CIT 593 | Assignment: Dynamic Memory & File I/O | 1
Setting up Codio for this HW:
1) Open the Codio assignment via Coursera
2) From the Codio File-Tree click on: lc4_memory.h and lc4_memory.c
Overview:
The goal of this HW is for you to write a program that can open and read in a .OBJ file created
by PennSim, parse it, and load it into a linked list that will represent the LC4’s program and data
memories (similar to what PennSim’s “loader” does). In the last HW, you created a .OBJ file. In
this HW, you will be able to read in a .OBJ file and convert it back to the assembly it came from!
This is known as reverse assembling (sometimes a disassembler).
RECALL: OBJECT FILE FORMAT
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, SYMBOL.
● Code: 3-word header (xCADE,
, ), n-word body comprising the instructions.
This corresponds to the .CODE directive in assembly.
● Data: 3-word header (xDADA,
, ), n-word body comprising the initial data
values. This corresponds to the .DATA directive in assembly.
● Symbol: 3-word header (xC3B7,
, ), n-character body comprising the
symbol string. Note, each character in the file is 1 byte, not 2. There is no null
terminator. Each symbol is its own section. These are generated when you create labels
(such as “END”) in assembly.
LINKED LIST NODE STRUCTURE:
In the file: lc4_memory.h, you’ll see the following structure defined:
The structure is meant to model a row of the LC4’s memory: a 16-bit address, & its 16-bit
contents. As you know, an address may also have a label associated with it. You will also recall
that PennSim always shows the contents of memory in its “assembly” form. So PennSim
reverse-assembles the contents and displays the assembly instruction itself (instead of the
binary contents).
CIT 593 | Assignment: Dynamic Memory & File I/O | 2
As part of this assignment, you will read in a .OBJ file and store each instruction in a NODE of
the type above. Since they’ll be an unknown # of instructions in the file, you’ll create a linked
list of the nodes above to hold all the instructions that are in the .OBJ file.
The details of how to implement all of this will be discussed in the sections of this document
that follow.
FLOW CHART: Overview of Program Operation
CIT 593 | Assignment: Dynamic Memory & File I/O | 3
IMPLEMENTATION DETAILS:
The first files to view in the helper file are lc4_memory.h and lc4_memory.c. In these files you
will notice the structure that represents a row_of_memory as referenced above (see the
section: LINKED_LIST_NODE_STRUCTURE above for the node’s layout). You will also see
several helper functions that will serve to manage a linked list of “rows_of_memory” nodes.
Your job will be to implement these simple linked list helper functions using your knowledge
from the last HW assignment.
Next, you will modify the file called: lc4.c It serves as the “main” for the entire program. The
head of the linked list must be stored in main(), you will see in the provided lc4.c file a pointer
named: memory will do just that. Main() will then extract the name of the .OBJ file the user has
passed in when they ran your program from the argv[] parameter passed in from the user.
Upon parsing that, it will call lc4_loader.c’s open_file() and hold a pointer to the open file. It
will then ask call lc4_loader.c’s parse_file() to interpret the .OBJ file the user wishes to have
your program process. Lastly it will reverse assemble the file, print the linked list, and finally
delete it when the program ends. These functions are described in greater detail below. The
order of the function calls and their purpose is shown in commends in the lc4.c file that you will
implement as part of this assignment.
Once you have properly implemented lc4.c and have it accept input from the command line, a
user should be able to run your program as follows:
./lc4 my_file.obj
Where “my_file.obj” can be replaced with any file name the user desires as long as it is a valid
.OBJ file that was created by PennSim. If no file is passed in, your program should generate an
error telling the user what went wrong, like this:
error1: usage: ./lc4
CIT 593 | Assignment: Dynamic Memory & File I/O | 4
Problem 1) Implementing the LC4 Loader
Most of the work of your program will take place in the file: called: lc4_loader.c. In this file, you
will start by implementing the function: open_file() to take in the name of the file the user of
your program has specified on the command line (see lc4_loader.h for the definition of
open_file()). If the file exists, the function should return a handle to that open file, otherwise a
NULL should be returned.
Also in lc4_loader.c, you will implement a second function: parse_file() that will read in and
parse the contents of the open .OBJ file as well as populate the linked_list as it reads the .OBJ
file. The format of the .OBJ input file has been in lecture, but its layout has been reprinted
above (see section: INPUT_FILE_FORMAT). As shown in the flowchart above, have the function
read in the 3-word header from the file. You’ll notice that all of the LC4 .OBJ file headers
consist of 3 fields: header type,
, . As you read in the first header in the file, store
the address field and the field into local variables. Then determine the type of header you
have read in: CODE/DATA/SYMBOL.
