processor on the left memory
103. 4:27in the middle and
104. 4:28input and the output connected to it
105. 4:31inside the processor there are two main
106. 4:33parts
107. 4:34we haven't talked really much about them
108. 4:36although we got some picture
109. 4:38about what they do there is a control
110. 4:41and there is a data path
111. 4:42inside our data path we have found
112. 4:45registers
113. 4:46and a special one which was the program
114. 4:48counter
115. 4:49and the arithmetic logic unit as
116. 4:53the most common functional unit that we
117. 4:56use
118. 4:56there may be other functional units but
119. 4:58we're going to
120. 4:59focus on the alu for now so
121. 5:03inside the microprocessor or the cpu
122. 5:08or cpu stands for essential processing
123. 5:10unit
124. 5:12there are two main components and again
125. 5:14cpu
126. 5:15is this active part of a computer where
127. 5:18the main action happens there are two
128. 5:21main parts in it
129. 5:22which are the data path and the control
130. 5:26the data path sometimes we may call it
131. 5:28the bronze
132. 5:29of the processor is the portion of a
133. 5:31processor that
134. 5:33has necessary hardware to execute all
135. 5:36the
136. 5:36instructions that we have in the isa now
137. 5:39there is one important point that i like
138. 5:41to make here we can build
139. 5:43a separate data path for every
140. 5:45instruction we have seen thirty
141. 5:47something instructions so far
142. 5:49in risk five integer base subset
143. 5:53but we don't build a separate
144. 5:56data path for each instruction we build
145. 5:59one data path
146. 6:01that executes many instructions or in
147. 6:04our case
148. 6:05all rb32i instructions
149. 6:08the data path can be configured to
150. 6:10optimally execute
151. 6:12every single one of these instructions
152. 6:14so the role
153. 6:15of the control or the brain of the
154. 6:17microprocessor
155. 6:19is to control the data path or tell the
156. 6:22data path
157. 6:23how to set itself such that it executes
158. 6:26each instruction correctly
159. 6:30what we have seen so far is
160. 6:33this subset of instructions that
161. 6:37comprise the rb32i
162. 6:38or the integer part of the base
163. 6:42instruction set for a 32-bit variant of
164. 6:44risk five
165. 6:45that is enough to to to allow us to
166. 6:49write
167. 6:49any c code so what we are going to do
168. 6:52now we are going to go through different
169. 6:56types of instructions
170. 6:57and build the data path and the
171. 6:59associated control that is needed to
172. 7:02execute
173. 7:03them in the end when we complete this
174. 7:06journey
175. 7:07we could build a processor like this
176. 7:10risk five processor that can run any
177. 7:13assembly or
178. 7:14any c code that we can write that's
179. 7:16quite exciting we're going to get into
180. 7:18that
181. 7:19just after the break