Lecture 4

RISC-V Instructions

Lecture

Slides (PDF, PPTX).

Outline

• Stored program concept.
• Memory layout.
• RISC-V ISA base and extensions.
• Instruction encoding (R, I, S, SB, U, and UJ formats).

Workshop

Outline

• Discuss the tasks from the previous seminar.
• Get familiar with the RISC-V ISA Specification (instruction types, encodings, etc.).
• Practice with encoding and decoding instructions.
• Get familiar with pseudoinstructions.
• Understand the use of labels and branch instructions.
• Practice writing programs with conditions and loops.

Tasks

1. Encode the following instructions to the binary format (see the rules in the ISA specification):

   • R-type:

     add  t3, t1, t2
     

   • I-type:

     ori  s0, t1, 0x123
     lw   t1, 0x8(t0)
     

   • S-type:

     sw   t1, 0x8(t0)
     

   • SB-type:

     beq t2, t3, 16
     beq t2, t3, -8
     

   • U-type:

     lui  t0, 0x12345
     

   • UJ-type:

     jal zero, 16
     

2. Decode the following hexademical values to instructions:

   0x98765437
   0x00744433
   0x0080006f
   0xfff37293
   0x00000463
   0x00032823
   

3. Get familiar with RISC-V pseudoinstructions (mv, li, la, b, j, etc). What instructions are used to replace them when a program is assembled:

   mv t0, t1
   li t0, 0x12345678
   li t0, 16
   b  label
   j  label
   la t0, label
   

   Note: Add label to some place in your code. See what code will be generated in the Execute panel of RARS.

4. Write a program that inputs an integer value x and prints -1 if it is negative, 0 if it equals 0, and 1 if it is positive.

   On if the solutions is this:

   main:
       li   a7, 5
       ecall
         
       mv   t0, zero
       beqz a0, done
       li   t0, 1
       blt  zero, a0, done  
       li   t0, -1
   done:
       li a7, 1
       mv a0, t0
       ecall
   

   Propose any other solution?

5. Write a program that inputs two integer values x and y and prints first the smallest of them and then the largest of them.

Homework

Task 1

Decompile the program (convert instructions from the binary representation into assembly source code), put the source code into RARS and run it. What is an output?

Rewrite the program using a smaller number of instructions.

0x6c6c6337
0x54830313
0x412033b7
0xc6f38393
0x00535e37
0xf43e0e13
0x10010437
0x00040413
0x00642023
0x00742223
0x01c42423
0x00400893
0x00800533
0x00000073

Task 2

Decompile the program, put the source code into RARS and run it. What arithmetic operation this program is doing?

Rewrite the program using a smaller number of instructions (hint: use RV32M extension instructions).

0x00500893  3 
0x00000073  4 
0x00a00333  5  # store "a"
0x01f55293  6  # store sign of "a"
0x00000073  8 
0x00a00eb3  9  # store "b"
0x01f55e13  10 # store sign of "b"
0x06030263  12
0x060e8063  13
0x00028663  15 # if "a" < 0 then inverse it
0xfff34313  16
0x00130313  17
0x000e0663  19 # if "b" < 0 then inverse it
0xfffece93  20
0x001e8e93  21
0x01d35863  24 # if "a" < "b" swap them
0x006eceb3  25
0x006ec333  26
0x006eceb3  27
0x000003b3  29
0x006383b3  31
0xfffe8e93  32
0xffd04ce3  33
0x01c2cfb3  36
0x000f8663  37
0xfff3c393  38
0x00138393  39 # t2 - result accumulator
0x00100893  41
0x00700533  42
0x00000073  43
0x00a00893  45
0x00000073  46
0x00100893  49 # if sign of "a" and "b" is different then negate the result
0x00000533  50
0x00000073  51
0x00a00893  52
0x00000073  53

References

• The RISC-V Instruction Set Manual. Volume I: Unprivileged ISA.
• RISC-V Assembly Programmer’s Manual
• Opcode (Wikipedia).
• Addressing Mode (Wikipedia).