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ECE391: Computer Systems Engineering Spring 2021
Machine Problem 1 Due: in Gitlab repository by 9:59 AM CST Thursday 18 February
(11:59PM in China on Thursday 18 February)
Text-Mode Fish Animation
Demo on Monday 22 February, 6 PM CST: UIUC campus and last name starts with A-J
Demo on Tuesday 23 February, 8 AM in China: ALL ZJUI campus students
Demo on Tuesday 23 February, 6 PM CST: UIUC campus and last name starts with K-Z
Note: Must commit to the master branch in your provided GitLab repository by assignment deadline, NOT YOUR
DEMO TIME. Demo queue will open up 15 minutes before start time on demo days. Demos will last until there are
no more students on the queue.
In recognition of lunar New Year, we shifted the deadline back (just past the all-campus UIUC holiday on 17 February).
Please manage your time, however, as PS2 will begin in the same week and be due on the same day as the MP1 demos.
We strongly suggest that you do not put off working on MP1 until the last minute.
In this machine problem, you will modify the Linux real-time clock (RTC) driver to toggle characters on the text-mode
video console from one ASCII character to another, with a user-settable toggle rate. This will serve a dual purpose:
first, it will be an exercise in writing x86 assembly, allowing you to gain experience with the x86 ISA. Second, it will
provide an introduction into how drivers accomplish tasks inside the Linux kernel.
Please read the entire document before you begin.
A Note On This Handout: The sections entitled “Linux Device Driver Overview,” “RTC Overview,” “Ioctl Functions,”
and “Tasklets” contain background Linux knowledge which is not critical for you to complete this MP. The
material described in these background sections will be covered in lecture in the next few weeks, but it may be helpful
to read these sections to familiarize yourself with the context of your code in this MP.
MP1 Assignment
You will add four new ioctls to the existing RTC driver, as well as a tasklet that will update the text-mode video
screen on every RTC interrupt.
Your code will reside in mp1.S, a GNU-style assembly file. Assembly files with a capital-S extension (.S) are preprocessed
using the standard C preprocessor before being assembled, so things like #include and #define are OK to
use. Your code must be implemented using GNU x86 assembly.
Note: Do not use # to start a comment at start of line.
MP1 Data Structure
The main structure you will be working with is mp1 blink struct.
struct mp1_blink_struct {
unsigned short location; /* Linear offset on text-mode buffer */
char on_char; /* Char to put during "on" period */
char off_char; /* Char to put during "off" period */
unsigned short on_length; /* Length of on period
* in number of RTC interrupts */
unsigned short off_length; /* Length of off period */
unsigned short countdown; /* Number of RTC interrupts left in period */
unsigned short status; /* Status word (on=1/off=0) */
struct mp1_blink_struct *next; /* pointer to next item in linked list */
}
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This structure definition is usable only in C programs. There are constants defined for you at the top of the provided
mp1.S that give you easy access to the fields in this struct from your assembly code. See the comments in mp1.S for
further information on how to use them.
To implement characters “blinking” on the text-mode video console, a linked list will be created by your modified
RTC driver that will allow any location on the text-mode video console to be toggling characters from off char to
on char and back, with toggle rates determined by on length and off length. A pointer to the first element in the
linked list (the head of the list) is defined in the mp1.S file as a global variable, mp1 list head. mp1 list head is
initialized to NULL (the value it holds is zero) to indicate that there are currently no blinking locations on the screen.
The tail element of the list will have its next field equal to NULL to indicate that it is the last element. A diagram
of this singly-linked list layout for a three-item list is shown on the following page. Example memory addresses of
structures and variables are shown in parentheses.
mp1_list_head (0x804a1b0)
(0x804b008) (0x804b020) (0x804b038)
next = 0x804b020 next = 0x804b038 next = 0x0
0x804b008
MP1 Tasklet
The first function you need to write is called mp1 rtc tasklet. The tasklet must update the state of the game. Its
C prototype is:
void mp1 rtc tasklet (unsigned long);
Every time an RTC interrupt is generated, mp1 rtc tasklet will be called. Your tasklet will walk down the
mp1 list head list, examining each mp1 blink struct structure. The function first decrements the countdown
field of the structure. If the countdown field has reached zero after the decrement, the tasklet will examine the
status field. If this field is equal to 1, that location currently has the on char character; if this field is 0, that location
currently has the off char character. The tasklet should put the opposite character (i.e. interchange the status
between on/off) out to video memory with a call to mp1 poke. For information on how to draw to the screen, see the
“Text-Mode Video” section. Finally, the tasklet updates the countdown field by copying the value from the opposite
length field to countdown. For example, if the character was currently off and you just turned it on, copy on length
to countdown. In this way, the toggle rate for each character is controlled by the length fields. The tasklet then must
move on to the next list element. The function returns when it reaches the end of the list.
