[CS61C FA20] Lecture 07.1 - RISC-V Intro: RISC-V Assembly Language — Transcript

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1. 0:00[Music]
2. 0:02[Applause]
3. 0:03[Music]
4. 0:09welcome to the next module in 61c we are
5. 0:12going to
6. 0:13dive down to the next level of
7. 0:15abstraction
8. 0:16and cover the risk 5 instruction set
9. 0:18architecture and the risk 5 assembly
10. 0:20language
11. 0:23remember early in the class we talked
12. 0:25about the
13. 0:26the ideas of layering different layers
14. 0:29of abstraction
15. 0:30to represent complex compute systems on
16. 0:33the top
17. 0:34there is a high level language and
18. 0:36hopefully people have mastered c
19. 0:37by now below that is the assembly
20. 0:40language below that is the
21. 0:42machine readable version of the assembly
22. 0:44language
23. 0:45which is the machine code and below that
24. 0:48are different implementations of the
25. 0:51architecture
26. 0:52implemented perhaps as block diagrams
27. 0:56that can execute that code below that
28. 0:59level of abstraction there are logic
29. 1:01gates that actually
30. 1:03replace things that are inside these
31. 1:05boxes in the block diagram
32. 1:08logic gates are not the very bottom of
33. 1:10the of the
34. 1:12compute stack they're also an
35. 1:14abstraction
36. 1:15uh they're built out of transistors
37. 1:16transistors are also an abstraction
38. 1:19um transistors consist of different
39. 1:22kinds of materials
40. 1:23they're also wires and all of them
41. 1:26are there to carry electrons in holes
42. 1:29and we can talk about deeper than the
43. 1:31deeper layers of abstraction than that
44. 1:34c is not the highest level language as
45. 1:36we already know
46. 1:38there are more complex things that are
47. 1:39built on top of it their operating
48. 1:41systems
49. 1:42their productivity languages their
50. 1:44software frameworks
51. 1:45and this entire world is built or at
52. 1:48least the software view of the world
53. 1:49is built on top of that
54. 1:53it is important to point out that there
55. 1:55are always well defined in
56. 1:57interfaces between these layers of
57. 1:59abstraction
58. 2:00typically we don't write assembly
59. 2:03language code
60. 2:05we can but most commonly it is being
61. 2:08produced by a compiler
62. 2:11assembler produces a machine readable
63. 2:14code
64. 2:15instruction set architecture is what
65. 2:17defines
66. 2:18what will be executed by the hardware
67. 2:21and
68. 2:22then there are different layers
69. 2:23different ways of implementing things
70. 2:25below that
71. 2:30when we are thinking about the assembly
72. 2:32language we think
73. 2:33about the instructions executed by the
74. 2:35processor
75. 2:36the basic job of a cpu is to execute
76. 2:39lots of instructions when i say
77. 2:41lots i mean lots when we speak about the
78. 2:44processor
79. 2:47that is running at among the speed
80. 2:49nowadays of one gigahertz
81. 2:51that means that one cycle in a processor
82. 2:54lasts for one
83. 2:55nanosecond and that's approximately a
84. 2:57time that it takes to execute one
85. 2:58instruction sometimes more sometimes
86. 3:00less but let's say
87. 3:01of the order so each
88. 3:05processor will execute 1 billion
89. 3:07instructions per second
90. 3:09and that's a lot these instructions
91. 3:13are primitive operations that cpu
92. 3:16executes
93. 3:17they essentially
94. 3:20constitute of some kind of an action
95. 3:23like a sentence
96. 3:24where the operations are like verbs
97. 3:27applied to operands which are objects
98. 3:29that are processed in sequence
99. 3:33now there are many different processor
100. 3:36architectures out there and different
101. 3:38sets of instructions
102. 3:40different cpus implement different sets
103. 3:42of
104. 3:43instructions
105. 3:46a particular set of instructions that
106. 3:48applies
107. 3:49to a processor or class of processors is
108. 3:52called the instruction set architecture
109. 3:54so when we you know there are many known
110. 3:57instruction set architectures out there
111. 4:00the most common that you'll encounter
112. 4:01nowadays is the arm
113. 4:04instruction set architecture and it is
114. 4:07deployed in
115. 4:08pretty much all of the cell phones that
116. 4:09are out there in the world and many many
117. 4:11other things
118. 4:13very popular is intel x86 that is
119. 4:16implemented in
120. 4:18laptops desktops and many of the of the
121. 4:21servers and cloud processors there is
