buglog.txt

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Checkpoint1:

PIC:
1.  Problem:    Slave Interrupt handler cannot be invoked.
    Reason:     The IRQ2 never get enabled.  
    Time used:  1h
    Solution:   Enable IRQ2 in i8259_init() to connect the master PIC and the slave PIC.

Keyboard:
2.  Problem:    Two characters are printed when one key pressed.
    Reason:     One key pressed two interrupts generated with two different scancodes.
    Time used:  2h
    Solution:   Check if the scancode received from PIC is within the number of scannodes(127)
                Discard the interrupt when the number is equal to or greater.
                
3.  Problem:    Only the first key pressed works, the remaining ones do not.
    Reason:     The interrupt handler never returns to the previous instruction address(EIP).
    Time used:  4h
    Solution:   Used a wrapper function written in assembly languages, to save all registers,
                to call the actual handler function written in C by the keyboard driver, and
                to use the iret instruction resumes to the previous state that were saved by
                the CPU before invoking the interruption.
Paging:
4.  Problem:    The machine crashed in page_init()
    Reason:     Used video memory as a array index directly
    Time used:  1h
    Solution:   Only use the most significant 8 bits of VIDEO as the memory index

5.  Problem:    page_init() will get the machine stuck
    Reason:     The video memory is not shifted when it is filled into the base address
    Time used:  1h
    Solution:   change "VIDEO" to "(VIDEO >> PTE_OFFSET)"
    
IDT:
6.  Problem:    First time loading handler into the IDT table, we could not find the handler.
    Reason:     1) I forgot that handler function could not take parameter
                2) I forgot to take the address of the handler.
                3) Also take time to form and wrap the function, and differentiate them into trap/interrupt/syscall
    Time usedL  2h
    Solution    1) Write indicivual function inorder to eliminate the parameter.
                2) Take address of the handler


RTC:
7.  Problem:    After call test_interrupts, the video memory space is static! Instead of 
                changing characters.
    Reason:     frequency not set correctly and therefore the characters cannot move.
    Time used:  a day
    Solution:   Adjust local function of set_RTC_freq to reject invalid frequencies
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Checkpoint2:

Terminal:
1.  Problem:    Terminal_read never break out of loop.
    Reason:     It only checks if the buffer has \n, not \r.
    Time used:  1h
    Solution:   Add \r to the if statement.


File System:
2.  Problem:    Page fault when initializing the file system driver.
    Reason:     Access a vitural address that should not be accessed.
    Time used:  30 min 
    Solution:   (1) When incrmenting the address of the starting location of 
                    the file system, I should check the type of the current 
                    pointer. If the type is inode_t *, I should only add 1 to
                    it so it will be added to the next 4kb address.
                (2) Initializing the file system driver before paging. 
                    (paging will refuse me to access the memory)

3.  Problem:    Directory Always show the same filename.
    Reason:     Did not update file position after reading.
    Time used:  30 min
    Solution:   Update f_pos after reading.

4.  Problem:    Read 0 bytes everytime calling file_read
    Reason:     inode is not the same as dirs index.
    Time used:  1 hour 
    Solution:   Remove the code that check if the current type is not regular file.
                Because in my implmentation, only reading regular files can enter this
                function.

5.  Problem:    Divide by zero exception occurs when reading regular files.
    Reason:     Divide by zero when calcuate The index of the data we want to read.
    Time used:  10 min
    Solution:   Divide by BLOCK_SIZE instead of the index of block because the index 
                could be zero.

6.  Problem:    system boot failed.
    Reason:     did not open the terminal before using it.
    Time used:  1 hour
    Solution:   Since we don't have the standard syscall now, I must init terminal by myself.
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Checkpoint3:

System calls Linkage
1.  Problem:    Page fault calling fputs 
    Reason:     (1) Access sys_open instead of sys_write
                (2) Forget to init file descriptor array for each process
    Time used:  1 hour
    Solution:   change the jump table offset or add a reset sys_reset at the beginning of the jump table.
                init processes' file descriptor array

2.  Problem:    syscall handler cannot return back to user mode (ring 3)
    Reason:     does not iret properly 
    Time used:  10 min 
    Solution:   pop and iret

3.  Problem:    Cannot read from sys_read
    Reason:     use unimplment function copy from user 
    Time used:  20 min
    Solution:   Discard the call to copy from user since we have direct mapping this time 

Process 
4.  Problem:    Shell stopped after typing a error command
    Reason:     Forget to kill the error process and resume shell process 
    Time used:  1 hour
    Solution:   when a error occur, context switch to shell


5.  Problem:    Process switch failed
    Reason:     Fail to switch back to parent when halt
    Time used:  2 days
    Solution:   Store parent esp and ebp before context switch and restore after child halts.

6.  Problem:    invalid command page fault
    Reason:     Did not remap the user space to the parent
    Time used:  1 day
    Solution:   Remap and reset tss esp0 and ss0

Terminal 
7.  Problem:    Backspace will remove the printed characters
    Reason:     Did not check if the buffer size is empty 
    Time used:  1 hour
    Solution:   Only perform backspace when terminal buffer is empty

8.  Problem:    CTRL-L will clear all of the screen
    Reason:     Did not redraw 391OS >
    Time used:  5 min
    Solution:   Redraw after clear when press CTRL-L
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Checkpoint4:

Process
1.  Problem:    Mutiple Shell cannot exit
    Reason:     My computer interprets new line character as \r
    Time used:  30 min
    Solution:   change \r to \n when terminal_read to adjust shell program

2.  Problem:    Init shell problem exit page fault
    Reason:     Did not restart the init shell properly
    Time used:  10 min
    Solution:   Free the init shell process and restart the init process

3.  Problem:    command line arguments parsing error 
    Reason:     more than one leading spaces should be ignored
    Time used:  30 min 
    Solution:   Use a while loop to skip leading zeros
    
4.  Problem:    store command arguments error
    Reason:     local char pointers will be lost after the function returns and stack frame cleared.
    Time used:  20 min 
    Solution:   Store the command arguments locally in array and later store it in the PCB

5.  Problem:    syserr test failed 
    Reason:     terminal and rtc: read/write/close wrong
    Time used:  2 hour
    Solution:   (1) terminal should never be able to close
                (2) terminal should only allow fd 0 to read
                (3) terminal should only allow fd 1 to write 
                (4) rtc should open a file to set its f_op

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Checkpoint5:

Scheduler
1.  Problem:    Page Fault when executing new process
    Reason:     Linked List linking error
    Time used:  1 h
    Solution:   avoid add node into run queue more than once and clear it only once

2.  Problem:    return abnormally from execute
    Reason:     must cli when performing list add/remove
    Time used:  2 h
    Solution:   add cli/sti to protect scheduling and sleep/wakeup

3.  Problem:    interactive process (shell) delayed response
    Reason:     (1). chosen times lice too small
                (2). init process occupy CPU
                (3). unused shells should go to sleep
    Time used:  2 h
    Solution:   (1). choose a more reasonable time slice number to avoid too much 
                context switches.
                (2). at this point, init does nothing but spinning and yield the 
                CPU, delete it for now greatly increase the response time
                (3). after a shell executing a new process, it should go to sleep,
                blocked by removing from the running queue. it wakes up as soon as
                its child returns or the another terminal switch to its one.


Terminal Switching
1.  Problem:    
    Reason:     
    Time used:  30 min
    Solution:   

1.  Problem:    
    Reason:     
    Time used:  30 min
    Solution:   


Video memory
1.  Problem:    Fish exited when terminal switching
    Reason:     
    Time used:  1 h
    Solution: