Assembly

Question 1

What decides how much work each instruction performs in an assembly setting?

Answer 1

The ISA.

Question 2

How many bytes does each RISC-V instruction have?

Answer 2

4.

Question 3

TRUE/FALSE: Despite different instructions in RISC-V having different purposes, each instruction has the same encoding structure.

Answer 3

FALSE. Each TYPE of instruction has an instruction set with the same encoding, but different parameters may be used for each.

Question 4

What is R-Type encoding? Where do parameters come from, and where does output go?

Answer 4

Arithmetic. Register to register.

Question 5

What is I-Type encoding? Where do parameters come from, and where does output go?

Answer 5

Immediate arithmetic. Register and immediate to register.

Question 6

What is S-Type encoding? Where do parameters come from, and where does output go?

Answer 6

Load/Store. Memory to register and register to memory.

Question 7

What is SB-Type (or B-Type) encoding? Where do parameters come from, and where does output go?

Answer 7

Branching. Register or immediate to Program counter.

Question 8

What is UJ-Type (or J-Type) encoding? Where do parameters come from, and where does output go?

Answer 8

Jump and Link. Register or immediate to Program counter.

Question 9

What is U-Type encoding? Where do parameters come from, and where does output go?

Answer 9

Load Upper immediate. Immediate to register.

Question 10

How many bits are used in B-type encoding for where to jump to? Which bits? Note that 5 bits are used for rs2, 5 bits are used for rs1, 3 bits are used for funct3, and 7 bits are used for the opcode.

Answer 10

12 bits are used for B-Type (gotten by 32 - total of others), and the bits used are 1:12 from 0:31. This is because the last two bits don’t actually matter in jumping (since you’ll only ever jump to a whole byte), and therefore dropping 0 is fine.

Question 11

Suppose I want to save the immediate value 0xffffffff into register x3. What instructions would I need, in what order?

Answer 11

1 instruction (if running on code):
Addi -1 (written as 0xfff but converted automatically by the processor to carry the negative) plus x0 to x3.

Question 12

Why do most instructions in RISC-V use 12-bits to store immediate values?

Answer 12

Performance, benchmarking.

Question 13

How many bytes does each x86 instruction have?

Answer 13

It varies. CISC is variable-width encoding.

Question 14

What rule must each instruction, RISC or CISC, follow?

Answer 14

The longest one must be shorter than the clock period.