122. 4:24ibm power
123. 4:25implemented in servers as well there
124. 4:27used to be very popular ibm motorola
125. 4:29power pc that was used in all their
126. 4:33macs there is mips
127. 4:36and there is risk five we're going to be
128. 4:38talking about risk five
129. 4:40and this module and beyond
130. 4:44there is a book that
131. 4:48actually was pointed to me by dan um
132. 4:51programming from the ground up and i'm
133. 4:52just gonna read a little bit from its uh
134. 4:56excerpt from its review the reviewer
135. 4:58says here
136. 4:59i found that the key difference between
137. 5:02mediocre and excellent programmers
138. 5:04is whether or not they know assembly
139. 5:07language
140. 5:08those that do tend to understand
141. 5:11computers themselves
142. 5:12at a much deeper level and this really
143. 5:14strikes deep
144. 5:16uh with me that's why we are trying to
145. 5:19cover
146. 5:20assembly language such that people
147. 5:22really understand
148. 5:24how is that software being executed also
149. 5:27many of the
150. 5:28um i had quite a bit of experience
151. 5:30within with the
152. 5:31assembly language of a particular
153. 5:33architecture of olden times
154. 5:35um it was old computers like an apple
155. 5:38computer just came with a basic
156. 5:40interpreter
157. 5:41didn't have really an operating system
158. 5:43um and if you wanted to do something
159. 5:45other than write code in basic that was
160. 5:48a
161. 5:49language of that time um you had to
162. 5:51write assembly language for example i
163. 5:53had to write a lot you know this was my
164. 5:55first computer
165. 5:56i still have it not quite sure if it
166. 5:58works it's a simpler zx spectrum
167. 6:00that was a british type of machine it
168. 6:03was very inexpensive you could plug it
169. 6:04into a tv
170. 6:06and just keep coding
171. 6:10it also had a basic but i really wanted
172. 6:13performance out of it so i had to write
173. 6:15a lot of assembly code
174. 6:20back to instruction set architectures
175. 6:24as the computers were developing
176. 6:26somewhere in the 70s and the 80s there
177. 6:28was a trend
178. 6:29to try to build more and more complex
179. 6:31instructions
180. 6:33that led something you know to
181. 6:37a division or to a trend of building
182. 6:39something that is called complete
183. 6:41instruction set architect
184. 6:43computers um or what's then what
185. 6:46you know as a as an acronym uh known as
186. 6:49sisk
187. 6:54there was in the early 80s
188. 6:57emerged uh or a little bit earlier than
189. 7:00that by john in ibm
190. 7:02to try to build very simple instruction
191. 7:05sets
192. 7:05very basic instruction sets that was
193. 7:08picked up and
194. 7:09really taken to the full extent by dave
195. 7:11patterson
196. 7:12berkeley and john hennessy at stanford
197. 7:16the idea there was to keep instructions
198. 7:18that's small and simple
199. 7:19and make it easier to build fast
200. 7:23hardware
201. 7:24so each instruction per cycle would do
202. 7:27less but you would have fast hard
203. 7:29hardware and will be able to
204. 7:31execute more instructions in a given
205. 7:34time
206. 7:35and then we'll use software to build
207. 7:38complicated operations by composing
208. 7:40simpler ones
209. 7:42um
210. 7:44person hennessy one um there was a
211. 7:47long debate but they eventually won and
212. 7:50even though there are cisc
213. 7:52instruction sets out there like x86 if
214. 7:54you look at the way
215. 7:56how the the processor is built
216. 7:59that is really in the essence of it is
217. 8:03a risk engine and these complex
218. 8:05instructions are added by
219. 8:07you know concatenating simpler ones
220. 8:10inside
221. 8:11that machine one would say
222. 8:15he who loves last laughs best person
223. 8:18hennessey
224. 8:19as we know won the touring award two
225. 8:22years ago
226. 8:24when teaching computer architecture
227. 8:28people you know both instructors and the
228. 8:31students have a tough choice
229. 8:32instructions
230. 8:32in particular need to choose an
231. 8:34instruction set architecture
232. 8:36to teach the architecture in an
233. 8:38architecture class
234. 8:42so the choice there is to try to teach
235. 8:45x86 but that's
236. 8:46very complex and there is often quite a
237. 8:49bit of a pushback by students in trying
238. 8:51to teach
239. 8:52us some subset of x86
240. 8:56alternatives to that are
241. 9:00to try to use some older instruction set