MP1 Ioctls
The next function you must write is called mp1 ioctl. Its C prototype is:
int mp1_ioctl (unsigned long arg, unsigned long cmd);
This function serves as a “dispatcher” function. It uses the cmd argument to determine which of the next four functions
to jump to. The table below gives a brief summary of cmd values, the corresponding core function, and a brief description
of what that core function does. Each of the core functions are described in the section entitled “Core Functions.”
Note that you must check this cmd value; if it is an invalid command, return -1.
cmd value Core function Description
0 mp1 ioctl add add a blinking location
1 mp1 ioctl remove remove a blinking location
2 mp1 ioctl find get information about a location
3 mp1 ioctl sync synchronize a new blink location with an existing one
other - Any value other than 0-3 is an error. Return -1.
The method used to jump to one of the core functions is to use assembly linkage without modifying the stack. A
picture of the stack at the beginning of mp1 ioctl is shown below.
3
return address
arg
command number
(previous stack)
ESP
Each of the core functions takes arg directly as its parameter. Since this parameter is passed to the mp1 ioctl function
as its first parameter, mp1 ioctl can simply jump directly to the starting point of one of the core functions without
modifying the stack. The arg parameter will already be the first parameter on the stack, ready to be used by the core
function. In this way, it will appear to the core functions as if they were called directly from the RTC driver using the
standard C calling convention without the use of this assembly linkage. Your mp1 ioctl must use a jump table—see
the section “Jump Tables” below.
Core Functions
You must implement each of the following four functions in assembly in the mp1.S file.
Note: A common task across these four ioctls is searching a linked list for a specific element that matches a particular
location. You must implement a separate function that performs a linked list search, and call this function from the
mp1 ioctl remove, mp1 ioctl find, and mp1 ioctl sync core functions. Designing the interface to this function
(in other words, what parameter(s) is/are passed to it, what value(s) is/are returned from it, and so forth) is up to you.
int mp1 ioctl add(unsigned long arg)
The add ioctl takes as its argument a user-level pointer to a mp1 blink struct structure. First, dynamically
allocate memory using the mp1 malloc function to store a copy of the structure. Copy the entire mp1 blink struct
from the user-space structure to the newly-allocated memory (use mp1 copy from user). Then set the countdown
field to be equal to the on length field, and set the status field to 1. Then insert this structure at the head of the
linked list using the mp1 list head pointer. Finally, make a call to mp1 poke with the correct register parameters
to immediately display the character on the text-mode video screen. This effectively turns the location “on.” After
countdown RTC interrupts have elapsed, your mp1 rtc tasklet will turn the location “off.” This function should
return 0 if a successful add was performed.
Your function must handle errors. If there is a memory allocation error (in which case mp1 malloc returns NULL),
return -1. Remember the semantics of mp1 copy from user. If it could not copy all the bytes requested, it will return
the number of bytes it was not able to copy. If this function returns anything other than 0, the copy has failed, and the
function should return -1. If the location is outside the valid range of 0 to 80*25-1, this function should return -1.
Finally, your error handling must prevent memory leaks. If you have allocated any memory using mp1 malloc, and
you find that there is an error condition, you must free the memory using mp1 free.
int mp1 ioctl remove(unsigned long arg)
The remove ioctl takes an integer location as its parameter. Traverse the mp1 list head list, looking for an element
whose location field matches the argument given to this function. If there is such an element, remove it from the
linked list and free its memory with a call to mp1 free, and return 0. If there is no element whose location matches,
return -1.
int mp1 ioctl find(unsigned long arg)
The find ioctl takes a pointer to a mp1 blink struct, like add. The only parameter it is concerned with as an input
is the location parameter, but you must validate that the pointer refers to a valid structure before reading from the structure.
