Help on the Lab programs
- Writing a program in SPIM
General hints and helps
- Move to coprocessor
Reverse syntax for this item
- Data Items
No error for faulty input for 0x items
- Ending a SPIM program
Need syscall to exit
Writing the Program
Use Notepad/Wordpad to write your program. PCSPIM
requires one blank line after your last line of code. So, ensure that you
press the carriage return key at least once at the end of your program. Save the
program with a ".s" extension. For example, Program1.s.
Using the Simulator
Double click on the PCSpim icon on your desktop. When the
simulator is started, you should see 4 windows titled "Messages",
"Data Segment", "Text Segment", and "Registers".
If you don't see them, click on the Window menu and select Tile.
Loading a Program
Select the Open file icon from the toolbar. Alternatively,
you can select from the menu bar: File->Open. A file open dialog box will
appear for you to select the appropriate assembly file. Select the appropriate
assembly file and click on the button labeled Open in the dialog box. If
simulator settings are not correct for the file, and it fails to load, PCSpim
will provide you an opportunity to change simulator settings and automatically
reload the file.
If you change your mind, click on the button labeled Cancel,
and PCSpim removes the dialog box. When you load an assembly file, PCSpim
removes dialog box, then loads your program and redraws the screen to display
its instructions and data. If you have not done so, change the view of the four
display windows to a tiled format by selecting from the menu bar:
Windows->Tile. You should be able to see the program in the Text segment
window display.
Each instruction in the Text segment window display is shown
on a line that looks like:
[0x00400000]
0x8fa40000 lw $4, 0 ($29); 89: lw $a0, 0($sp)
The first number on the line, in square brackets, is the
hexadecimal memory address of the instruction. The second number is the
instruction's numerical encoding, again displayed as a hexadecimal number. The
third item is the instruction's mnemonic description. Everything following the
semicolon is the actual line from your assembly file that produced the
instruction. The number 89 is the line number in that file. Sometimes nothing is
on the line after the semicolon. This means that the instruction was produced by
SPIM as part of translating a
pseudoinstruction.
Running a Program
Click on the Go button in the toolbar. Alternatively, you can
select Simulator->Go from the menu bar. Your program will begin execution. If
you want to stop the execution of your program, select Simulator->Break from
the menu bar. Alternatively, you can type Control-C when PCSpim application
window is in focus. A dialog box will appear and ask if you want to continue
execution. Select No to break the execution. Before doing anything, you can look
at memory and registers contents in the Register display window to find out what
your program was doing. When you understand what happened, you can either
continue the program by selecting Simulator->Continue or stop your program by
selecting Simulator-> Break from the menu bar.
If your program reads or writes from the terminal, PCSpim
pops up another window called the console. All characters that your program
writes appear on the console, and everything that you type as input to your
program should be typed in this window.
Debugging a Program
SPIM has two features that help debug your program. The first
is single-stepping, which allows you to run your program an instruction at a
time. Select Simulator->Single_Step to execute only one instruction.
Alternatively, you can press the F10 function key to single step. Each time you
step through a program, PCSpim will execute the next instruction in your
program, update the display, and return control to you. You can also choose the
number of instructions in your program to step by selecting Simulator->Multiple_Step
instead of single stepping through your program. A dialog box will appear and
ask you the number of instructions to step.
If your program runs for a long time before the bug arises, a
better alternative is to use a breakpoint, which tells PCSpim to stop your
program immediately before it executes a particular instruction. Select
Simulator->Breakpoints from the menu bar. The PCSpim program pops up a dialog
box window with two boxes. The top box is for you to enter breakpoint address
and the second box is a list of active breakpoints. Type in the first box the
address of the instruction at which you want to stop. Or, if the instruction has
a global label, you can just type the name of the label. Labeled breakpoints are
a particularly convenient way to stop at the first instruction of a procedure.
To actually set the breakpoint, and click on the button labeled Add. When you
are done adding breakpoints, click on the button labeled Close. You can then run
your program. When the simulator is about to execute the breakpointed
instruction, PCSpim pops up a dialog box with the instruction's address and asks
if you want to continue the execution. The Yes button continues running your
program and the No button stops your program. If you want to delete a
breakpoint, you can select Simulator-> Breakpoints from the menu bar, click
on the address in the dialog box, and click on the button labeled Remove.
Fixing Bugs in a Program
Single-stepping and setting breakpoints will probably help
you find a bug in your program quickly. To fix a bug, go back to the editor that
you used to create your program and change your source file. After you have made
the changes to your source file, simply reload it into PCSpim for Windows by
choosing Simulator-> Reload<filename> from the menu bar. This causes
PCSpim to clear its memory and registers and return the processor to the state
it was in when PCSpim first started. Once the simulator has reinitialized
itself, it will reload your recently modified file.
2002-02-10
Spim has a reverse syntax for the move to coprocessor. The symptoms are a
syntax error and the program will not
load. The fix is to reverse the order of
the operands. A sample of an instruction
that will assemble is:
mtc1 $a0, $f12
# Move to co-processor
The instruction copies the contents
of CPU register $a0 to coprocessor 1,
register $f12. The order is the
same as the store to memory
instructions. The documentation in
this case is incorrect at least for some
versions of SPIM. To check out the
version of SPIM that you are using, you
can write a simple two or three line
program to see if it assembles and that
it functions.
Spim continues to display some C like oddities. It has been noticed
that if you declare a data item like
.word 0x12345
that the x must be lower case. If it is upper case, the word will assemble as
all zeros with no error message.
The newest version of SPIM requires a syscall to accomplish an orderly return
back to the SPIM simulator. Previous versions allowed the program to run
off the end with no unusual behavior.
Use the following sequence to end your program
lw $v0,
10 # Setup register for exit
syscall
# Exit to SPIM simulator
The sequence is similar to the "call exit" used by some other
languages.
