IS1200/IS1500
Lab1 – Assembly Programming
2015-09-04
Introduction
Welcome to the first lab in the course! In this laboratory exercise you will learn the fundamentals of
programming in an assembly language. After finishing this lab, you should be able to
1. analyze MIPS assembly code by using a simulator.
2. write short assembly code functions in MIPS assembler.
3. execute and test a compiled program on a PIC32 microcontroller.
Lab Environment
The first part of this lab is using MARS (MIPS Assembler and Runtime Simulator). You can
download the software here http://courses.missouristate.edu/KenVollmar/MARS/.
In the second part of the lab, you will use a chipKIT development board, which includes a 32-bit
MIPS microprocessor. The teaching assistants will hand out these development boards during the
lab.
Preparations
You must do the lab work in advance, except for a few specific tasks. The teachers will examine
your skills and knowledge at the lab session.
We recommend that you book a lab-session well in advance, and start preparing at least a week
before the session.
Assignments 1 through 6 must be prepared completely before the start of your lab session.
Assignments 7 and 8 are performed during your lab session.
You can ask for help anytime before the lab session. There are several ways to get help; see the
course website for details and alternatives.
During the lab session, the teachers work with examination, but can also offer help. Make sure to
state clearly that you want to ask questions, rather than being examined.
Sometimes, our teachers may have to refer help-seekers to other sources of information, so that
other students can be examined.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 1
Resources
You will use the MARS simulator for Assignments 1 through 4. The simulator allows you ro run
your programs without access to MIPS hardware.
The MCB32 Tools will be used for Assignments 5 through 7. These tools allow you to run your
code on the Chipkit Uno32 board. Assignment 5 describes how to get started with the MCB32
Tools.
Get files analyze.asm, hexmain.asm, and timetemplate.asm from the course website. These files
contain code to get you started with your first Assignments.
Also get the compressed archive time4mips.zip from the course website.
Reading Guidelines
Review the following course material while you are preparing your solutions.
• Lectures 1, 2, and 3.
• The course web page named Reading Guidelines that lists specific chapters in the text
books. See information about module 1.
• The MIPS reference sheet that is available from the Literature page on the course web site.
General description of the files in the compressed archive time4mips.zip
• COPYING: Copyright information. Add lines with your name/names when you edit or add
one or more files.
• Makefile: Standard file telling the system how to compile MIPS programs. Don't change this
file.
• mipslab.h: Contains declarations of functions and variables. Included by C files. If you want
to change this file, you are probably doing something wrong.
• mipslabdata.c: Contains declarations of some arrays and matrices used for the display. Don't
change this file.
• mipslabfunc.c: Contains functions used for the labs. Don't change this file.
• mipslabmain.c: Contains start-up code, that initializes the display and shows the welcome
message. Following that, this file calls your lab-specific functions. Don't change this file.
• mipslabwork.c: This file gets your own code started, in the first lab. You may need to
change this file as part of a surprise assignment, and also for later labs and projects. Add
lines with your name and the date whenever you change this file.
• stubs.c: Contains interrupt-related C code. For later labs and projects, you may need to
change this file.
• vectors.S: Contains interrupt-related assembly code. For later labs and projects, you may
need to change this file.
Examination
Examination is grouped into parts. Each part is usually a group of several assignments. Examining
one part takes 5–15 minutes. Make sure to call the teacher immediately when you are done with an
assignment.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 2
Please state clearly that you want to be examined, rather than getting help.
The teacher checks that your program behaves correctly, and that your code follows all coding
requirements and is easily readable. In particular, all assembly-language code must conform to the
calling conventions for MIPS-32 systems.
The teacher will also ask questions, to check that your knowledge of the design and implementation
of your program is deep, detailed, and complete. When you write code, make detailed notes on your
choices of algorithms, data-structures, and implementation details. With these notes, you can
quickly refresh your memory during examination, if some particular detail has slipped your mind.
You must be able to answer all questions. In the unlikely case that some of your answers are
incorrect or incomplete, we follow a specific procedure for re-examination. This procedure is
posted on the course website.
Part 1 – Assignments 1 and 2.
