MARS MIPS simulator is an assembly language editor, assembler, simulator & debugger for the MIPS processor, developed by Pete Sanderson and Kenneth Vollmar at Missouri State University (src).
Before assembling, the environment of this simulator can be simplisticly split to three segments: the editor at the upper left where all of the code is being written, the compiler/output right beneath the editor and the list of registers that represent the "CPU" for our program.
After assembling (by simply pressing F3) the environment changes, with two new segments getting the position of the editor: the text segment where
i) each line of assembly code gets cleared of "pseudoinstructions" (we'll talk about those in a sec) at the "basic" column and
ii) the machine code for each instruction at the "code" column,
and the data segment where we can have a look at a representation of the memory of a processor with little-endian order.
Now, let's see the example code from above and explain each line:
.text .globl main main: #main function li $v0, 11 #11=system code for printing a character, $v0=register that gets the system code for printing as value la $a0, 'a' #'a'=our example character, $a0=register that accepts the character for printing syscall #Call to the System to execute our instructions and print the character at the a0 register li $v0, 10 #11=system code for terminating, $v0=register that gets the system code for terminating (optional, but desirable) syscall #Call to the System to terminate the execution
MARS accepts and exports files with the .asm filetype
But the code above prints just a character, what about the good ol' "Hello World"? What about, dunno, adding a number or something? Well, we can change what we had a bit for just that:
.data #data section str: .asciiz "Hello world\n" number: .word 256 .text #code section .globl main main: li $v0, 4 #system call for printing strings la $a0, str #loading our string from data section to the $a0 register syscall la $t0, number #loading our number from data section to the $t0 register lw $s1, 0($t0) #loading our number as a word to another register, $s1 addi $t2, $s1, 8 #adding our number ($s1) with 8 and leaving the sum to register $t2 sw $t2, 0($t0) #storing the sum of register $t2 as a word at the first place of $t0 li $v0, 10 # system call for terminating the execution syscall
Before illustrating the results through MARS, a little more explanation about these commands is needed:
System calls are a set of services provided from the operating system. To use a system call, a call code is needed to be put to $v0 register for the needed operation. If a system call has arguments, those are put at the $a0-$a2 registers. Here are all the system calls.
li (load immediate) is a pseudo-instruction (we'll talk about that
later) that instantly loads a register with a value.
address) is also a pseudo-instruction that loads an address to a
li $v0, 4 the $v0 register has now
la $a0, str loads the string of
str to the
A word is (as much as we are talking about MIPS) a 32 bits sequence, with bit 31 being the Most Significant Bit and bit 0 being the Least Significant Bit.
lw (load word) transfers from the memory to a register, while
(store word) transfers from a register to the memory. With the
lw $s1, 0($t0) command, we loaded to
$s1 register the value that was
at the LSB of the
$t0 register (thats what the
0 symbolizes here,
the offset of the word), aka
$t0 here has the address, while
$s1 has the value.
sw $t2, 0($t0) does just the opposite
MARS uses the Little Endian, meaning that the LSB of a word is stored to the smallest byte address of the memory.
MIPS uses byte addresses, so an address is apart of its previous and next by 4.
By assembling the code from before, we can further understand how memory and registers exchange, disabling "Hexadecimal Values" from the Data Segment:
or enabling "ASCII" from the Data Segment:
Start it like this
$ java -jar Mars4_5.jar
Create this file and save it.
.text main: li $s0,0x30 loop: move $a0,$s0 # copy from s0 to a0 li $v0,11 # syscall with v0 = 11 will print out syscall # one byte from a0 to the Run I/O window addi $s0,$s0,3 # what happens if the constant is changed? li $t0,0x5d bne $s0,$t0,loop nop # delay slot filler (just in case) stop: j stop # loop forever here nop # delay slot filler (just in case)
Press F3 to assembly it and then press run. Now you are started compiling and executing MIPS code.