如何统计下述MIPS程序执行的总指令条数？

Alright, let's walk through how to count the total number of instructions executed by this MIPS program, step by step.

First, we'll separate the one-time setup code from the repeating loop logic:

Initialization (Executed Once) #

These two instructions run only at the start of the program, no repeats:

• ADDI R1, R0, #1
• SW R1, 2000 (R0)
  Total for initialization: 2 instructions

Loop Body (Repeats Until Termination) #

Next, count every instruction inside the LOOP block (from the LOOP: label all the way to BNEZ R8, LOOP):

1. LW R1, 2000 (R0)
2. MULT R2, R1, #4
3. ADDI R3,R2, #5000
4. LW R4, 0 (R3)
5. LW R5, 1500 (R0)
6. ADD R6, R4, R5
7. LW R1, 2000 (R0)
8. MULT R2, R1, #4
9. ADDI R1, R1, #1
10. SW R6, 0(R7)
11. SW R1, 2000 (R0)
12. ADDI R1, R1, #1
13. SW R1, 2000 (R0)
14. ADDI R1, R1, #1
15. SW R1, 2000 (R0)
16. LW R1, 2000 (R0)
17. ADDI R8, R1, #-101
18. BNEZ R8, LOOP
    Total instructions per loop iteration: 18 instructions

Now we need to figure out how many times the loop runs:

• The initial value of R1 (stored at address 2000) is 1.
• Each loop iteration increments R1 three times (via three ADDI R1, R1, #1 instructions), so after each full loop, the stored R1 value goes up by 3.
• The loop stops when BNEZ R8, LOOP doesn't jump. This happens when R8 = 0, and since R8 = R1 - 101 (from ADDI R8, R1, #-101), this requires R1 = 101.

Wait a second—there's a critical issue here: Starting from 1, adding 3 each time will never reach 101 exactly (101 minus 1 is 100, which isn't divisible by 3). That means the program will run infinitely, cycling through values like 1, 4, 7, ..., 100, 103, 106, etc., and never hitting 101. In this case, there's no finite total instruction count.

If We Assume a Minor Logic Fix #

If we guess the termination condition was meant to trigger when R1 reaches 100 (e.g., ADDI R8, R1, #-100 instead of -101), then:

• The number of loop iterations is (100 - 1)/3 = 33 (since 1 + 3*33 = 100).
• Total instructions = Initialization instructions + (Loop instructions per iteration * Number of iterations)
• Total = 2 + (18 * 33) = 2 + 594 = 596 instructions

If the program is written exactly as provided, it loops forever with no end. But if there's a small typo in the termination condition, we can calculate a finite total as shown above.

内容的提问来源于stack exchange，提问作者user3710257