After extracting the location parameter from the user-level structure, search the mp1 list head list for an element
that matches the location. Then copy the entire element, which is a
mp1 blink struct, to the user-level structure pointed to by the parameter that was passed in (use mp1 copy to user).
In this way it uses the parameter as both an input and an output. If there is no matching location in the list, return -1,
otherwise return 0. Similar error conditions apply to this function as in the previous two.
int mp1 ioctl sync(unsigned long arg)
The sync ioctl’s unsigned long argument is really two two-byte unsigned short integers, packed into one
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four-byte argument. The first integer is stored in the upper 16 bits of arg, and the second integer is stored in the lower
16 bits. You must extract these two integers from the single argument.
The sync ioctl synchronizes two existing locations on the screen. The first integer represents the location of the first
blinking character, and the second integer represents the location of the second blinking character that will become
synchronized with the first. Search the mp1 list head list, looking for elements with locations that match the two
integers, respectively. Then copy the timing information (the on length, off length, countdown, and status
fields) from the first element to the second element. After copying these fields, call mp1 poke to immediately update
the display using the correct character (that is, either the on char or the off char, depending on status) to the
screen for the second location. This function should return 0 on success, and -1 on failure. Similar failure cases apply.
Synchronization Constraints
The code (both user-level and kernel) for MP1 allows the tasklet to execute in the middle of any of the ioctls, so you
must be careful to order the updates properly in some of the operations. Since the tasklet does not modify the list, the
main constraint is that any ioctl that modifies the list does so in a way that never leaves the list in an unusable state.
In particular, mp1 ioctl add must fill in the newly allocated structure, including the next field, before changing the
head of the list to point to the new structure. Similarly, mp1 ioctl remove must remove the element from the list
before freeing it; copying the structure’s next pointer into a register is not sufficient, since the tasklet could try to read
the structure after the call to mp1 free. Updates in the other calls can not lead to major problems.
Suggested Order of Writing Functions
Below is our suggested order of writing functions. If you write the functions in this order, you can test them as you go
along and the expected output should be what is described. This way, you can have some confidence in the portions of
code you write instead of writing everything all at once and then testing it only to find out something does not work
and you have to look through every single line of every single function to track the bug. Feel free to deviate from this
list if it is more convenient. And of course, do write your own test cases that individually test the functions you have
written (formally called unit testing).
1. IOCTL dispatcher - There won’t be any output when this is finished. This is just to set up the calls to the core
functions.
2. ADD - An ASCII picture of a fish should appear if this is working correctly
3. Tasklet - The ASCII fish should blink between the two frames
4. FIND/SYNC - An I/M should appear after a few seconds and the “I/M” blinks should sync up with the rest of
the fish background after some time.
5. REMOVE - After the “I/M” have synced with the rest of the fish background, the “M” will be removed and so
the blinking will stop and the “I” will be left over
5
Getting Started
Be sure that your development environment is set up from MP0. In particular, have the base Linux kernel compiled
and running on your test machine. Begin MP1 by following these steps:
• We have created a Git repository for you to use for this project. The repository is available at
https://gitlab.engr.illinois.edu/ece391 sp19/mp1
and can be accessed from anywhere.
• Access to your Git repositories will be provisioned shortly after the MP is released. Watch your @illinois.edu
email for an invitation from Gitlab.
• To use Git on a lab computer, you’ll have to use Git Bash on Windows, not the VM. You are free to download
other Git tools as you wish, but this documentation assumes you are using Git Bash. To launch Git Bash,
click the Start button in Windows, type in git bash, then click on the search result that says Git Bash.
