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Computer Systems & Programming Machine Problem 1

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ECE220: Computer Systems & Programming
Machine Problem 1
Printing Subroutines
Your task this week is to write two
subroutines to support printing of a student’s
daily schedule. One subroutine prints an
hour, such as “0600” or “1200”, preceded by
three spaces and followed by two spaces.
The second subroutine prints an arbitrary
string centered in nine spaces, truncating the
string or adding extra space characters as
necessary. Together, the two subroutines
require about 100 lines of LC-3 assembly,
including comments. In the next two MPs,
you will make use of these subroutines.
The objective for this week is to give you some experience with decomposing tasks into algorithms, with
writing LC-3 assembly code, and with formatting output.
The Task
The first subroutine is PRINT_SLOT. A number from 0 to 14 is passed to your subroutine in R1. The
number corresponds to one of the rows of a schedule, as shown in the figure above. Your subroutine must
print the time corresponding to the specified slot. If R1=0, for example, your subroutine must print “0600”
preceded by three spaces (ASCII x20) and followed by two trailing spaces.
The second subroutine is PRINT_CENTERED. A string (the address of the first ASCII character in
sequence terminated by an ASCII NUL, x00) is passed to your subroutine in R1. Your subroutine must
print exactly nine characters. If the string is longer than nine characters, your subroutine must print the first
nine characters. If the string is shorter than nine characters, your subroutine must print additional spaces
around the string to bring the total length to nine characters. If the number of spaces needed is odd, the
subroutine must use one more leading space than trailing space.
To make your life easier in the later MPs, neither of these subroutines may change any register values. In
other words, except for R7, which is modified by JSR, both subroutines must preserve all bits in all other
registers. Note that you are probably going to want to use TRAP instructions in both subroutines, so you
should also preserve the return address (in R7) and restore it before executing the RET (JMP R7) instruction.
You may find it useful to develop look-up tables for one or both of these subroutines. A look-up table
specifies a function from a small, contiguous set of integers to an arbitrary result type. For example, in a
later MP, you might use a look-up table to translate an integer from 0 to 4 into one of the day names at the
top of the schedule in the figure (“MONDAY”, “TUESDAY”, and so forth). If each element in a look-up
table requires a single memory address, one can add the index to the base address for the table to find the
address at which the desired element is stored.
You may want to use a look-up table to translate string length into a count of leading spaces, for example.
You may also find it tempting to use a look-up table of strings for slots. While such an approach is
acceptable, we encourage you to think about other ways to generate the hour output.
As you develop your subroutines, you may want to include code to test them at the start of the assembly
file. Since the LC-3 will begin execution at the start of the file by default, you can then easily test your
subroutines with the LC-3 tools.
Specifics
 Your code must be written in LC-3 assembly language and must be contained in a single file called
mp1.asm. We will not grade files with any other name.
 Your subroutine for printing one of the schedule slot hours must be called PRINT_SLOT.
o You may assume that R1 (the slot number) holds a value from 0 to 14 when your subroutine
is called.
o Your subroutine must not change any register values other than R7.
o Your subroutine must output exactly nine characters to the display, including three leading
spaces, an appropriate hour number (R1+6 as a two-digit number, possibly including a
leading zero), two zeroes, and two trailing spaces.
 Your subroutine for printing a string centered in nine characters must be called
PRINT_CENTERED.
o You may assume that the string starting at the address in R1 is valid and is terminated by
an ASCII NUL character (x00, extended to x0000 in LC-3 memory).
o You may NOT make any assumptions about string length. In particular, string length can
be any non-negative integer, including 0.
o Your subroutine must not change any register values other than R7.
o Your subroutine must output exactly nine characters to the display. For strings longer than
nine characters, the first nine characters should be displayed. For strings shorter than nine
characters, leading and trailing spaces must be added to output nine total characters. For
short, even-length strings, use one more leading space than trailing space.
 Your code must not execute data—any bits generated using .FILL, .STRINGZ, .BLKW, or other
data-generating pseudo-ops or directives.
 Except for loads (NOT stores) from the string passed to PRINT_CENTERED, neither subroutine
may access LC-3 memory outside of that allocated by your mp1.asm program. If you need space,
use .FILL or .BLKW to create it, and use labels to address it.
 You may not assume anything about how many times either subroutine is called, nor about the
sequence of calls to your subroutines, nor about the input values other than as specified here.
 Your code must be well-commented, and must include a table describing how registers are used
within each subroutine. Follow the style of examples provided to you in class and in the textbook.
 You may leave any code that you have used for testing at the start of your file, provided that it does
not in any way interfere with your subroutines’ functionality.

Testing
Testing is your responsibility. We have provided a file called test1.asm that performs a basic test on
your code. Instructions on how to use it are included at the start of the file, and you may want to generalize
the approach to suit your own purposes in testing your code.
We suggest that you write a loop to output all possible values of the slot number for PRINT_SLOT. There
are only 15 of them.
We also suggest that you test PRINT_CENTERED with strings of various lengths.
Grading Rubric
Functionality (65%)
 PRINT_SLOT
o 15% – produces correct output for any slot number from 0 to 14 (in R1)
o 10% – does not modify any register value other than R7 when called, and does not access
memory outside of mp1.asm
 PRINT_CENTERED
o 30% – produces correct output for any string length (string in R1)
o 10% – does not modify any register value other than R7 when called, only reads the string
provided, and otherwise does not access memory outside of mp1.asm
Style (20%)
 10% – PRINT_SLOT not organized as nested conditionals (a look-up table is ok; 16 chunks of
code selected by conditionals are not)
 10% – PRINT_CENTERED not organized as nested conditionals (use a look-up table or a right
shift)
Comments, Clarity, and Write-up (15%)
 5% – each subroutine has a paragraph explaining what it does and how it must be called (these are
given to you; you just need to document your work)
 10% – code is clear and well-commented
Note that some categories in the rubric may depend on other categories and/or criteria. For example, if
your code does not assemble, you will receive no functionality points. Note also that the remaining LC-3
MPs (two of them) will build on these subroutines, so you may have difficulty testing those MPs if your
code does not work properly for this MP. You will also be penalized heavily if your code executes data.
Finally, your subroutines must be written so that they can be called any number of times in any order with
any legal input values, and we will deduct points if such is not the case.

Computer Systems & Programming Machine Problem 1
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