If you have read in a CODE header in the .OBJ file, from the file format for a .OBJ file, you’ll
recall the body of the CODE section is -words long. As an example, see the hex listed below,
this is a sample CODE section, notice the field we should correlate with n=0x000C, or decimal:
12. This indicates that the next 12-words in the .OBJ file are in fact 12 LC-4 instructions. Recall
each instruction in LC4 is 1 word long.
CA DE 00 00 00 0C 90 00 D1 40 92 00 94 0A 25 00 0C 0C 66 00 48
01 72 00 10 21 14 BF 0F F8
From the example above, we see that the first LC-4 instruction in the 12-word body is: 9000.
(that happens to be a CONST assembly instruction if you convert to binary). Allocate memory
for a new node in your linked list to correspond to the first instruction (the section above:
LINKED LIST NODE STRUCTURE, declares a structure that will serve as a blue-print for all your
linked list nodes called: “row_of_memory”). As it is the first instruction in the body, and the
address has been listed as 0000, you would populate the row_of_memory structure as follows.
address 0000
label NULL
contents 9000
assembly NULL
next NULL
CIT 593 | Assignment: Dynamic Memory & File I/O | 5
In a loop, read in the remaining instructions from the .OBJ file; allocate memory for a
corresponding row_of_memory node for each instruction. As you create each
row_of_memory add these nodes to your linked list (you should use the functions you’ve
created in lc4_memory.c to help you with this). For the first 3 instructions listed in the sample
above, your linked list would look like this:
The procedure for reading in the DATA sections would be identical to reading in the CODE
sections. These would become part of the same linked list, as we remember PROGRAM and
DATA are all in one “memory” on the LC-4, they just have different addresses.
For the following SYMBOL header/body:
C3 B7 00 00 00 04 49 4E 49 54
The address field is: 0x0000. The symbol field itself is: 0x0004 bytes long. The next 4 bytes: 49
4E 49 54 are ASCII for: INIT. This means that the label for address: 0000 is INIT. Your
program must search the linked list: memory, find the appropriate address that this label is
referring to and populate the “label” field for the node. Note: the field: tells us exactly how
much memory to malloc() to hold the string, however you must add a byte to hold the NULL. 5
bytes in the case of: INIT. For the example above, the node: 0000 in your linked list, would be
updated as follows:
address 0000
label INIT
contents 9000
assembly NULL
next
Once you have read the entire file; created and added the corresponding nodes to your linked
list, close the file and return to main(). If you encounter an error in closing the file, before
exiting, print an error, but also free() all the memory associated with the linked list prior to
exiting the program.
CIT 593 | Assignment: Dynamic Memory & File I/O | 6
Problem 2) Implementing the Reverse Assembler
In a new file: lc4_disassembler.c: write a third function (reverse_assemble) that will take as
input the populated “memory” linked list (that parse_file() populated) – it will now contain the
.OBJ’s contents. reverse_assemble() must translate the hex representation of some of the
instructions in the LC4 memory’s linked list into their assembly equivalent. You will need to
reference the LC4’s ISA to author this function. To simplify this problem a little, you DO NOT
need to translate every single HEX instruction into its assembly equivalent. Only translate
instructions with the OPCODE: 0001 (ADD REG, MUL, SUB, DIV, ADD IMM)
As shown in the flowchart, this function will call your linked list’s “search_by_opcode()” helper
function. Your search_by_opcode() function should take as input an OPCODE and return the
first node in the linked list that matches the OPCODE passed in, but also has a NULL assembly
field. When/if a node in your linked list is returned, you’ll need to examine the “contents” field
of the node and translate the instruction into its assembly equivalent. Once you have
translated the contents filed into its ASCII Assembly equivalent, allocate memory for and store
this as string in the “assembly’ field of the node. Repeat this process until all the nodes in the
linked list with an OPCODE=0001 have their assembly fields properly translated.
As an example, the figure below shows a node on your list that has been “found” and returned
when the search_by_opcode() function was called. From the contents field, we can see that
the HEX code: 128B is 0001 001 010 001 011 in binary. From the ISA, we realize the sub-opcode
reveals that this is actually a MULTIPLY instruction. We can then generate the string MUL R1,
R2, R3 and store it back in the node in the assembly field. For this work, I strongly encourage
you to investigate the switch() statement in C (any good book on C will help you understand
how this works and why it is more practical than multiple if/else/else/else statements). I also
remind you that you must allocate memory strings before calling strcpy()!
CIT 593 | Assignment: Dynamic Memory & File I/O | 7
Problem 3) Putting it all together
As you may have realized main() should do only 3 things: 1) create and hold the pointer to your
memory linked list. 2) Call the parsing function in lc4_loader.c. 3) Call the disassembling
function in lc4_dissassembler.c. One last thing to do in main() is to call a function to print the
contents of your linked list to the screen. Call the print_list() function In lc4_memory.c; you will
need to implement the printing helper function to display the contents of your lc4’s memory
list like this: