CSSE232 : Computer Architecture I

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HW3 solution

(10 points) Consider the following 32-bit binary items.

0000 0000 1000 0100 0000 0100 0011 0011
0100 0000 0101 0100 1000 0100 0011 0011

a. Assuming the items above are instructions, what RISC-V assembly instructions do they represent? (Include all operands to the instruction in your answer.)

add x8, x8, x8
sub x8, x9, x5

b. Assuming the items above are data (i.e. base 10 integers), what value do they represent?

8651827 or 0x0084 0433
1079280691 or 0x4054 8433

c. How can you tell if the bits represent instructions or data? Explain your answer.

You cannot without more information on the context.

d. Could the bit patterns represent anything else besides instructions and integers? If so, give some examples.

Many possibilities: 4 chars each, 2 shorts, 1 float each, 1 double, bit masks...

(Continues on next page)

(6 points) Convert the following C code to RISC-V assembly instructions. Use the minimum number of instructions necessary. Assume that variables f, g and h are 32- bit integers stored in registers x5, x6 and x7 respectively and that the base address of arrays A and B are in registers x8 and x9 respectively. A and B are arrays of 4-byte integers (this is important). If you need to store temporary values, use one of the other registers.
a. f = 1; A[f] = 0;
```
addi x5, x0, 1
slli  x10, x5, 2
add  x10, x8, x10
sw   x0, 0(x10)
```
b. B[1] = A[f-5];
```
slli  x10, x5, 2    #make offset for A[f]
add  x10, x8, x10  #add base to offset for A[f]
lw   x10, -20(x10)  #get value for A[f-5]
sw   x10, 4(x9)
```
(4 points) Show the hexadecimal representation of the following RISC-V instructions. Show your work.
a. addi x7, x7, 4

Needs to show the decomposition into imm12 | rs1 | f3 | rd | op for full credit.
4 | 7 | 0 | 7 | 0x13

0x0043 8393

b. lw x9, 8(x5)

Needs to show the decomposition into imm12 | rs1 | f3 | rd | op for full credit.
8 | 5 | 2 | 9 | 0x03

0x0082 A483