Part 2 – Assignments 3 through 7.
Part 3 – Assignment 8, the surprise assignment.
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Assignments
When you run these assignments int the MARS simulator, make sure that pseudo-instructions are
turned on, and that delayed branching is turned off. There are check-boxes for these items in the
settings – see the screenshot:
The recommended settings (on or off) for all options are shown in the screenshot.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 4
Assignment 1: Analyzing assembly code
Get file analyze.asm from the course website. Start the MARS simulator. Load file analyze.asm.
This file contains a small program, which prints the ASCII characters from 0 through Z in the
"Run I/O" console window in Mars. After "Z", the program stops. See the screenshot:
Assemble the code by clicking on the “screwdriver and wrench” icon:
Single-step through the program, then run it at full speed. While single-stepping, please note how
the contents of registers $s0 and $a0 change during execution.
Now, change the program so that only every third character is printed. The program must still stop
after "Z". The screenshot below shows the expected output:
Questions for assignment 1
The following questions aim to help you check that you understand the code well. At the
examination, the teacher may choose to ask questions which are not on this list.
• Consider the large loop in the the original program – from the label loop to the branch
bne loop. How many iterations are executed in this loop?
• How many lines of code had to be changed? Why?
Assignment 2: Writing your first assembly-language function
Get file hexmain.asm from the course website. Start the MARS simulator. Load file hexmain.asm.
You will now write a small subroutine, that converts numbers in the range 0 through 15 into a
printable ASCII-coded character: '0' through '9', or 'A' through 'F', depending on the number.
For numbers not in the range 0 through 15, some bits will be ignored.
In file hexmain.asm, add an assembly-language subroutine with the following specification.
Name: The subroutine must be called hexasc.
Parameter: One, in register $a0. The 4 least significant bits specify a number, from 0 through 15.
All other bits in register $a0 can have any value and must be ignored.
Return value: The 7 least significant bits in register $v0 must be an ASCII code as described below.
All other bits must be zero when your function returns.
Required action: The function must convert input values 0 through 9 into the ASCII codes for digits
'0' through '9', respectively. Input values 10 through 15 must be converted to the ASCII codes for
letters 'A' through 'F', respectively. An ASCII code table is available at the last page in the
document.
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Important note for all subroutines: When a subroutine returns, registers $s0–$s7, $gp, $sp, $fp, and
$ra must have the same contents as when the subroutine was called. If a subroutine uses any of
these registers, the original contents must be saved, and restored before the subroutine returns. As a
special case, the contents of registers $k0 and $k1 may not be modified at all; these registers are
reserved for interrupt-handling code. All other register contents may be modified by any subroutine.
Run the file and explain the output. Change the constant on the line marked "input here" and re-run
the program. Run the program for all values from 0 through 15 and check that the digits 0 through 9
and the letters A through F appear correctly in the "Run I/O" console in MARS.
Questions for assignment 2
The following questions aim to help you check that you understand the code well. At the
examination, the teacher may choose to ask questions which are not on this list.
• Assume that your subroutine hexasc is called with the integer-value 5 as the argument in
register $a0. Go through each code-line just as the processor would do when executing the
program. For each code line, why is that code-line necessary?
• Repeat the previous question for argument values 14 and 17.
• The subroutine returns a 32-bit value in register $v0. Consider a case where the value
returned is 0000 0000 0000 0000 0000 0000 0100 0001. Try to calculate what the argument
value in $a0 must have been when the subroutine was called. Is there exactly one possible
value, or more than one possible value, or no possible value? If more than one value was
possible, state at least two such values.
Assignment 3: Printing the time
a) Get file timetemplate.asm from the course website. The file timetemplate.asm contains most
of the parts of a clock-program.
The variable mytime is initialized to 59:57, to be read as 59 minutes and 57 seconds. The subroutine
tick increments the contents of the mytime variable, once for each iteration of the loop (from
main: to b main).
The variable mytime stores time-info as four separate digits. Each digit uses four bits, allowing
values from 0 through 9. The values 10 through 15 are also technically possible, but are not used.
