[CS61C FA20] Lecture 19.5 - Single-Cycle CPU Datapath II: Adding JAL — Transcript

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1. 0:01[Music]
2. 0:08hi
3. 0:08welcome back to our design of risk 5 cpu
4. 0:12this time we'll add support for the jowl
5. 0:15instruction gel instruction
6. 0:16is stands for jump and link and is the
7. 0:20only
8. 0:20instruction that we have in the j format
9. 0:23g format
10. 0:24has a 20 bit wide immediate
11. 0:28and then for field for a destination
12. 0:31register and the op code
13. 0:33it does two updates to the state of the
14. 0:36processor it saves the value of pc plus
15. 0:404
16. 0:41in the destination register so we have
17. 0:43the return address
18. 0:45and sets the new value of a program
19. 0:48counter to be equal to the
20. 0:50current value of program counter plus
21. 0:52the offset
22. 0:53and the offset is set by the immediate
23. 0:58so it performs a program counter
24. 1:01relative jump
25. 1:02where we can jump anywhere that that
26. 1:05these 20 bits of the immediate allow us
27. 1:07to do
28. 1:08um so the target will be anywhere
29. 1:12within two to plus minus two to the 19
30. 1:15locations and these locations are said
31. 1:17to be
32. 1:17two bytes apart to support the
33. 1:19compressed instruction set
34. 1:21which means that we can jump anywhere
35. 1:23from
36. 1:24minus 2 to the 18th to plus to the
37. 1:272 to the 18th 32-bit words
38. 1:31in our code
39. 1:35we have all the hardware that we need to
40. 1:37implement
41. 1:38jump in link we just need to have a way
42. 1:41to generate a j
43. 1:43type immediate so if you look at the
44. 1:46data path we'll see that we really
45. 1:49actually have everything we need
46. 1:51the only update is immediate generation
47. 1:53has to become
48. 1:54of j type and j type is similar to a b
49. 1:57type immediate we just need to add a few
50. 2:00more
51. 2:01multiplexers to multiplexing
52. 2:04more most significant bits the top eight
53. 2:07significant
54. 2:08bits so
55. 2:11when we look at the data path we have
56. 2:13the two mechanisms that we are
57. 2:15borrowing from gel
58. 2:18r and branch to implement jowl
59. 2:22so the first thing that we need to do
60. 2:25we need to save the value of pc plus 4
61. 2:29in the destination register rd we do
62. 2:32that by using the same mechanism that we
63. 2:34have added
64. 2:35for gel r we
65. 2:39send the pc plus 4 value to the right
66. 2:42back select register
67. 2:43right back select is set to have the
68. 2:46value of 2
69. 2:47that enables us to write into the
70. 2:50destination register rd the second
71. 2:53mechanism we are borrowing from the
72. 2:55branch
73. 2:56what we need to do we need to make the
74. 2:57alu add the value of bc
75. 3:00at its input a
76. 3:04with the new immediate value and
77. 3:07send that result back to the program
78. 3:09counter we're going to set
79. 3:11the pc select signal to be always taken
80. 3:15and that's it let's light up the jowl
81. 3:18data path
82. 3:20we start with the instruction fetch and
83. 3:23we proceed to the decode
84. 3:26after so few things that i would like to
85. 3:28highlight here we are sending the pc
86. 3:31to the a select multiplexer
87. 3:35and we are sending the pc plus 4 to the
88. 3:38right back select when we decode the
89. 3:41instruction
90. 3:42we set the control bits appropriately
91. 3:46simultaneously well that will
92. 3:49generate the immediate that is of a j
93. 3:52type
94. 3:53and then we have the right operands in
95. 3:55front of the alu
96. 3:56the alu multiplexer inputs are set
97. 4:00to pick the program counter
98. 4:03and the immediate and the output of the
99. 4:06alu
100. 4:07will be sent back to the pc cell
101. 4:09multiplexer
102. 4:11pc cell multiplexer also has a pc plus 4
103. 4:15but it will not take it because pc
104. 4:18cell is all signal is always sent set
105. 4:21to be taken finally we
106. 4:25just use the right back select
107. 4:28multiplexer
108. 4:29to pick pc plus 4 to be written back in
109. 4:32the destination register rd
110. 4:35on the next clock tick
111. 4:38the destination register and the program
112. 4:40counted there
113. 4:41are updated to the new values
114. 4:44and that is it that is what chal does
115. 4:48see you in a bit with the long
116. 4:51immediates