CS-343 Assignment 5

Fall 2004

Due October 26

Your assignment is to write a MIPS assembly language program that adds up the integers in a 5-word array and writes the sum to the console. Here is a skeleton assembly language file you can start with:

              .globl    main
              .data
      alpha:  .word     [put five "interesting" integers here]
      
              .text
      main:   [write your code here]
      
      # Exit program
              li      $v0, 10
              syscall
              .end

Use pseudo-instructions to make your code easier to write. For example, use li ("load immediate") to initialize registers, and blt ("branch less than") for the test at the end of the loop.

Test your program using the SPIM simulator to make sure it works correctly. Examine the code window carefully and make sure you understand all four columns contents fully. (Help: The first column contains the memory address inside square brackets, the second column contains the memory contents, the third column gives the assembly language representation of the instructions generated by the assembler with pseudo-instructions converted to real instructions and with register names converted to register numbers, and the fourth column contains the assemly language statements you wrote.

Once your program is working and you have studied the code section of the SPIM window, do the following:

  1. Tell what the lw pseudo-instruction at the top of the loop expands to, and explain what those three instructions do.
  2. Tell what instruction li turns into, and tell another real instruction the assembler could have used to accomplish the same thing.
  3. Explain the code generated for the blt instruction. Explain why the address field of the bne instruction is different for columns 2 and 3 of the code window. Tell exactly how the branch target address is computed for this instruction.
  4. Exactly how many clock cycles will it take to run this program from the time the instruction at main is executed to the time that the loop exits? That is, ignore the four instructions for printing the results and exiting the program.
  5. Extend Figure 5.22 of the textbook by adding a columns for the following I-Format instructions: addi, andi, ori, slti.
  6. Write the pseudocode for a function that that implements the control unit of the single-cycle MIPS processor developed in the first part of Chapter 5. The argument passed to this function is a 32-bit integer with the value of the IR. The result returned by the function is a "control word" with bits 8:0 set to the values of RegDst, ALUSrc, etc. for that instruction. (See below).
  7. Optional: Translate your pseudocode into a real program and run it with column 2 of your SPIM code window as input values. Here's what my version produced:
    8. 
File: addem.ccv
0x34080000  ori $8, $0, 0
     ori: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=1 AOp1=1 AOp0=0
0x34040000  ori $4, $0, 0
     ori: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=1 AOp1=1 AOp0=0
0x34090014  ori $9, $0, 20
     ori: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=1 AOp1=1 AOp0=0
0x3c011001  lui $1, 4097
     lui: Jmp=? RgDs=? ASrc=? M2Rg=? RgWr=? MRd=? MWr=? Br=? AOp1=? AOp0=?
0x00280821  addu $1, $1, $8
  r_type: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=0 AOp1=1 AOp0=0
0x8c250000  lw $5, 0($1)
      lw: Jmp=0 RgDs=0 ASrc=0 M2Rg=0 RgWr=1 MRd=1 MWr=1 Br=1 AOp1=0 AOp0=0
0x00852020  add $4, $4, $5
  r_type: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=0 AOp1=1 AOp0=0
0x21080004  addi $8, $8, 4
    addi: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=1 AOp1=1 AOp0=0
0x0109082a  slt $1, $8, $9
  r_type: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=0 AOp1=1 AOp0=0
0x1420fffa  bne $1, $0, -24
     bne: Jmp=? RgDs=? ASrc=? M2Rg=? RgWr=? MRd=? MWr=? Br=? AOp1=? AOp0=?
0x34020001  ori $2, $0, 1
     ori: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=1 AOp1=1 AOp0=0
0x0000000c  syscall
  r_type: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=0 AOp1=1 AOp0=0
0x3402000a  ori $2, $0, 10
     ori: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=1 AOp1=1 AOp0=0
0x0000000c  syscall
  r_type: Jmp=0 RgDs=1 ASrc=0 M2Rg=0 RgWr=0 MRd=1 MWr=0 Br=0 AOp1=1 AOp0=0

    Legend: The signal names are abbreviated to keep the lines short. A stands for ALU, M stands for Mem, Rg stands for Reg, Rd stands for Read, Wr stands for Write, Ds stands for Dst, and Jmp stands for Jump (see below). A value of '?' for a signal means that the instruction can't be handled by the datapath designed in the book; it would need additional features. And a value of 'X' (there are none in the sample output above) means the value of the signal doesn't matter, but a real controller would have to generate either a 1 or a 0 in place of the X.

    Confessions: I captured the SPIM simulator's code window in a text file, and edited it so each line contained just columns 2 (the address) and 3 (the assembly language) of one instruction, and my program prints this line as it reads it. My program outputs a value for the Jump control signal developed just after Figure 5.22 in the book. The syscall instructions look like R-type instructions to my program, but that's not very meaningful. And the immediate format instructions won't work unless the datapath is modified to get the correct function code into the ALU; just setting ALUOp to 102 won't actually work. (How would you have to change the datapath to handle the immediate format instructions?)