• Run the following commands to make sure the line endings are set to LF (Unix style):
git config --global core.autocrlf input
git config --global core.eol lf
• Switch the path in git-bash into your Z: drive by running the command: cd /z
• If you do NOT have a ssh-key configured, clone your git repo in Z: drive by running the command (it will
prompt you for your NETID and AD password):
git clone https://gitlab.engr.illinois.edu/ece391 sp19/mp1 .git mp1
If you do have a ssh-key configured, clone your git repo in Z: drive by running the command:
git clone git@gitlab.engr.illinois.edu:ece391 sp19/mp1 .git mp1
In your devel machine:
• Change directory to your MP1 working directory (cd /workdir/mp1). In that directory, you should find a file
called mp1.diff. Copy the file to your Linux kernel directory with
cp mp1.diff /workdir/source/linux-2.6.22.5
• Now change directory to the Linux kernel directory (cd /workdir/source/linux-2.6.22.5). Apply the
mp1.diff patch using
cat mp1.diff | patch -p1
The last argument contains a digit 1, not the lowercase letter L. This command prints the contents of mp1.diff
to stdout, then pipes stdout to the patch program, which applies the patch to the Linux source. You should
see that the patch modified three files, drivers/char/Makefile, drivers/char/rtc.c, and
include/linux/rtc.h. Do NOT try to re-apply the patch, even if it did not work. If it did not work, revert
all 3 files to their original state using SVN (svn revert ). After that, you may try to apply
the patch again.
• Change directory back to /workdir/mp1. You are now ready to begin working on MP1.
• Do not commit the Linux source or the kernel build directory. The number of files makes checking out
your code take a long time. If during handin, we find the whole kernel source, any object files or the build
directory in your repository, you will lose points. We have added a .gitignore file to your initial repository.
This file contains all the Git ignore rules that tells Git to not commit the specified file types. The Linux source
and kernel build directory are one such example of files that are ignored. Try and explore the .gitignore file to
see what other file types are ignored.
Be sure to use your repository as you work on this MP. You can use it to copy your code from your development machine
to the test machine, but it’s also a good idea to commit occasionally so that you protect yourself from accidental
loss. Preventable losses due to unfortunate events, including disk loss, will not be met with sympathy.
6
Testing
Due to the critical nature of writing kernel code, it is better to test and debug as much as possible outside the kernel.
For example, let’s say that a new piece of code has a bug in it where it fails to check the validity of a pointer passed in
to it before using it. Now, say a NULL pointer is passed in and the code attempts to dereference this NULL pointer.
When running in user space, Linux catches this attempt to dereference an invalid memory location and sends a signal,1
SEGV, to the program. The program then terminates harmlessly with a “Segmentation fault” error. However, if this
same code were run inside the kernel, the kernel would crash, and the only recourse would be to restart the machine.
In addition, debugging kernel code requires the setup you developed in MP0—two machines, connected via a virtual
TCP connection, with one running the test kernel and the other running a debugger. In user space, all that’s necessary
is a debugger. The development cycle (write-compile-test-debug) in user space is much faster.
For these reasons, we have developed a user-level test harness for you to test your implementation of the additional
ioctls and tasklet. This test harness compiles and runs your code as a user-level program, allowing for a much faster
development cycle, as well as protecting your test machine from crashing. Using the user-level test harness, you can
iron out most of the bugs in your code from user space before integrating them into the kernel’s RTC driver. The
functionality is nearly identical to the functionality available if your code were running inside the kernel.
The current harness tests some of the functionality for all the ioctls, but it is not an exhaustive test. It is up to you to
ensure that all the functionality works as specified, as your code will be graded with a complete set of tests.
Note: For this assignment, a test harness is provided to you that can test some of the functionality of your code prior
to integration with the actual Linux kernel. Future assignments will place progressively more responsibility on you,
the student, for developing test methods. What this means is that a complete test harness will not be provided for every
MP, and it will be up to you to design and implement effective testing methods for your code. We encourage you to
look over how the user-level test harness works for this MP, as its design may be of use to you in future MPs. This
test harness is fully functional, and uses some advanced programming techniques to achieve a complete simulation
of how your code will execute inside the Linux kernel. You need not understand all of these techniques; however,
understanding the important ideas is useful. Questions on Piazza as to how this test harness works are welcome as
well.
Running the user-level test program: To run the user-level test program, follow these steps:
• Type cd /workdir/mp1 to change to your MP1 working directory.
• Type make to compile your code and the test harness.
• Type su -c ./utest to execute the user-level test program as root (you will need to type root’s password).
You can also type su -c "gdb utest" to run gdb on the user-level test harness to debug your code. Debugging
the kernel code will be difficult. Use the disas (disassemble) command on mp1 rtc tasklet or mp1 ioctl to see
the start of your code (feel free to add more globally visible symbols), then use explicit addresses to see the rest of
it. Be sure to start any disassembly with the starting byte of an instruction rather than an address in the middle of
an instruction. With non-function symbols (such as those in your assembly code), and with addresses, you need an
asterisk when identifying a breakpoint. For example, break *mp1 ioctl or break *0x12345678.
