[CS61C FA20] Lecture 12.3 - RISC-V Instruction Formats II: J-Format — Transcript

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1. 0:00[Music]
2. 0:08welcome back
3. 0:09to list five instruction formats we are
4. 0:11almost done we have covered five
5. 0:13different instruction formats we
6. 0:14just have one more to cover that's the
7. 0:16one that covers jumps
8. 0:17in risk five and guess what there is
9. 0:19only one jump instruction
10. 0:21which is jowl so
11. 0:24the format for gel is very similar to
12. 0:28our u format that we have seen before it
13. 0:32essentially needs room for
14. 0:35a long immediate and a destination
15. 0:39register
16. 0:39and then of course the opcode so the
17. 0:41format is
18. 0:42goes like this we have the op code that
19. 0:44is 7-bit the destination register
20. 0:47that is rd that is 5 bits and the 20-bit
21. 0:50offset
22. 0:51however shuffled a bit to look more like
23. 0:54what we have
24. 0:55seen in branches because we would like
25. 0:58this
26. 0:58to behave more like branches
27. 1:03um what does gel do just to remind
28. 1:06ourselves it saves the next instruction
29. 1:09which is the value in pc plus 4 which
30. 1:11points to the next instruction it will
31. 1:12be executed
32. 1:14[Music]
33. 1:15in the memory so it is it will save pc
34. 1:18plus 4
35. 1:18in the destination register that's our
36. 1:20return address
37. 1:21after the function call and sets the
38. 1:25program counter to the program counter
39. 1:27plus offset
40. 1:29because gel and risk 5
41. 1:33support pc relative addressing
42. 1:36for jumps as well as for branches and
43. 1:39the code
44. 1:40is can be relocated
45. 1:44across the memory locations
46. 1:48important things to to remember here the
47. 1:52range here is actually 21 bits because
48. 1:56we follow the same convention that we
49. 1:57have seen
50. 1:58in in branches
51. 2:02where we will be jumping only to even
52. 2:05addresses in the memory and that's where
53. 2:07we are going to find valid instructions
54. 2:09there is no need
55. 2:10to jump to odd addresses because we will
56. 2:14not
57. 2:14find the start of either 16 or 32-bit
58. 2:17instruction at an odd address so we are
59. 2:20going to jump
60. 2:22only to even addresses therefore
61. 2:25the least significant bit of the offset
62. 2:28has to be zero
63. 2:30so we are using 20 bits
64. 2:33to specify to cover the range that
65. 2:36is 21 bits wide so the target is
66. 2:40anywhere within uh to the 19th locations
67. 2:44uh which are two bytes apart two to the
68. 2:47plus
69. 2:47minus two to the location is 2 bytes
70. 2:50apart
71. 2:50which means that is plus minus 2 to the
72. 2:5318th
73. 2:5432-bit instructions is our jump range
74. 2:57relative to the current
75. 3:00value of the program counter that gives
76. 3:03us
77. 3:03plus minus megabyte worth of code
78. 3:07which is quite a bit um
79. 3:11the only other thing to to to keep
80. 3:14in mind here is that immediate encoding
81. 3:16here is optimized in the same way
82. 3:19as we had in the b format
83. 3:22we've also seen the commonly used pseudo
84. 3:26instruction
85. 3:27j which is a jump where we do not save
86. 3:30the return address
87. 3:32so the way not to save the return
88. 3:35address
89. 3:36is to specify the rd as x0
90. 3:39which would discard the value stored
91. 3:42there
92. 3:44a couple of examples of using
93. 3:47a jowl here is a j's to the instruction
94. 3:50[Music]
95. 3:52j label is the same as
96. 3:56jowl with the without saving the return
97. 4:00address
98. 4:01to an address specified as a label
99. 4:04[Music]
100. 4:06and then so that label will be
101. 4:09calculated
102. 4:10relative to the current pc and then um
103. 4:14our other common use of gel is to call a
104. 4:17function within 2 to the 18 instructions
105. 4:20of a pc so this would be done
106. 4:23as jowl ra with function name we are
107. 4:27going to save the return address
108. 4:29in ra so we know where to return and
109. 4:31then function name would be a label
110. 4:34that is within the reach of of this
111. 4:37jumping link and that's it there is not
112. 4:40much more to it we have seen it before
113. 4:43there is one more jump instruction but
114. 4:45we said
115. 4:47we have had only one instruction in the
116. 4:49j format
117. 4:50that's because jump and link register is
118. 4:53of i
119. 4:53type it needs room for both the
120. 4:56destination register and the source
121. 4:58register
122. 4:59so it uses the i format so gel r
123. 5:03is going to have its op code destination
124. 5:06register
125. 5:07and the source register and we'll be
126. 5:09using only a 12-bit offset
127. 5:12it could have had wider offset but
128. 5:14decided
129. 5:16not to develop to add one more
130. 5:19type of instruction format to risk 5.
