MIPS Assembly Language Programming

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2.3. USING SPIM 23 # end of add.asm 2.3 Using SPIM At this point, we should have a working program. Now, it’s time to try running it to see what happens. To run SPIM, simply enter the command spim at the commandline. SPIM will print out a message similar to the following 5 : % spim SPIM Version 5.4 of Jan. 17, 1994 Copyright 1990-1994 by James R. Larus (larus@cs.wisc.edu). All Rights Reserved. See the file README a full copyright notice. Loaded: /home/usr6/cs51/de51/SPIM/lib/trap.handler (spim) Whenever you see the (spim) prompt, you know that SPIM is ready to execute a command. In this case, since we want to run the program that we just wrote, the first thing we need to do is load the file containing the program. This is done with the load command: (spim) load "add.asm" The load command reads and assembles a file containing MIPS assembly language, and then loads it into the SPIM memory. If there are any errors during the assembly, error messages with line number are displayed. You should not try to execute a file that has not loaded successfully– SPIM will let you run the program, but it is unlikely that it will actually work. Once the program is loaded, you can use the run command to execute it: (spim) run The program runs, and then SPIM indicates that it is ready to execute another command. Since our program is supposed to leave its result in register $t0, we can verify that the program is working by asking SPIM to print out the contents of $t0, using the print command, to see if it contains the result we expect: 5 The exact text will be different on different computers.
24 CHAPTER 2. MIPS TUTORIAL (spim) print $t0 Reg 8 = 0x00000003 (3) The print command displays the register number followed by its contents in both hexadecimal and decimal notation. Note that SPIM automatically translates from the symbolic name for the register (in this case, $t0) to the actual register number (in this case, $8). 2.4 Using syscall: add2.asm Our program to compute 1+2 is not particularly useful, although it does demonstrate a number of important details about programming in MIPS assembly language and the SPIM environment. For our next example, we’ll write a program named add2.asm that computes the sum of two numbers specified by the user at runtime, and displays the result on the screen. The algorithm this program will follow is: 1. Read the two numbers from the user. We’ll need two registers to hold these two numbers. We can use $t0 and $t1 for this. 2. Compute their sum. We’ll need a register to hold the result of this addition. We can use $t2 for this. 3. Print the sum. 4. Exit. We already know how to do this, using syscall. Once again, we start by writing a comment. From what we’ve learned from writing add.asm, we actually know a lot about what we need to do; the rest we’ll only comment for now: # Daniel J. Ellard -- 02/21/94 # add2.asm-- A program that computes and prints the sum # of two numbers specified at runtime by the user. # Registers used: # $t0 - used to hold the first number. # $t1 - used to hold the second number. # $t2 - used to hold the sum of the $t1 and $t2.
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