CIT 593 – Module 11 Assignment Instructions  
CIT 593 – Module 11 Assignment  
Making the LC4 Assembler Instructions  
Contents  
Assignment Overview 3  
Learning Objectives 3  
Advice 3  
Getting Started 4  
Codio Setup 4  
Starter Code 4  
Object File Format Refresher 4  
Requirements 5  
General Requirements 5  
Assembler 5  
assembler.c: main 5  
asm_parser.c: read_asm_file 6  
asm_parser.c: parse_instruction 6  
asm_parser.c: parse_add 6  
asm_parser.c: parse_xxx 6  
asm_parser.c: str_to_bin 7  
asm_parser.c: write_obj_file 7  
Extra Credit 8  
Suggested Approach 8  
High Level Overview 8  
Great High Level Overview, but I really need a Slightly More Detailed Overview 10  
Part 0: Setup the main Function to Read the Arguments 10  
Part 1: Read the .asm File 10  
Part 2: Parse an Instruction 1  
Part 3: Parse an ADD Instruction 1  
Part 4: Converting the binary string to an hexadecimal formatted integer 1  
Part 5: Writing the .obj object file 1  
Testing 1  
Validate Output with PennSim 1  
Files for Testing 1  
Unit Testing 1  
GDB for Debugging 1  
Submission 1  
Submission Checks 1  
The Actual Submission 1  
Page 1 of 24CIT 593 – Module 11 Assignment Instructions  
Grading 1  
Assembler 1  
Extra Credit 1  
An Important Note of Plagiarism 1  
FAQ 1  
Quick Hints 1  
Formatting 1  
Endianness 1  
Resources 1  
Page 2 of 24CIT 593 – Module 11 Assignment Instructions  
Assignment Overview  
From lecture you’ve learned that C is file-oriented and that working with files represents I/O  
devices in C.  
C files fall into two categories: "text" and "binary". In this assignment you’ll work with both types  
by reading in a text file and writing out a binary file.  
You will read an arbitrary .asm file (a text file intended to be read by PennSim) and write a .obj  
file (the same type of binary file that PennSim would write out).  
Aside from reading and writing out the files, your task will be to make a mini-LC4- Assembler!  
An assembler is a program that reads in assembly language and generates its machine  
equivalent.  
This assignment will require a bit more programming rigor than we’ve had thus far, but now that  
you’ve gained a good amount of programming skill in this class and in others, it is the perfect  
time to tackle a large programming assignment (which is why the instructions are so many  
pages).  
Learning Objectives  
This assignment will cover the following topics:  
● Review the LC4 Object File Format  
● Read text files and process binary files  
● Assemble LC4 programs into executable object files  
● Use debugging tools such as GDB  
Advice  
● Start early  
● Ask for help early  
● Do not try to do it all in one day  
Page 3 of 24CIT 593 – Module 11 Assignment Instructions  
Getting Started  
Codio Setup  
Open the Codio assignment via Canvas. This is necessary to link the two systems.  
You will see many directories and files. At the top-level workspace directory, the mail files are  
asm_parser.h, asm_parser.c, assembler.c, and PennSim.jar.  
Do not modify any of the directories or any file in any of the directories.  
Starter Code  
We have provided a basic framework and several function definitions that you must implement.  
assembler.c - must contain your main function.  
asm_parser.c - must contain your asm_parser functions.  
asm_parser.h - must contain the definition for ROWS and COLS  
- must contain function declarations for read_asm_file,  
parse_instruction, parse_reg, parse_add, parse_mul,  
str_to_bin, write_obj_file, and any helper function you  
implement in asm_parser.c  
test1.asm - example assembly file  
PennSim.jar - a copy of PennSim to check your assembler  
Object File Format Refresher  
The following is the format for the binary .obj files created by PennSim from your .asm files. It  
represents the contents of memory (both program and data) for your assembled LC-4 Assembly  
programs. In a .obj file, there are 3 basic sections indicated by 3 header “types” = Code , Data,  
and Symbol:  
● Code: 3-word header (xCADE, <address>, <n>), n-word body comprising the instructions.  
○ This corresponds to the .CODE directive in assembly.  
● Data: 3-word header (xDADA, <address>, <n>), n-word body comprising the initial data  
values.  
○ This corresponds to the .DATA directive in assembly.  
● Symbol: 3-word header (xC3B7, <address>, <n>), n-character body comprising the  
symbol string. These are generated when you create labels (such as “END”) in  
assembly. Each symbol is its own section.  
○ Each character in the file is 1 byte, not 2 bytes.  
○ There is no NULL terminator.  
Page 4 of 24CIT 593 – Module 11 Assignment Instructions  
Requirements  
General Requirements  
● You MUST NOT change the filenames of any file provided to you in the starter code.  
● You MUST NOT change the function declarations of any function provided to you in the  
starter code.  
● You MAY create additional helper functions. If you do, you MUST correctly declare the  
functions in the appropriate header file and provide an implementation in the appropriate  
source file.  
● Your program MUST compile when running the command make.  
● You MUST NOT have any compile-time errors or warnings.  
● You MUST remove or comment out all debugging print statements before submitting.  
