Exercise 7

Objective

Consolidate RISC-V assembly learning and understand how procedure-support instructions are executed. The following C expression is implemented as a procedure call using jal and jr ra:

int f = (g + h) - (i + j);

Instructions

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

# Exercise 07
# C expression: int f = (g + h) - (i + j);

    .text
    j main

leaf_example:
    add  a5, a1, a2       # a5 = g + h
    add  a6, a3, a4       # a6 = i + j
    sub  a0, a5, a6       # f = (g + h) - (i + j), result in a0
    jr   ra               # return to caller

main:
    addi a1, zero, 4      # g = 4
    addi a2, zero, 7      # h = 7
    addi a3, zero, 2      # i = 2
    addi a4, zero, 1      # j = 1
    jal  leaf_example     # call the function
    nop                   # a0 now holds the return value (f)

Step-by-step Execution

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.
For pseudo-instructions, the simulator shows the real instruction in the Basic column and the original form in the Source column. Copy the Basic value into the Instruction column and the Source value into the Pseudo-instruction column. Leave the pseudo column blank for non-pseudo instructions.
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	Pseudo-instruction	x10	x11	x12	x13	x14	x15	x16	x1
			(a0 (f))	(a1 (g))	(a2 (h))	(a3 (i))	(a4 (j))	(a5)	(a6)	(ra)
			0x00000000	0x00000000	0x00000000	0x00000000	0x00000000	0x00000000	0x00000000	0x00000000
0x00400000	jal x0 20	j main

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