Exercise 3

Objective

Consolidate the use of the conditional branch instruction in RISC-V. The following C expression is translated into assembly using beq:

if (i != j)
    f = g + h;
f = f - i;

Instructions

Open the RISC-V simulator.
Transcribe the code below into the editor:

# Exercise 03
# C expression:
#   if (i != j)
#       f = g + h;
#   f = f - i;

    .text
main:
    addi s0, zero, 15     # f = 15
    addi s1, zero, 10     # g = 10
    addi s2, zero, 20     # h = 20
    addi s3, zero, 1      # i = 1
    addi s4, zero, 2      # j = 2

    beq  s3, s4, L1       # if (i == j) goto L1
    add  s0, s1, s2       # f = g + h

L1:
    sub  s0, s0, s3       # f = f - i

Step-by-step Execution - Scenario 1 (i ≠ j)

With the initial values i = 1 and j = 2, the branch is not taken and all instructions execute.

Run the program step by step using the Run One Step button. For each instruction, fill in the table below whenever a register is modified.

Fill in the table below

Fill in the PC with the instruction address and copy the Instruction column exactly as it appears in the simulator. Only fill in fields that are modified.
If you need more rows, click Add row. Verification only runs when all rows are filled, if nothing appears after clicking Check, it means there are still empty rows.

Before		After executing the instruction
PC	Instruction	x8	x9	x18	x19	x20
		(s0)	(s1)	(s2)	(s3)	(s4)
		0x00000000	0x00000000	0x00000000	0x00000000	0x00000000
0x00400000	addi s0 zero 15	0x0000000F

Tip: in the print dialog, select A2 paper size for best results.

Step-by-step Execution - Scenario 2 (i = j)

Now change the addi instruction that sets s4 to assign j = 1 (same as i):

    addi s4, zero, 1      # j = 1

Click Reset to reload the program, then run it step by step again. With i = j = 1, the branch is taken and the add instruction is skipped.

Fill in the table below

Fill in the PC with the instruction address and copy the Instruction column exactly as it appears in the simulator. Only fill in fields that are modified.
If you need more rows, click Add row. Verification only runs when all rows are filled, if nothing appears after clicking Check, it means there are still empty rows.

Before		After executing the instruction
PC	Instruction	x8	x9	x18	x19	x20
		(s0)	(s1)	(s2)	(s3)	(s4)
		0x00000000	0x00000000	0x00000000	0x00000000	0x00000000
0x00400000	addi s0 zero 15	0x0000000F

Tip: in the print dialog, select A2 paper size for best results.

Compare both tables and analyze the difference in the instruction flow (observe the PC sequence) and the final value of s0.