1. Linux Memory Management
Initialization

2. Some concept Address translation -logical address
User space vs Kernel space
- linear address
0~4GB (x86) ie.Alpha 2^64 byte
- physical address
ie: logical linear physical
User view
Physical
situation

3. - local descriptor table LDT for user space every task has one LDT
Logical Address
- local descriptor table LDT
for user space
every task has one LDT
ie: struct desc_struct* ldt ;
global descriptor table GDT
for kernel space
- sgment descriptor
entry in LDT/GDT
- selector
decide a segment descriptor in LDT/GDT

4. GDTR LDTR selector offset
Seg descriptor ,,
GDT[0]=null
GDT[1]=kernel code
seg descriptor
GDT[1]=kernel data ,,
base
limit
logical
Seg descriptor
LDTR
LDT[0]=null
LDT[1]=user code
seg descriptor
LDT{2]=user data
selector
CS /DS
offset
memory
EIP

5. Logicasl address to Linear adderss - user space
segment descriptor=(0 ,0xc )
Task_size 變數 = 0xc
linear address=0x 0xc
- kernel space :
segment descriptor=(0xc ,4GB)
linear address=3GB4GB

6. Task Logical address Linear address 0GB User space 3GB Kernel Space
Segment
hardware
3GB
Kernel
Space
4GB

7. - paging hardware mechanisinm - x86 is two layer
Linear to Physical
- paging hardware mechanisinm
- x86 is two layer
DIR
TABLE
OFFSET
10bit
10bit
12bit

8. Physcial memory page index Linear address page index table directory
Page hardware
page
index
Linear address
TABLE
page
DIR
index
table
directory
index
Dir base

9. - in th memory initial stats
Some important points
- swapper_pg_dir
- in th memory initial stats
- the first 768 entry in swapper_pg_dir =0
above 768 , mapping 3GB~4GB to physcial
- all Task’page directory has the “same” value
as swapper_pg_dir from entry 768 to why??
768*4MB= ~=3GB
let all task’s kernel space(3GB~4GB)
can map to the same physical memory

10. - Task from user sapce to kernel space 1. Prcoess ‘s seg_base= 0x00
Example
- Task from user sapce to kernel
space
1. Prcoess ‘s seg_base= 0x00
page_dir private to process
2. make system call
3.seg_bsae=0xc
page_dir has the same above 768 value as
swapper_pg_dir
4. Noe we run kernel space using the common page
tables
ie: run the same physical space

11. 1.when task/process runs in user
Concluds
1.when task/process runs in user
sapce ,it can run in arbitrary Physical
space according to it’s page_dir(0~768entry)
2. but runs in kernel space,
process just can run the common
physical space (the place we put kernel)

12. Memory initialization
arch/i386/kernel/HEAD.S
int/main.c
start_kernel() { .
paging_init(); call free_area_init()
mem_init(); }

13. Major work
paging_init()
- Initial the page tables of
swapper_pg_dir (from 768 entry)
free_area_init()
- loacate swap_cache space
initial free_list

14. - mem_init()
contruct the mem_map in the
memory to reflect the physcal
memory usage
start_memory , end_memory
paremeter

15. Map of physcial memory before any user process are executed
Kernel can’t look page[0]
address
terms
Relevant routine or variable or explains
Map of physcial memory before any user process are executed