[CS61C FA20] Lecture 07.3 - RISC-V Intro: RISC-V add/sub Instructions — Transcript

Full transcript

1. 0:00[Music]
2. 0:14so
3. 0:25when i was a kid i was a fan of bruce
4. 0:27lee's movies
5. 0:28that's where i picked this up 30 frames
6. 0:31per second
7. 0:32help catch the scene here
8. 0:36but i can't do any other bruce lee's
9. 0:38thing tricks
10. 0:41we finally got to talking about real
11. 0:44risk five
12. 0:45instructions and we're going to cover
13. 0:47the next few minutes the first two of
14. 0:48them
15. 0:49for addition and subtractions between
16. 0:52the registers
17. 0:53so let's take a look at them generally
18. 0:55assembly instructions
19. 0:56are have a very rigid format so
20. 0:59arithmetic
21. 0:59instructions and logic instructions
22. 1:01always look
23. 1:03follow the same rigid syntax
24. 1:061 space 2 comma
25. 1:09space 3 comma space
26. 1:134 for example we have add
27. 1:17x1 comma x2 comma x3
28. 1:21where this one is the operation
29. 1:24the name of the operation 2 is the
30. 1:27destination
31. 1:293 is the first source and 4
32. 1:32is the second source
33. 1:37the syntax is rigid meaning
34. 1:40that it really doesn't very many there
35. 1:42are many instructions that follow
36. 1:44exactly the same
37. 1:45syntax because the hardware needs to be
38. 1:47made simple
39. 1:48remember hardware is expected to decode
40. 1:51these
41. 1:51instructions and find out exactly what
42. 1:53are we trying to do
43. 1:54and we want to make it help it by
44. 1:57simplifying the format such that it
45. 1:58doesn't have to look
46. 2:00all over the place it always finds the
47. 2:02sources and destinations
48. 2:04at the same place
49. 2:08take a look um a little bit
50. 2:11deeper into how do we use additional
51. 2:14subtraction operations
52. 2:17for example here is the addition in
53. 2:19assembly
54. 2:20and the command is simple add adb
55. 2:24easy to remember you can make a mistake
56. 2:26that that
57. 2:27easily add x1 comma x2 comma x3
58. 2:33this is equivalent to a c instruction a
59. 2:36equals to b plus c where
60. 2:39the c variables correspond to the values
61. 2:43stored in particular registers in
62. 2:45a risk 5 processor so a would be stored
63. 2:49in the register x1
64. 2:51b is stored in the register x2
65. 2:55and c is stored in a register x3
66. 2:58subtraction works almost the same with
67. 3:02one
68. 3:02slide subtle difference but just let's
69. 3:05take a look at it
70. 3:07so subtraction operation is sub sub
71. 3:11um and in this case if you would like to
72. 3:14subtract
73. 3:14the values stored in registers x
74. 3:184 and x 5 and store the result in
75. 3:21x 3 we would write sub x 3 comma x
76. 3:254 comma x 5. this equivalent to a c
77. 3:29instruction d equals to e minus f
78. 3:33where the value of d would be in
79. 3:35register x3
80. 3:38value of e would be in register x4 and
81. 3:40value of f
82. 3:41would be in the register x5
83. 3:44the key difference between add and
84. 3:46subtract to you know just to pay
85. 3:47attention to that
86. 3:49um in addition addition is a commutative
87. 3:52operation so you can store um it doesn't
88. 3:55matter where you
89. 3:56where do you store the operands um
90. 4:00b and c in which registers you do that
91. 4:04source 1 or source 2
92. 4:06they'll anyways be added in subtraction
93. 4:08it matters
94. 4:09you know because we are subtracting the
95. 4:11value of the operand of the source
96. 4:13operand 2
97. 4:15from the value of the source operand one
98. 4:17how we do how do we do a little bit more
99. 4:19complex
100. 4:21arithmetic operations for example if we
101. 4:23have only two
102. 4:25registers where you can store sources
103. 4:27how do we do
104. 4:28addition uh and subtraction of four
105. 4:32different operands well we have to use
106. 4:35some temporary variables
107. 4:36and here is an example how we would do a
108. 4:38equals to b
109. 4:39plus c plus d minus c
110. 4:42would break it up into multiple
111. 4:44instructions so one
112. 4:45c command gets broken up into multiple
113. 4:48we would use x 10
114. 4:52as in this case a running sum
115. 4:55however would be accumulating the
116. 4:56results so we'd say we'll add the values
117. 4:59of
118. 4:59x1 and x2 where the values b and c are
119. 5:02stored and store it
120. 5:04in x10 then we would add the value of x3
121. 5:08to that temporary um running sum
122. 5:11that is stored in x10 and finally we'll
123. 5:13subtract
124. 5:14x4 from that value to stored in
125. 5:18in the register extent store it back in
126. 5:21the in the register
127. 5:22x10 and that is our final answer
128. 5:25so things to keep in mind a single
129. 5:28line of um higher level language like c
130. 5:32can break up can generate when compiled
131. 5:35to and often will uh compile to multiple
132. 5:39um instructions uh or several lines of
133. 5:42risk five assembly code and um not this
134. 5:46year
135. 5:46uh as we have seen before everything
136. 5:48that goes after the hash mark
137. 5:50um in in each line is not being compiled
138. 5:54by the compiler
139. 5:54those are just the comments
140. 5:58a little bit more complex example um
141. 6:00trying to use
142. 6:01multiple temporary variables so how do
143. 6:03we compile this instrument this
144. 6:05line in c f is equal to g
145. 6:08plus h minus the sum of i plus j
146. 6:12um the way how that's done we use
147. 6:15intermediate temporary registers
148. 6:17um x5 and x6 first we'll add the values
149. 6:22of g and h that are stored in registers
150. 6:25x20 and x21
151. 6:28right there and then
152. 6:32we would uh make another temporary
153. 6:35sum of x22 and x23 that store the values
154. 6:39of i plus j
155. 6:40and finally we would subtract them and
156. 6:42get the final
157. 6:44result in the register x19
158. 6:48and that's it that was the gist of
159. 6:51register to register arithmetic
160. 6:53instructions
161. 6:54we are going to see a few more
162. 6:58different instruction types after the
163. 7:01break