[CS61C FA20] Lecture 19.6 - Single-Cycle CPU Datapath II: Adding U-Types — Transcript

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1. 0:01[Music]
2. 0:09hello
3. 0:10welcome back to design over s5 cpu so
4. 0:13far we have designed
5. 0:14a data path that supports r type i type
6. 0:19s type b type and j type instruction so
7. 0:21there is only one type of instructions
8. 0:23that is missing
9. 0:24which is the u type this five base is a
10. 0:29has six instruction types
11. 0:32and that u type is something that we
12. 0:34have seen before
13. 0:35it is to support those unusually long
14. 0:39immediates which we call upper
15. 0:40immediates the format is fairly
16. 0:42straightforward it has
17. 0:4420 bits of an upper immediate in
18. 0:48stored in the most significant bits of
19. 0:51the instruction
20. 0:52followed by the destination register and
21. 0:54the op code
22. 0:55there are two instructions of that type
23. 0:58louis and now epc
24. 1:00louis simply loads that 20-bit
25. 1:04long upper immediate in the destination
26. 1:06register and clears the lower
27. 1:0912 bits our epc
28. 1:13adds that upper immediate to the program
29. 1:15counter
30. 1:16and stores the result in destination
31. 1:18register rd
32. 1:21we already have almost all the hardware
33. 1:23that we need
34. 1:24we just need to add the hardware that is
35. 1:27going to
36. 1:28be supporting that u type immediate
37. 1:32we didn't have that in that kind of
38. 1:33intermediate before this one deals with
39. 1:35upper immediates and it's fairly simple
40. 1:37to support we just need to add a
41. 1:38multiplexer that is going to be
42. 1:41sending those upper 20
43. 1:44immediate bits into the
44. 1:47immediate bypassing various kinds of
45. 1:52sign extension and so on
46. 1:57so let's convince ourselves that we got
47. 1:59what we wanted that we can execute both
48. 2:01louis and our apc so let's light up
49. 2:05the louis data path we'll first fetch
50. 2:09the instruction by pointing the program
51. 2:11counter
52. 2:12to the the the
53. 2:15instruction memory we'll
54. 2:19increment the program counter to point
55. 2:21to the next instruction
56. 2:22at the same time the this instruction
57. 2:24will point to the destination register
58. 2:26in the register file and we'll decode it
59. 2:28not going to show various control
60. 2:31signals
61. 2:31we all know how to set them
62. 2:34appropriately
63. 2:36go through the immediate generation and
64. 2:38all what we need to do is to make sure
65. 2:41that this immediate makes its way
66. 2:45all the way to the
67. 2:48destination register rd so we are going
68. 2:51to
69. 2:52set the b cell to one and then we need
70. 2:55to set
71. 2:55the alu such that it simply copies the
72. 2:58value of b
73. 2:59to its output over here and then
74. 3:03we send set right back select to one
75. 3:06and we have a new datum ready to be
76. 3:09written into destination register
77. 3:11rd on the next clock tick we are going
78. 3:14to
79. 3:14update the program counter to pc plus 4
80. 3:18and write that upper immediate
81. 3:22into destination register rd let's do
82. 3:24the same thing
83. 3:25with our pc we're going to fetch the
84. 3:29instruction
85. 3:31increment the program counter point to
86. 3:33the destination register
87. 3:35generate the immediate but at the same
88. 3:37time
89. 3:38we are lighting up this spot where we
90. 3:41are sending the program counter
91. 3:43and selecting it as the a input to the
92. 3:45alu
93. 3:46the b input to the alu will be that
94. 3:48immediate we'll add them together
95. 3:52select the write back select to pick
96. 3:54that result
97. 3:55and write it into a destination register
98. 3:57the destination register
99. 3:59and the program counter get updated
100. 4:03on the next clock deck and as the new
101. 4:05format
102. 4:06completes all of our instruction formats
103. 4:10in risk five see in a sec to wrap this
104. 4:14up