Introduction to Mars

This is a short guide on how to launch and use Mars. Mars will run on any system (including Windows) as long as you have Java installed. If you prefer, you may download and install Mars on your private computer.

Launch Mars

Mars should now start and you should see something similar to this.

IDE overview

Mars is an Integrated Development Environment (IDE) for Mips Assembly Language Programming.

At the top you find the top level menu: File, Edit, Run, Settings, Tools and Help.

Tools and operations

Under the top level menu a collection of icons show some of the most commonly used tools and operations. The most important of these controls are described in the below table.

Control	Description
	Load a file.
	Assemble the program in the Edit tab.
	Save the current file.
	Run the assembled program to completion (or breakpoint).
	Execute a single instruction (single-step).
	Undo the last instruction (single-step backwards).
	Adjust the execution speed.

Edit and Execute

In the middle left you find two tabs: Edit and Execute.

The Edit tab will be used to edit assembly code.
The Execute tab shows the Text Segment (machine instructions) and Data Segment during execution.

Registers

To the right you find the registers pane. Here the contents of all registers are shown. There are three register tabs:

General purpose registers.
Coprocessor 0 registers.
Coprocessor 1 registers.

Mars Messages and Run I/O

In the lower left corner there are two tabs: Mars Messages and Run I/O.

The Mars Messages tab is used for displaying assembly or runtime errors and informational messages. You can click on assembly error messages to highlight and set focus on the corresponding line of code in the editor.

The Run I/O tab is used at runtime for displaying console output and entering console input as program execution progresses.

The mips-examples repository

Before you continue you should already have cloned the mips-examples repository.

Load hello.s into Mars

Among the examples programs in the misp-exmples repository you find hello.s. Open the file hello.s in the Mars simulator by selecting Open from the File menu. You should now see something similar to this.

After you loaded a program, the source code is available for edit in the Edit pane.

Change font colors

The default font colors might not be the most pleasing on your eyes. Especially the font color for comments may be hard to read. From the top menu, follow these steps to change the font colors:

Settings
Editor …
Now a window named Text editor settings will open.
To the right you find the Syntax styling options.
Un-check the checkbox to override the default font.
Click on the button to the right of the font preview to change the color of the font.
Click on the button Apply and close in the lower left corner of the window to apply your settings.

Assemble

To translate the Mips assembly source code to executable machine instructions an assembler is used.

Assemble the file by clicking on the icon with the screwdriver and wrench.

Mars Messages

You should see something similar to the following in the Mars Messages display pane.

Assemble: assembling /path/to/file/hello.s

Assemble: operation completed successfully.

Execute pane

After a successful assembly, the generated machine code instructions together with the source code instructions are shown in the Execute pane.

Run to completion

Click on the play icon to run the program to completion.

Run I/O

In the the Run I/O display window you should see the following output.

Hello World!
-- program is finished running --

Symbol table

From the Settings menu, select Show Label Window (symbol table). Now, the following window should appear.

The symbol table shows the actual memory addresses of all labels. For example, we see that the label main is a mnemonic for the memory address 0x00400000. When you click on a label in the symbol table, the address of the label is highlighted in the text or data segment.

Text segment

In the symbol table, click on the label main. Now the following row should be highlighted with a blue border in the Source code column in the Text segment area in the Execute tab.

li $v0, 4 # Systemcall code print_str

If you look at the source code (press the Edit tab) you see that this is the instruction following directly after the label main.

Data segment

In the symbol table, click on the label msg. Now address 0x10010000 should be highlighted with a blue border in the Data Segment area in the Execute tab. The value store at this address is 0x6c6c6548.

If you study the data segment in detail you see that the string "Hello World!" is stored byte for byte starting at address 0x10010000.

Breakpoints and debugging

A very usefull feature of Mars is the ability to set breakpoints. Breakpoints together with single-stepping and backward single-stepping are very powerful when trying to understand or debugging a program.

Assemble the file.

Make sure to view the Execute tab. In the Text segment, click the Bkpt (breakpoint) checkbox at address 0x00400000.

This will set a breakpoint on the syscall instruction.

Click on the play icon to run the program.

The execution will now halt at the breakpoint (the syscall instruction).

Click on the single-step icon to continue the execution one instruction at the time.

Click on the single-step-backwards icon to run the execution backwards one instruction at the time.

Study the example programs

You are now ready to continue and study the other example programs in the mips-examples repository.