● You MUST NOT use externs or global variables.  
● You MAY use string.h, stdlib.h, and stdio.h.  
● You SHOULD comment your code since this is a programming best practice.  
● Your program MUST be able to handle .asm files that PennSim would successfully  
assemble. We will not be testing with invalid .asm files.  
● Your program MUST NOT crash/segmentation fault.  
● You MUST provide a makefile with the following targets:  
○ assembler  
○ asm_parser.o  
○ all, clean, clobber  
Assembler  
assembler.c: main  
● You MUST not change the first four instructions already provided.  
● The main function:  
○ MUST read the arguments provided to the program.  
■ the user will use your program like this:  
./assembler test1.asm  
○ MUST store the first argument into filename.  
○ MUST print an error1 message if the user has not provided an input filename.  
○ MUST call read_asm_file to populate program[][].  
○ MUST parse each instruction in program[][] and store the binary string equivalent  
into program_bin_str[][].  
○ MUST convert each binary string into an integer (which MUST have the correct  
value when formatted with "0x%X") and store the value into program_bin[].  
○ MUST write out the program into a .obj object file which MUST be loadable by  
PennSim's ld command.  
Page 5 of 24CIT 593 – Module 11 Assignment Instructions  
asm_parser.c: read_asm_file  
This function reads the user file.  
● It SHOULD return an error2 message if there is any error opening or reading the file.  
● It MAY try to check if the input program is too large for the defined variables, but we will  
not be testing outside the provided limits.  
● It MUST read the exact contents of the file into memory, and it MUST remove any  
newline characters present in the file.  
● It MUST work for files that have an empty line at the end and also for files that end on an  
instruction (i.e. do not assume there will always be an empty line at the end of the file).  
● It MUST return 0 on success, and it MUST return a non-zero number in the case of  
failure (it SHOULD print a useful error message and return 2 on failure).  
asm_parser.c: parse_instruction  
This function parses a single instruction and determines the binary string equivalent.  
● It SHOULD use strtok to tokenize the instruction, using spaces and commas as the  
delimiters.  
● It MUST determine the instruction function and call the appropriate parse_xxx helper  
function.  
● It MUST parse ADD, MUL, SUB, DIV, AND, OR, XOR instructions.  
○ It MUST parse ADD IMM and AND IMM if attempting that extra credit.  
● It MUST return 0 on success, and it MUST return a non-zero number in the case of  
failure (it SHOULD print a useful error message and return 3 on failure).  
asm_parser.c: parse_add  
This function parses an ADD instruction and provides the binary string equivalent.  
● It MUST correctly update the opcode, sub-opcode, and register fields following the LC4  
ISA.  
● It SHOULD call a helper function parse_reg, but we will not be testing this function.  
● It MUST return 0 on success, and it MUST return a non-zero number in the case of  
failure (it SHOULD print a useful error message and return 4 on failure).  
asm_parser.c: parse_xxx  
You MUST create a helper function similar to parse_add for the other instruction functions  
required in parse_instruction.  
● They MUST correctly update the opcode, sub-opcode, and register fields following the  
LC4 ISA.  
● They SHOULD call a helper function parse_reg, but we will not be testing this function.  
● They MUST return 0 on success, and they MUST return a non-zero number in the case  
of failure (it SHOULD print a useful error message and return a unique error number on  
failure).  
Page 6 of 24CIT 593 – Module 11 Assignment Instructions  
asm_parser.c: str_to_bin  
This function converts a C string containing 1s and 0s into an unsigned short integer  
● It MUST correctly convert the binary string to an unsigned short int which can be verified  
using the "0x%X" format.  
● It SHOULD use strtol to do the conversion.  
asm_parser.c: write_obj_file  
This function writes the program, in integer format, as a LC4 object file using the LC4 binary  
format.  
● It MUST output the program in the LC4 binary format described in lecture and in the  
Object File Format Refresher section.  
● It MUST create and write an empty file if the input file is empty  
● It MUST change the extension of the input file to .obj.  
● It MUST use the default starting address 0x0000 unless you are attempting the .ADDR  
extra credit.  
● It MUST close the file with fclose.  
● It MUST return 0 on success, and they MUST return a non-zero number in the case of  
failure (it SHOULD print a useful error message and return 7 on failure).  
● The generated file MUST load into PennSim (and you MUST check this before  
submitting), and the contents MUST match the .asm assembly program  
 Page 7 of 24CIT 593 – Module 11 Assignment Instructions  
Extra Credit  
You may attempt any, all, or none of these extra credit options. You MUST test using your own  
generated examples (we will not provide any).  
Option 1: modify your read_asm_file function to ignore comments in .asm files. You MUST  
handle all types of comments for credit.  
Option 2: modify your program to handle ADD IMM and AND IMM instructions. Both MUST work  
completely for credit.  
Option 3: modify your program to handle the .CODE and .ADDR directives.  
Option 4: modify your program to handle the .DATA, .ADDR, and .FILL directives.  
Suggested Approach  
This is a suggested approach. You are not required to follow this approach as long as you  
follow all of the other requirements.  
High Level Overview  
Follow these high-level steps and debug thoroughly before moving on to the next.  
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