[CS61C FA20] Lecture 07.4 - RISC-V Intro: RISC-V Immediates — Transcript

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1. 0:00[Music]
2. 0:08welcome back
3. 0:09we are continuing to go through our risk
4. 0:12five assembly instructions
5. 0:14we have covered throwing two
6. 0:15instructions so far add and sub
7. 0:17that perform addition and subtraction
8. 0:20over the contents of the registers
9. 0:22inside of a processor we
10. 0:25add assembly instructions to a processor
11. 0:28to support common cases that we find
12. 0:30in higher level languages so one thing
13. 0:32that we
14. 0:33frequently do in higher level languages
15. 0:36like c
16. 0:37is we add a constant to a variable
17. 0:40so there is a a special instruction that
18. 0:43does
19. 0:44this addition of a constant to a
20. 0:46variable
21. 0:48numerical constants in assembly are
22. 0:49called immediates
23. 0:52they often very they they appear often
24. 0:55in the code
25. 0:55so it does make sense to have support
26. 0:57for them in the assembly language
27. 1:00the instruction that risk 5 uses for
28. 1:03adding immediates is called
29. 1:05adbi which stands for ad immediate
30. 1:08it makes sense that it is called that
31. 1:10way because in a catalog of the
32. 1:11instructions so
33. 1:12in in the green card you add immediate
34. 1:15is going to be
35. 1:16standing right next to the register add
36. 1:21its format is also very similar to what
37. 1:23we have seen as a format for the add
38. 1:25instruction
39. 1:27instead of having add destination
40. 1:31register
41. 1:32source 1 source 2 the format of the add
42. 1:35immediate is addi
43. 1:37destination register source register and
44. 1:40the value of the immediate
45. 1:45it looks similar to the add instruction
46. 1:48because we do
47. 1:48want to reuse actually most of the
48. 1:51hardware that we'll be
49. 1:52using to build this processor to
50. 1:55implement these two instructions
51. 1:58a few other interesting things about the
52. 2:00risk 5
53. 2:01and the immediates although there are
54. 2:04separate add and sub instructions
55. 2:06because
56. 2:07sub instruction you know substitution
57. 2:10is not communicated commutative so
58. 2:14it makes sense to have a sub instruction
59. 2:16there is no sub immediate
60. 2:19i'll give you a second to think about
61. 2:20why we don't need
62. 2:22sub immediate in assembly
63. 2:30of course well because we really really
64. 2:33don't need it
65. 2:35we can implement it we can replace it by
66. 2:37using add immediate
67. 2:39where that immediate would be a negative
68. 2:41value
69. 2:42implemented in two's complement in risk
70. 2:44five so
71. 2:45our instruction that would implement f
72. 2:48equals to g minus 10 in c would look
73. 2:51like
74. 2:52add immediate x 3 x 4 minus 10
75. 2:55is to in two's complement in risk five
76. 3:00so we don't need it notice though that
77. 3:03if we wanted to implement
78. 3:04f equals 10 minus g
79. 3:08we would need two instructions to do
80. 3:10that
81. 3:12you can think about how to do that
82. 3:16keep in mind that the most common
83. 3:18immediate that we are going to encounter
84. 3:20in code
85. 3:20is zero that's why it makes sense
86. 3:24to keep that zero handy inside of a
87. 3:26processor
88. 3:27by hardwiring one of its registers and
89. 3:30in
90. 3:30the risk five it is the x zero register
91. 3:34so no matter what you do to this x0
92. 3:36register it
93. 3:37always holds the value of zero because
94. 3:40processor does
95. 3:40like to have a zero handy at all times
96. 3:47so if you do things with x0 keep that in
97. 3:50mind
98. 3:51for example add x3 x4
99. 3:54x0 would simply move the contents of a
100. 3:58register x4
101. 4:00to register x3 like
102. 4:03f equals g and c
103. 4:06now we can try to use
104. 4:11the the register x0 as a destination
105. 4:13register but that
106. 4:14doesn't make sense because if we say
107. 4:17that add
108. 4:19if we write an instruction add x0
109. 4:22x3 x4 no matter what the value
110. 4:26or what the values are stored in the
111. 4:28registers x3 and x4
112. 4:31this instruction will not change the the
113. 4:33contents of x0
114. 4:34x0 is going to stay zero and that's it
115. 4:38what we have to say for now
116. 4:39about the immediate um now it's a good
117. 4:42time to
118. 4:43take a little break because we're going
119. 4:45to get into
120. 4:46slightly more slightly
121. 4:50different instructions in the next
122. 4:53segment
123. 4:54see you then