1. Topic 2e High-Level languages and Systems Software
(Memory Layout)
Introduction to Computer Systems Engineering
(CPEG 323)
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2. Reading List Slides: Topic2e Operating System and Compiler Books
Other papers as assigned in class or homeworks
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3. Several Topics Object File Format GP register and GP area
Run-time Stack
Virtual / Physical memory
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4. Object File Format
Compiler or assembler translates the program into an object file, which is consequently linked into a executable file. These "object" files and "executable" files have a specific format.
Several common formats are:
a.out: assembler and linker output format
COFF: Common Object File Format
ECOFF: Extended Common Object File Format
ELF: Executable and Linking Format
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5. Object File Format(Cont.)
a.out: assembler and linker output format
A fairly primitive format, lacking some key features to enable easy shared libraries, etc.
On UNIX boxes, a.out is the default output format of the system assembler and the linker. The linker makes a.out executable files. A file in a.out format consists of: a header, the program text, program data, text and data relocation information, a symbol table, and a string table (in that order).
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6. Object File Format(Cont.)
Common Object File Format (COFF) binary files
COFF is a portable format for binary applications on UNIX System V
Extended Common Object File Format (ECOFF) binary files
Under Windows, Visual C, C++ and every Windows compiler generates ECOFF files.
MIPS
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7. Object File Format(Cont.)
ELF: Executable and Linking Format
ELF and COFF formats are very similar but ELF has greater power and flexibility
Become the standard in file format
ELF representation is platform independent
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8. Object File Format(Cont.)
Three main types of ELF files
executable file
supplies information necessary for the operating system to create a process image.
relocatable file
describes how it should be linked with other object files to create an executable file or shared library.
shared object file
contains information needed in both static and dynamic linking.
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9. ELF Object File Format ELF Format Linking and Execution Views
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10. ELF Object File Format(Cont.)
The ELF Header
ELF Header is always the first section of the file(The other sections can be in any order)
What does the ELF Header describe?
● the type of the object file
● target architecture
● The location of the Program Header table, Section Header table, and String table
● number and size of entries for each table the ELF
● the location of the first executable instruction
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11. ELF Object File Format(Cont.)
The Program Header Table
only important in executable and shared object files
It is an array of entries
each entry is a structure describing a segment in the object file
The OS copies the segment into memory according to the location and size information
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12. ELF Object File Format(Cont.)
The Section Header Table
Has pointers to all sections in object files
It is similar to the program header
Each entry correlates to a section in the file.
Each entry provides the name, type, memory image starting address, file offset, the section’s size, alignment, and how the information in the section should be interpreted.
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13. ELF Object File Format(Cont.)
The ELF Sections
Hold code, data, dynamic linking information, debugging data, symbol tables, relocation information, comments, string tables, and notes.
Sections are treated in different ways
● loaded into the process image
● or provide information needed in the building of a process image
● or are used only in linking object files
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14. ELF Object File Format(Cont.)
The ELF Segments
Group related sections
● text segment groups executable code,
● data segment groups the program data,
● dynamic segment groups information relevant to dynamic loading.
Each segment consists of one or more sections.
A process image is created by loading and interpreting segments.
The OS logically copies a file’s segment to a virtual memory segment according to the information provided in the program header table.
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15. GP Register and GP Area Stack Heap BSS positive offset
0xEFFFFFFF
Stack
Heap
BSS
positive
offset
Global data area
64k GP 0x
negative
offset
Data
Text 0x
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16. GP Register and GP Area(Cont.)
Why Global Data Area ?
Load variable x to r10
Without GP: 3 instructions
Li r9, x low 16-bit of x
Addiu r9, x high 16-bit of x
Lw r10, 0(r9) -- load
With GP: 1 – instruction
LW r10, 24(GP) -- load
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17. What should be put into Global Data Area ?
GP Register and GP Area(Cont.)
What should be put into Global Data Area ?
Most Frequently Access Data
How to Use Global Data Area ?
• Global Data Area requires linker support
• $gp register must be correctly initialized (by the startup routine)
• assembly code must not modify the $gp register
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18. Runtime Stack Stack organization High memory argumentn …… argument1
Virtual frame
Pointer($fp)
argument1
Frame offset
Local & temporaries
Saved registers
(including returnreg)
framesize
Procedure call
Argument area
static
Pointer($sp) ……
low memory
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19. Stack and Frame Example 1: int total;
int sum_all(int a1, int a2, int a3, int a4,int a5, int a6, int a7, int a8) {
return a1+a2+a3+a4+a5+a6+a7+a8; }
Main()
total=sum_all(1,2,3,4,5,6,7,8);
printf(“total = %d\n”, total);
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20. Stack and Frame – enter main()
$fp=0
0x7fffffff
subu $sp, $sp, ?