242. 9:02like maps
243. 9:03or an old risk or to come up with a made
244. 9:06up instruction set
245. 9:09the advantage of teaching x86 you know
246. 9:11whoever survives the class is that
247. 9:13then they can run any software on that
248. 9:15architecture that they will understand
249. 9:17because there is a rich x86
250. 9:19you know system built out there but
251. 9:24then it's easier to understand older
252. 9:26instruction sets or
253. 9:27made up instruction sets but the problem
254. 9:30there is
255. 9:31there is no really software that you can
256. 9:33get to run on that
257. 9:34it's really hard to even get a basic
258. 9:36compiler running
259. 9:39so there came risk 5. about 10 years ago
260. 9:42it was conceptualized
261. 9:44it really you know took off in the past
262. 9:47four or five years
263. 9:50it is open source and license free what
264. 9:53means there
265. 9:54is that anybody can use it you don't
266. 9:57have to pay anybody
267. 9:59the rights to use those instructions so
268. 10:04it became very popular in both teaching
269. 10:07but it immediately jumped into a
270. 10:08commercial world
271. 10:10so we have now a rich commercial
272. 10:12ecosystem
273. 10:13that builds risk fire from processors
274. 10:15but there is a well-supported software
275. 10:17stack
276. 10:18so we got everything now a simple
277. 10:20instruction set
278. 10:21that can be extended and is being
279. 10:23extended that we can teach in classes
280. 10:25and there is a soft there is a rich
281. 10:28collection of software
282. 10:29that will run on it risk 5 comes in
283. 10:32different kinds of variants to support
284. 10:34from tiny microcontrollers to
285. 10:36big computers it comes with
286. 10:39in 32 64-bit 128 bit variants if you
287. 10:43like
288. 10:44that says how big are these words that
289. 10:47the
290. 10:48processor operates on 32 bits are
291. 10:52four bytes that we have seen before 64
292. 10:55bits
293. 10:56are eight bytes that it operates on at a
294. 10:58time
295. 11:00in class we are using 32 bits because
296. 11:03believe me here projects
297. 11:06you know project 3 is a lot easier to
298. 11:08implement when it's 32 bits
299. 11:10than when it's 64 bits but the book
300. 11:13covers
301. 11:14a 64-bit version of the architecture
302. 11:16that is called
303. 11:17rb64 our variant is called rb32
304. 11:22um it's what we covering class really
305. 11:24fits on one page
306. 11:26this is what i'm going to be talking
307. 11:27about over the next
308. 11:29few of these video segments that one
309. 11:32page
310. 11:32is the captured entire architecture the
311. 11:36definition of risk five um and it's
312. 11:38called the
313. 11:39green card that is named after the ibm
314. 11:43360 green card which was very popular
315. 11:45back in the 60s
316. 11:46i actually had an opportunity to program
317. 11:50ibm 360 and when
318. 11:53i was in high school and use the punch
319. 11:55cards and
320. 11:56line printers and really understood what
321. 12:00does that
322. 12:00slash n mean in uh in c code
323. 12:04otherwise it will not return the line if
324. 12:07it does
325. 12:07if you don't put that in
326. 12:12risk five just a few words about where
327. 12:15it came from and where it's going
328. 12:17um it started in summer 2010 to support
329. 12:21classes
330. 12:21exactly like this one 61c it was built
331. 12:24for 61c
332. 12:25but also for 152 and 151
333. 12:29but a lot of the instructions that
334. 12:30you'll find there are
335. 12:33the same instructions that you found in
336. 12:35the old risk one
337. 12:37and raise 2 in the 80s
338. 12:41as risk 5 project matured
339. 12:45it started getting adopted by industry
340. 12:48and it got moved into an open source you
341. 12:50know a
342. 12:52non-profit foundation that maintains its
343. 12:55open source description it's called the
344. 12:58risk5.org
345. 13:01and there are as i said many uh research
346. 13:04projects based on risk five
347. 13:06and their products that may be open
348. 13:08source but maybe
349. 13:09pro proprietary
350. 13:12people say they'll be next year about a
351. 13:14billion
352. 13:15processor called course built in risk
353. 13:17five that is quite
354. 13:18exciting you can read more about the
355. 13:21history and the lineage
356. 13:23in on the risk5.org page
357. 13:27and i'll jump in the next
358. 13:30video segment i'm going to jump into the
359. 13:33elements of
360. 13:34the architecture see you there