131. 5:24so we have seen that instruction haven't
132. 5:27studied it in detail so
133. 5:28here it is in the detail it is jowl
134. 5:31register
135. 5:33specifying the destination the source
136. 5:34register and the immediate value where
137. 5:37immediate is
138. 5:38a 12-bit value so
139. 5:41it again does two things it writes
140. 5:44the next address are the the return
141. 5:48address
142. 5:49of pc plus four and the rd in the
143. 5:52destination register so we know
144. 5:54how to return back and then
145. 5:58sets the program counter to a new value
146. 6:00which is equal to the
147. 6:02value of the source register plus the
148. 6:05immediate so important thing to notice
149. 6:07here
150. 6:07this is the way to make a jump to an
151. 6:10absolute address we will put
152. 6:12a value in the source register and
153. 6:14specify any offset with respect to that
154. 6:18another important distinction immediates
155. 6:21here are not like in gel
156. 6:23and in branches because we are using the
157. 6:27immediate format
158. 6:28we can't just um
159. 6:32um assume that the least significant bit
160. 6:36is zero it is there in the instruction
161. 6:38you know it is specified in in the
162. 6:40offset it is
163. 6:42right there there
164. 6:47so there is no multiplication by two bit
165. 6:50two bytes so our range is a bit reduced
166. 6:54just keep that in mind so the way how it
167. 6:56does it it calculates
168. 6:58a sum of the sign extended immediate
169. 7:01like we
170. 7:02always do in the immediate in the i
171. 7:04format
172. 7:05so we sign extend the immediate add it
173. 7:07to the
174. 7:08source register and then we always by
175. 7:11convention
176. 7:12set the least significant bit to zero
177. 7:14such that we always
178. 7:16jump to even addresses here a couple of
179. 7:19examples of use of jalar
180. 7:23so the one that we have already seen is
181. 7:26a return
182. 7:28or or j r jump register
183. 7:31with the return address that is
184. 7:33essentially jalar
185. 7:35x 0 r a and a 0. so
186. 7:39our source is the ra and we don't have
187. 7:42to our return address we're returning to
188. 7:45our return address we don't need to save
189. 7:47the
190. 7:52destination we don't have to save the
191. 7:54current pc
192. 7:55and our offset is 0.
193. 7:59if we want to call function at any
194. 8:0232-bit absolute address
195. 8:04we can do that with gel r with the help
196. 8:07of louie
197. 8:08so if we lui upper 20 bits
198. 8:13of our our target absolute address
199. 8:16into register x1 and then gel r
200. 8:21with the value of x1 and
201. 8:24lower 12 bits that are going to be now
202. 8:27added
203. 8:28to that then we are there we have
204. 8:33called um a function that
205. 8:37is 30 you know that is specified by a
206. 8:4032-bit absolute address
207. 8:43now jalar can also do
208. 8:46um relative addressing the way how we do
209. 8:49that
210. 8:50well you said jalar isn't that
211. 8:52contradiction you just said the jlr
212. 8:54always always uses absolute addresses
213. 8:56but we can take it
214. 8:58right we can load the current value of a
215. 9:01pc
216. 9:02by using our epc so we can load
217. 9:06upper 20 bits in x1 and then
218. 9:10jlr with the lower 12 bits
219. 9:13to to our
220. 9:16relative destination with full 32-bit
221. 9:20offset and that's it that sums up
222. 9:23all this five instruction formats when i
223. 9:25come back i'll just sum up