CS201 Lab: Introduction to PCSpim

	SPIM is a simulator for the MIPS.
	In the lab, we are going to use PCSpim.
	For more information about SPIM, check the link:
	http://www.cs.wisc.edu/~larus/spim.html

Start up PCSpim

Create an Assmbly Lauguage Program

Let's assume that you have created a program called hello.s by using NotePad on a PC.

##
## 	hello.s - prints out "hello world"
##
##      a0 - points to the string
##

#################################################
#                                               #
#               text segment                    #
#                                               #
#################################################

        .text
        .globl __start
__start:                # execution starts here

        la $a0,str      # put string address into a0
        li $v0,4        # system call to print
        syscall         # out a string

        li $v0,10	# Exit
        syscall         # Bye!


#################################################
#                                               #
#               data segment                    #
#                                               #
#################################################

        .data
str:    .asciiz "hello world\n"

##
## end of file hello.s

Open and Run the Program

To open the program, click on the "File" menu from PCSpim screen and find the program that you want to open.
 

Then, you would see the following screen:
 

Now select "Go" from the "Simulator" menu:
 

To run the program, click on "OK".
 

Here is the program output on the "Console".
 

Now let's analyze the program and see how it works.

Analyze the Program

The MIPS assembly language program using PCSpim is usually held in a file with .s at the end of the file name.
For example hello.s. This file is processed line by line by the SPIM program. In this file,

	A sharp sign # is used to begin inline comments.
	i.e. Everything from the # to the end of the line is 
	     ignored by the assembler.

	An identifier is a sequence of alphanumeric characters, 
	underbars/underscores(_),and dots(.) that do not begin with a number.

	A label is declared by putting identifiers at the beginning of a line 
	followed by a colon.

	An operation field contains either a machine instruction or 
	an assembler directive.

	One or more operands are required by some machine instructions.
	Assembler directives may also require operands.

	A constant is a value that does not change during program assembly
	or execution. A string contant is delimited by double quotes(").
	For example:  "Hello World\n"

The program hello.s prints out the message hello world.
It sets up the string in the data segment and makes a system call to print out this string in the text segment,
followed by a system call to exit the program.

	.text  

		-  a directive which tells the assembler to place 
			what follows in the text segment.

	.globl __start
__start:

		-  These two lines attach the label __start
			to the first instruction so that the assembler
			can identify where the execution should begin.
			All the exercises in the lab will have these two 
			lines unchanged.

SPIM System Call (syscall)

SPIM provides a set of operating-system-like services through the system call (syscall) instruction.
To request a service, a program loads the system call code into register $v0 and arguments into registers $a0, $a1, $a2, or $a3.
Here is a summary of the system calls for your reference.

Function Code ($v0)	Function	Other Important Registers	Sample Code
1	Print an Integer	INPUT $a0 : the integer to be printed	li $a0, 12345 li $v0, 1 syscall
2	Print a Float	INPUT $f12 : the Floating point number to be printed.	.data fvar: .float 1.125 .text l.s $f12, fvar li $v0, 2 syscall
3	Print a Double	INPUT $f12 : the Double to be printed.	.data fvar: .double 1.2345678901234567 .text l.d $f12, fvar li $v0, 3 syscall
4	Print a String	INPUT $f12 : the address of the string to be printed.	.data str1: .asciiz "I'm a string!" .text la $a0, str1 li $v0, 4 syscall
5	Read an Integer	OUTPUT $v0 : the integer that was read	li $v0, 5 syscall
6	Read a Float	OUTPUT $f0 : the Floating point number that was read	li $v0, 6 syscall
7	Read a double	OUTPUT $f0 : the double that was read	li $v0, 7 syscall
8	Read a String	INPUT $a0 : Address of input buffer in memory $a1 : size of buffer (in bytes) OUTPUT Input buffer has user's input loaded into it.	.data str1: .space 255 .text la $a0, str1 li $a1, 255 li $v0, 8 syscall li $v0, 4 syscall
9	Dynamically allocate n bytes of memory.	INPUT $a0 : n The number of bytes to allocate. OUTPUT $v0 : A pointer to the bytes allocated.	li $a0, 255 li $v0, 9 syscall # Get memory move $a0, $v0 li $a1, 255 li $v0, 8 syscall # Read String li $v0, 4 syscall # Print string
10	Exit.		li $v0, 10 syscall

A Few Special Characters