Project 2 Roadmap
Segmentation Review

Intel docs, fig. 3-1 and 3-5
Spawn()
Starts a user program



in src/geekos/user.c
triggered by SYS_SPAWN
Read_Fully()


Parse_ELF_Executable()



fill Exe_Format data structure; (Project 1)
ignore the bogus program headers (size=0)
Load_User_Program()


read an executable file from disk into memory
will set up the memory image for the new process and create a
User_Context with the loaded program
Start_User_Thread()

with the new User_Context
Load_User_Program()
Prepares a new address space for the program and creates a
User_Context


in src/geekos/userseg.c
allocate the memory image of the new process


find highest virtual address: scan Exe_Format
find args size:



size =





Get_Argument_Block_Size()
this is not sizeof(Argument_Block) !
Round_Up_To_Page(highest virtual address) +
DEFAULT_USER_STACK_SIZE +
args size
create the User_Context…
Fill in remaining User_Context fields
Create_User_Context()
Allocates space for a User_Context,fill in its fields. User_Context
stores all the information needed to setup and run a user thread






in src/geekos/userseg.c
create an LDT for the process
add a descriptor to the GDT that describes the location of the LDT
create a selector that contains the location of the LDT descriptor
within the GDT
create descriptors for the code and data segments of the user
program and add these descriptors to the LDT
create selectors that contain the locations of the two descriptors
within the LDT
User_Context
Create_User_Context()
struct Segment_Descriptor
ldt[0]
5
struct Segment_Descriptor
*ldtDescriptor
ushort_t ldtSelector
Selector
2
Selector
struct Segment_Descriptor
5
ldt[1]
GDT
4
1
3
LDT descriptor
ushort_t csSelector
4
6
ushort_t dsSelector
6
int stackAddr
int programSize
char * program
1
Create an User_Context structure; it will store all the information that will
be needed to setup and run the user thread
Malloc()
2
Allocate an LDT descriptor in the GDT for the process
Allocate_Segment_Descriptor()
3
Initialize the LDT descriptor in GDT with the location of the LDT
Init_LDT_Descriptor()
4
Create a selector that contains the location of the LDT descriptor within
the GDT
Selector()
5
Initialize the descriptors in LDT as code/data descriptors
Init_Code_Segment_Descriptor()/
Init_Data_Segment_Descriptor
6
Create selectors that contain the locations of the two descriptors within the
LDT
Selector()
6
Selector
Start_User_Thread
Create a thread, setup the stack to look as if it
was interrupted, make it runnable.

in src/geekos/kthread.c
Create_Thread()
 call Setup_User_Thread() which will push
values (specified in description)



Look at Setup_Kernel_Thread()for similar code
Make_Runnable_Atomic()
Command Line Arguments
In Load_User_Program(), create an Argument_Block
data structure and attach it to the process image. This
data structure contains the argc and argv arguments
that are passed to the main() function of the user
program


Get #of bytes and argc from
Get_Argument_Block_Size()
Call Format_Argument_Block() to


build Argument_Block data structure in user memory
fix argv up (make all user space pointers)
System Calls

INT90
put args in registers on user side
 recover them on kernel side
 call Sys_xxx accordingly
 return result/error code


Use g_CurrentThread to get info about
current thread
Sys_Null
Just add a print statement in syscall.c
 Your first test for user mode:


spawn /c/null.exe
Sys_Exit
simple call to Exit()
 but funs called from Exit() must clean
up the thread

Free_Segment_Descriptor
 Free userContext, memory space allocated
for the program

Sys_PrintString
Copy_From_User() the string
 Put_Buf()

Sys_GetKey

Wait_For_Key()

the shell takes care of the rest

see libc/conio.c
Sys_SetAttr/Sys_GetCursor/Sys_PutCursor
 straightforward
calls to name-similar
kernel funs

Sys_GetCursor() requires a
Copy_To_User() !
Sys_Spawn

Calls Spawn()

Command needs a Copy_From_User() !
Sys_Wait
Wait for specified PID
 Use Join()

Sys_GetPID
 just
return current thread’s PID
Copy_From_User/Copy_To_User

Caller allocates space (dstInKernel)

Validate memory pointer
is within bounds?
 convert

If yes copy
 Make generic, not specific to strings
 Ensure proper ‘\0’ termination for
SYS_PRINTSTRING and SYS_SPAWN

User_Context
struct Segment_Descriptor
ldt[0]
GDT
struct Segment_Descriptor
ldt[1]
struct Segment_Descriptor
*ldtDescriptor
ldtSelector
LDT descriptor
. . .
ushort_t ldtSelector
ushort_t csSelector
ushort_t dsSelector
. . .