The test code changes the display location to the start of video memory. If you do not see a prompt after the code
finishes (correctly or otherwise), pressing the Enter key will usually return the display to normal. Note also that
gdb will return the display to its usual location, after which you will not be able to see any of the animation (while
debugging).
Note: When running the user test under gdb, the debugger stops your program whenever a signal (such as SIGUSR1
or SIGALRM) occurs. To turn off this behavior and make it easier to debug your program, type the following commands
in gdb:
handle SIGUSR1 noprint
handle SIGALRM noprint
1Think of a signal as a user-level (unprivileged) interrupt for now.
7
Testing your code in the kernel: Once you are confident that your code is working, you need to build it in the kernel.
• If you logged in as root to test, log out and back in again as user. If you have not already done so, commit your
changes to the MP1 sources.
• Type cp /workdir/mp1/mp1.S /workdir/source/linux-2.6.22.5/drivers/char to copy your
mp1.S file to your kernel source directory.
• Type cd ∼/build to change to the Linux build directory.
• Type make to build the kernel with your changes. If you have applied the mp1.diff file as described in the
“Getting Started” section of this handout, the kernel will build and link properly.
• Follow the procedure described in MP0, “Preparing Your Environment,” to install your new kernel onto the test
virtual machine and run it. We suggest that you execute the test kernel under gdb when debugging.
• In the test machine, navigate to your mp1 directory using the command cd /workdir/mp1, then type make
clean and make.
• Type su -c ./ktest to execute the kernel test program as root (you will need to type root’s password).
Both test programs should produce the exact same behavior.
Moving Data to/from the Kernel
Virtual memory allows each user-level program to have the illusion of its own memory address space, separate from
any other user-level program and also separate from the kernel. This affords each program a level of protection, such
that user-level programs cannot write to memory owned by other programs, or worse, owned by the kernel. Therefore,
when passing memory addresses between a user-level program and the kernel (such as in an ioctl system
call) a translation is needed so that the kernel can correctly reference the user-level memory address being passed
to it to get at the data. This translation is performed by the mp1 copy to user and mp1 copy from user functions,
which are wrappers around the real Linux kernel functions copy to user and copy from user defined in
asm-i386/uaccess.h.
The declarations for these two functions are:
unsigned long mp1 copy to user (void *to, const void *from, unsigned long n);
unsigned long mp1 copy from user (void *to, const void *from, unsigned long n);
The semantics of mp1 copy to user and mp1 copy from user are similar to those of memcpy, for those of you
familiar with it. These functions take two pointers to memory areas, or buffers, to and from, and a length n. Each
function copies n bytes from the from buffer to the to buffer. As can be inferred from their names, mp1 copy to user
copies data from a kernel buffer to a user-level buffer, and mp1 copy from user copies data from a user-level buffer
to the kernel. All user- to kernel- address translations are taken care of by these functions. Each of these functions
returns the number of bytes that could not be copied, which should be 0. Bad user-level pointers can cause return
values greater than zero. For example, if you pass a NULL pointer in as the user-level parameter to one of these
functions (such as the to parameter in mp1 copy to user), it checks the pointer and memory area, sees that it points
to an invalid buffer, and returns n, since it could not copy any data.
You’ll need these functions in any of the core functions which take pointers to user-level structures. Each ioctl takes
an “arg” parameter, so you will need to look at the documentation for each ioctl to figure out which ones are actually
pointers to user-level structures.
One final important note: When copying data to a buffer in the kernel, you should not use statically-allocated global
buffers. In multiprocessor systems, for example, multiple calls to your ioctl functions may be going on at the same
time. Using a statically-allocated storage area, like a global variable, is a bad idea because the separate calls to the
ioctl would be contending for using this same storage area. You should use either local variables on the stack or
dynamically-allocated memory. Refer to the Course Notes for information on allocating local variables on the stack.
The section below has information on dynamic memory allocation in the Linux kernel.
