MARS Primer

Some things you need to know about MARS

• Just under the toolbar are two tabs labeled "Edit" and "Execute"
  • in the Edit window you can type in your program and make changes to it
  • in the Execute window you can run your program and inspect the state of the machine
    • breakpoints (details below) allow you to make the program stop right before any instruction that you choose
    • you can also run the program one instruction at a time using the single-step button
    • you can even single step backwards
• Down near the bottom are two tabs labeled "Mars Messages" and "Run I/O"
  • MARS will print assembly results (success, errors) in the Mars Messages window
  • when your program prints things, they will appear in the Run I/O window
• the Execute window has several subwindows that you need to be familiar with:
  • the text-segment window is at the top left; it displays the instructions in your program, one per line and in five columns:
    • the first column contains checkboxes; each checked box marks the instruction on that line as a breakpoint; when the program encounters a breakpoint, it will stop just before fetching and executing that instruction
    • the second column displays the memory address of the instruction on that line
    • the third column displays the assembled instruction in hexadecimal
    • the fourth column displays the instruction source with fakes replaced by real code
    • the fifth column displays the instruction source as you typed it but with macros expanded
    • when the fourth and fifth columns differ in any material way, that is a sure sign that you are looking at a fake of some kind
    • usually, when this is the case, the fourth column will have more lines than the fifth column
  • the data-segment window is right under the text-segment window; it displays the data segment of your program in nine columns
    • in each row, the first column contains an address; each of the other eight columns contains one data word
    • the address in the first column is that of the first data word in that row (the one in the next column)
    • as you move from left to right across a row, each column's address is 4 more than the column before it
    • this is indicated by the column headers in the very first row
    • the address of each cell is the sum of the number in that row's address column and the number in that column's header
    • each word is made up of four bytes
    • each word is made up of eight hex digits, so each byte is made up of two hex digits (4 bits each)
    • the bytes appear in right-to-left order in their word because MARS is little-endian
  • the registers window is at the far right
    • each register is identified by name and by number and its contents are displayed
    • when you single-step through a program, the last register changed is highlighted in green
    • watching to see that registers change as you expect them to is a principal debugging technique
  • the labels window is between the text-segment window and the registers window
    • to see it, you must choose "Show Labels Window", which is the first item in the Settings menu
    • it displays the value assigned to each of the labels in your program
    • each of these values is an address
    • they can change as you make changes to your program; stay alert to this
    • addresses in the data segment begin at 0x10010000
    • addresses in the text segment begin at 0x00400000