You will complete the clock-program, by writing the subroutines delay and time2string; both
subroutines are described below.
b) Copy your function hexasc from the previous assignment. Paste the copied code at the end of
file timetemplate.asm.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 6
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 1 0 0 1 0 1 0 1 0 1 1 1
not used
4 digits, NBCD-coded
minutes
(2 digits)
seconds
(2 digits)
c) Write a small function, like this, at the end of file timetemplate.asm.
delay:
jr $ra
nop
This is a temporary version of the delay function. You will soon write a more useful version.
d) You will now write a function that converts time-info into a string of printable characters, with a
null-byte as an end-of-string-marker.
At the end of file timetemplate.asm, add an assembly-language subroutine with the following
specification.
Name: The subroutine must be called time2string.
Parameters (two): Register $a0 contains the address of an area in memory, suitably large for the
output from time2string. The 16 least significant bits of register $a1 contains time-info, organized
as four NBCD-coded digits of 4 bits each. All other bits in register $a1 can have any value and
must be ignored. Example: register $a0 can contain the address 0x10010004, and register $a1 can
contain the value 0x00001653.
Return value: None.
Required action: The following sequence of six characters must be written to the area in memory
pointed to by register $a0.
1. Two ASCII-coded digits showing the number of minutes, according to the two more
significant NBCD-coded digits of the input parameter. Example: '1', '6' (ASCII 0x31, 0x36).
2. A colon character (ASCII :, code 0x3A).
3. Two ASCII-coded digits showing the number of seconds, according to the two less
significant NBCD-coded digits of the input parameter. Example: '5', '3' (ASCII 0x35, 0x33).
4. A null byte (ASCII NUL, code 0x00).
Note: You must use the function hexasc to convert each NBCD-coded digit into the corresponding
ASCII code. Test your program using the MARS simulator.
Hint: The macros PUSH and POP are useful for saving and restoring registers.
Pitfall: Even when calling your own subroutine hexasc, you must save and restore registers just as
if you didn't know anything about the internals of hexasc.
Important note for all subroutines (reprise): When a subroutine returns, registers $s0–$s7, $gp, $sp,
$fp, and $ra must have the same contents as when the subroutine was called. If a subroutine uses
any of these registers, the original contents must be saved, and restored before the subroutine
returns. As a special case, the contents of registers $k0 and $k1 may not be modified at all; these
registers are reserved for interrupt-handling code. All other register contents may be modified by
any subroutine.
e) When testing your function, insert a breakpoint at the instruction j main. The breakpoint will
cause MARS to pause your program at the breakpoint, just before jumping back for the next
iteration. To insert a breakpoint, check the box in the Bkpt column for the instruction j main. See
the screenshow below.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 7
Questions for assignment 3
The following questions aim to help you check that you understand the code well. At the
examination, the teacher may choose to ask questions which are not on this list.
• The time2string subroutine does not return any value in register $v0. So where is the
result?
• Did you use the macros PUSH and POP? Why, or why not?
• The time2string subroutine must call hexasc, and the jal instruction (for subroutine call)
will always change register $ra. Where can the original contents of $ra be saved? Is there
exactly one possible location, or more than one possible location, or no possible location? If
more than one location is possible, state at least two such locations.
• Consider the possibility that the argument value in register $a0 is not divisible by 4 – that is,
the address is not word-aligned. How is the execution of your subroutine time2string
affected in this case? Are there large effects, small effects, or no effects at all on the
execution of time2string? Why?
IS1200/IS1500 – Lab 1 – Assembly Programming Page 8
Assignment 4: Programming a simple delay
You will now write a simple delay function. This kind of code can be used when a computer should
wait a while between two different actions. However, using a program loop for delay is inefficient
in many ways. In lab 3, we will use a timer-device to write a much-improved delay function.
a) Rewrite the following C function as an assembly-language subroutine with the same behavior.