8
Allocating and Freeing Memory
User-level C programs make use of the malloc() and free() C library functions to allocate memory needed for
storing dynamic structures such as linked list elements. Linux kernel code uses a number of different memory allocation
functions that you will learn later in the semester. Since your code must run in the kernel, you must use the
memory allocation services provided there. To abstract the details away (for now), the MP1 distribution contains two
memory allocation functions that behave similarly to malloc() and free(). Their prototypes are:
void* mp1 malloc(unsigned long size);
void mp1 free(void* ptr);
mp1 malloc takes a parameter specifying the number of bytes of memory to allocate. It returns a void*, called a
“void pointer,” which is the memory address of the newly-allocated memory.
mp1 free takes a pointer to a block of memory that was allocated with mp1 malloc() and releases that memory back
to the system. It does not return anything.
Text-Mode Video
Each character on the text display comprises two bytes in memory. The low byte contains the ASCII value for the
character to be display. The high byte is an attribute byte, which holds information about the color of that particular
character on the screen.
The screen is divided into rows and columns, with the upper-left character position referred to as row 0, column 0.
Each row is 80 characters wide, and there are 25 rows. The screen is stored linearly in video memory, with each
successive row stored directly after the one above it. For example, row 1, column 0 immediately follows row 0,
column 79 in memory, row 2, column 0 immediately follows row 1, column 79, and so forth. Thus, to write a pixel at
row 12, column 15 on the screen, you first need to calculate the row offset: row 12 × 80 characters per row × 2 bytes
per character = 1920. Then, add the column offset: column 15 × 2 bytes per character = 30. So, row 12 column 15 on
the screen is 1920 + 30 = 1950 bytes from the start of video memory.
mp1 poke: Due to Linux’s virtualization of the screen buffer and of video memory, the start of video memory is
not a constant, so writing to video memory is somewhat more complicated than using a mov instruction. To simplify
things for this MP, a function has been defined called mp1 poke. This function, defined in assembly in mp1.S, takes
care of finding the starting address of video memory and writing a single byte to an offset from that starting address.
mp1 poke does not make use of the C calling convention discussed in the Course Notes. Instead, to use mp1 poke,
make a function call with the following parameters:
%eax offset from the start of video memory
%cl ASCII code of character to write
mp1 poke then finds the correct starting address in memory and writes the character in CL to the location specified by
EAX.
Note: For mp1 poke, EDX is a caller-saved register (in other words, mp1 poke clobbers EDX). If you need to preserve
the value of EDX across a call to mp1 poke, you must save its value on the stack. This preservation can be accomplished
by pushing the register’s value onto the stack with pushl %edx before making the call, and then popping the
value back into EDX with popl %edx. All other registers are callee-saved (that is, mp1 poke preserves their values).
9
Jump Tables
You must use a jump table to perform the “dispatching” operation in mp1 ioctl. A jump table is a table in memory
containing the addresses of functions (called function pointers). Each function pointer is a 32-bit memory address,
just like any other pointer. The memory addresses that you want to put in the jump table are the labels of the start of
each function. Let’s say you have three functions in an assembly (.S) file, with labels function0, function1, and
function2 as the names of each. You can define a jump table as follows:
function0:
# function 0 body
function1:
# function 1 body
function2:
# function 2 body
jump_table:
.long function0, function1, function2
The jump table provides an easy way to access those three functions. If you view the jump table as a C-style array of
pointers:
void* jump_table[3];
jump table[0] (in other words, the memory location at jump table + 0 bytes) holds the address of function0,
jump table[1] (at jump table + 4 bytes) holds the address of function1, and jump table[2] (at jump table
+ 8 bytes) holds the address of function2. In these examples, the number inside the brackets is the “index” into the
jump table.
In this MP, the cmd parameter should serve as the index into the jump table, and you should be able to easily jump to
each of the five core functions by creating a table similar to that shown above.
Linux Device Driver Overview
The first important concept in Linux device drivers is the fact that Linux makes all devices look like regular disk files.
If you list the files in the /dev directory (using ls), you can see some devices that may be present on the machine.
Each device is associated with one of the files. For example, the first serial port is associated with the device file
/dev/ttyS0. For this MP, you will be dealing with the /dev/rtc device file, which is the device file associated with
the real-time clock.
Since everything looks like a file, Linux drivers must support a certain set of standard file operations on their associated
device files. These operations should seem familiar, as they are the operations available for normal disk files:
open, close, read, write, lseek (to move to arbitrary locations within the file), and poll (to determine if data is
available for reading or writing). In ad