void delay( int ms ) /* Wait a number of milliseconds, specified by the parameter value. */
{
int i;
while( ms > 0 )
{
ms = ms – 1;
/* Executing the following for loop should take 1 ms */
for( i = 0; i < 4711; i = i + 1 ) /* The constant 4711 must be easy to change! */
{
/* Do nothing. */
}
}
}
Important note for all subroutines (reprise): When a subroutine returns, registers $s0–$s7, $gp, $sp,
$fp, and $ra must have the same contents as when the subroutine was called. If a subroutine uses
any of these registers, the original contents must be saved, and restored before the subroutine
returns. As a special case, the contents of registers $k0 and $k1 may not be modified at all; these
registers are reserved for interrupt-handling code. All other register contents may be modified by
any subroutine.
b) Replace the three-line delay subroutine from the previous Assignment, with your own
assembly-language subroutine delay in file timetemplate.asm.
c) Test your program using MARS. Adjust the constant in the for loop to get a delay of 1000 ms
when delay is called with a parameter value of 1000, and a delay of 3000 ms when delay is called
with a parameter value of 3000.
Questions for assignment 4
The following questions aim to help you check that you understand the code well. At the
examination, the teacher may choose to ask questions which are not on this list.
• Consider the case that the argument value in register $a0 is zero. Which instructions in your
subroutine are executed in this case? Why?
• Consider the case that the argument value in register $a0 is 2. Which instructions in your
subroutine are executed in this case? How many times each? Why?
• Now consider the case that the argument is a negative number: -1. Roughly, how many
milliseconds will the subroutine execute in this case?
IS1200/IS1500 – Lab 1 – Assembly Programming Page 9
Assignment 5: Delayed branching
You will now prepare your program for actual Mips hardware, which always has delayed
branching. Therefore, please change the MARS setting so that delayed branching is turned on.
The new recommended settings (on or off) for all options are shown in the screenshot.
Check that you have a nop instruction after every branch or jump. Run your program in Mars with
the new settings, and verify that it still works.
Assignment 6: Move to the MCB32 environment
For this assignment, you need to have the MCB32 toolchain installed. You must also have an
MCB32 Terminal open: "[mcb32]" should be shown as part of the promt-string in the Terminal
window. Please see the course webpage “Software for Labs” for more information.
a) Get the zip file time4mips.zip from the course website. Unpack (unzip) the contents.
b) Copy your assembly-code for hexasc, delay and time2string from the previous assignment to
the end of the file labwork.S (note the Capital S).
Please make sure that the directives .global, .data, and .text are placed correctly in the file.
Since MARS doesn't use the .global directive, code that worked in MARS may need to be
modified slightly when ported to the MCB32 environment.
c) In the Terminal window, at the "[mcb32]" prompt, change directory to the folder with the
unzipped contents of file time4mips.zip. Then type
make
You will see a few lines of messages. Look for warnings and errors, and correct them all.
Note: The assembly-language dialects are slightly different for MARS and MCB32. Macros, in
particular, must be rewritten (by you) when you move your code from one to the other.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 10
Questions for assignment 6
The following questions aim to help you check that you understand the code well. At the
examination, the teacher may choose to ask questions which are not on this list.
• What is the effect of the assembler directive .global? Why is the directive particularly
important in this assignment?
Assignment 7: At the lab session
For this assignment, you must have an MCB32 Terminal open: "[mcb32]" should be shown as part
of the promt-string in the Terminal window. Please see the course webpage “Software for Labs” for
more information. In the Terminal window, at the "[mcb32]" prompt, change directory to the folder
with your code.
a) Connect the Uno32 + Basic IO Shield combo to your computer with an USB cable. Wait until the
green LED (marked LD4) has stopped flashing, then type
make
followed by
make install
in order to compile and run your project on the board.
If this doesn't work right away, you need to figure out the name of the USB serial port.
• Linux: This is normally /dev/ttyUSB0
• Windows: In MSYS2, this is normally /dev/ttyS2
• Mac: This is normally /dev/cu.usbserial-* (replace * with something)
Ask the laboratory teachers for help if necessary.
When you know the path to your serial device, issue the command
make install TTYDEV=/dev/ttyUSB0, substituting the correct path of the device.
During the flashing process, the LEDs LD4 and LD5 on the Uno32 board should start blinking.
When the process in done, your code should be running on your Uno32!
Mac users may have to press the Reset button on the Uno32 board, and then directly run "make
install".
Note: The command
make
will only compile the code, and the command
make install
will only download into the board whatever is already compiled. Therefore, you need to use both
commands in order to compile and run an update of your code.
b) Update the constant in the for loop of the delay subroutine, so that the time is updated correctly.
An error of less than +/- 10% is acceptable – that's 6 seconds per minute. Test and run your project
on the board to adjust and demonstrate this.
c) Disconnect the USB connection from the board, count to three, and plug it back in. What
happens? Why?
IS1200/IS1500 – Lab 1 – Assembly Programming Page 11
Questions for assignment 7
The following questions aim to help you check that you understand the code well. At the
examination, the teacher may choose to ask questions which are not on this list.
• When you move your code from the simulator to the lab-board, why do you need to change
the value of the constant in the delay subroutine?
Assignment 8: Surprise assignment
You will get a surprise assignment at the lab session.
This assignment must be completed during the session.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 12
ASCII table – hexadecimal codes and corresponding ASCII characters
0x00..NUL
0x01..SOH
0x02..STX
0x03..ETX
0x04..EOT
0x05..ENQ
0x06. ACK
0x07..BEL
0x08.....BS
0x09....HT
0x0A....LF
0x0B....VT
0x0C.....FF
0x0D...CR
0x0E....SO
0x0F......SI
0x10..DLE
0x11..DC1
0x12..DC2
0x13..DC3
0x14..DC4
0x15. NAK
0x16..SYN
0x17..ETB
0x18. CAN
0x19....EM
0x1a..SUB
0x1b..ESC
0x1c.....FS
0x1d.....GS
0x1e.....RS
0x1f.....US
0x20.....SP
0x21........!
0x22........"
0x23........#
0x24........$
0x25......%
0x26.......&
0x27.........'
0x28........(
0x29........)
0x2a........*
0x2b.......+
0x2c.........,
0x2d........-
0x2e........ .
0x2F......../
0x30........0
0x31........1
0x32........2
0x33........3
0x34........4
0x35........5
0x36........6
0x37........7
0x38........8
0x39........9
0x3a.........:
0x3b.........;
0x3c........<
0x3d.......=
0x3e........>
0x3f.........?
0x40......@
0x41.......A
0x42.......B
0x43.......C
0x44.......D
0x45.......E
0x46.......F
0x47.......G
0x48.......H
0x49........I
0x4a........J
0x4b.......K
0x4c.......L
0x4d......M
0x4e.......N
0x4f.......O
0x50.......P
0x51.......Q
0x52.......R
0x53.......S
0x54.......T
0x55.......U
0x56.......V
0x57......W
0x58.......X
0x59.......Y
0x5a.......Z
0x5b........[
0x5c.........\
0x5d........]
0x5e........^
0x5f........_
0x60........`
0x61........a
0x62........b
0x63........c
0x64........d
0x65........e
0x66........f
0x67........g
0x68........h
0x69.........i
0x6a.........j
0x6b........k
0x6c.........l
0x6d.......m
0x6e........n
0x6f........o
0x70........p
0x71........q
0x72........r
0x73........s
0x74.........t
0x75........u
0x76........v
0x77.......w
0x78........x
0x79........y
0x7a........z
0x7b........{
0x7c.........|
0x7d........}
0x7e........~
0x7f...DEL
NUL = null character, used as filler.
BEL = bell, makes your computer beep when printed.
BS = backspace, moves the cursor left one step.
LF = line feed, moves the cursor down one line (often without moving to the leftmost column).
FF = form feed, starts a new page on a hard-copy printout (and sometimes also on screen).
CR = carriage return, moves the cursor to the beginning of the current line.
ESC = escape, starts a sequence of control characters.
DEL = delete, sometimes used to delete one character.
SP = space between words.
SOH, STX, ETX, EOT, ENQ, ACK, HT, VT, SO, SI, DLE, DC1, DC2, DC3, DC4, NAK, SYN,
ETB, CAN, EM, SUB, FS, GS, RS, US are control characters; we don't use them in this lab.
IS1200/IS1500 – Lab 1 – Assembly Programming Page 13