sw $31, 36($sp)
sw $fp, 32($sp)
move $fp, $sp ?
li $2, 0x ?
sw $2, 16($sp)
li $2, 0x ?
sw $2, 20($sp)
li $2, 0x7
sw $2, 24($sp)
li $2, 0x8
sw $2, 28($sp)
li $4, 0x1
li $5, 0x2
li $6, 0x3
li $7, 0x4
jal sum_all …
$sp=0x7ffff7fe8
$31 ($ra)
$30 ($fp) 8 7 6 5
$sp=0x7ffff7fc0
$fp=$sp 0x
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21. Stack and Frame – enter sum_all()
0x7fffffff
subu $sp, $sp, 8
sw $fp, 0($sp)
move $fp, $sp
sw $4, 8($sp)
sw $5, 12($sp)
sw $6, 16($sp)
sw $7, 20($sp) …
move $sp, $fp
lw $fp, 0($sp)
addu $sp, $sp, 8
j $31
0x7ffff7fe8
$31 ($ra)
$30 ($fp) 8 7 6 5 4 3 2
$sp=0x7ffff7fc0 1
$fp=$sp
$30 ($fp)
$sp=0x7ffff7fb0
$fp=$sp 0x
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22. Stack and Frame – exit sum_all()
0x7fffffff
subu $sp, $sp, 8
sw $fp, 0($sp)
move $fp, $sp
sw $4, 8($sp)
sw $5, 12($sp)
sw $6, 16($sp)
sw $7, 20($sp) …
move $sp, $fp
lw $fp, 0($sp)
addu $sp, $sp, 8
j $31
0x7ffff7fe8
$31 ($ra)
$30 ($fp) 8 7 6 5 4 3 2 1
$fp
$sp
$30 ($fp)
$sp=0x7ffff7fb0
$fp=$sp 0x
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23. Stack and Frame – exit main()
0x7fffffff …
move $sp, $fp
lw $31, 36($sp)
lw $fp, 32($sp)
addu $sp, $sp, 40
j $31
$sp
$fp =0
$31 ($ra)
$30 ($fp) 8 7 6 5 4 3 2 1
$fp
$sp
$30 ($fp) 0x
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24. Stack and Frame Do we really need store $4 to $7 onto stack?
An optimized version
sum_all:
lw $3, 16($sp)
lw $8, 20($sp)
lw $9, 24($sp)
lw $2, 28($sp)
addu $4, $4, $5
addu $4, $4, $6,
addu $4, $4, $7
addu $4, $4, $3,
addu $4, $4, $8
addu $4, $4, $9,
addu $2, $4, $2
j $31
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25. Stack and Frame
Example 2: where are the local variables (automatic variable)
int sum_all(int a1, int a2, int a3, int a4,int a5, int a6, int a7, int a8) {
int total;
total=a1+a2+a3+a4+a5+a6+a7+a8;
return total; }
int test()
return sum_all(1,2,3,4,5,6,7,8);
main()
total=test();
printf(“total = %d\n”, total);
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26. Stack and Frame – main()
0x7fffffff
$sp
total
$31 ($ra)
$30 ($fp) r7 r6 r5 r4
$fp
$sp 0x
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27. Stack and Frame – test()
0x7fffffff
total
$31 ($ra)
$30 ($fp) r7 r6 r5 r4
$fp
$sp
$31
$fp 8 7 6 5
$fp
$sp 0x
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28. Stack and Frame – sum_all()
0x7fffffff
total
$31 ($ra)
$30 ($fp)
main r7 r6 r5 r4
$31
$fp 8 7 6
test 5
$fp
sum_all
total
$fp
$sp 0x
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29. Run-time Stack Fib(3) SP Fib(2) ret1 ret2 Fib(1) $s1 - unknown
sub $sp, $sp, 12
sw $s0, 4($sp)
sw $s1, 8($sp)
sw $ra, 0($sp)
beq $a0, $0, L1
mov $t0, 1
beq $a0, $t0, L1
mov $s0, $a0
sub $a0, $a0, 1
jal fib
mov $s1, $v0
subi $a0, $s0, 2
jal fib
add $v0, $v0, $s1
j L2
L1:
addi $v0, $0, 1
L2:
lw $s1, 8($sp)
lw $s0, 4($sp)
lw $ra, 0($sp)
add $sp, $sp, 12
j $ra
Fib(3) SP
$s1 - unknown
$s0 - unknown
$ra – return to main
Fib(2)
$s1 - unknown
ret1
$s0 - unknown
$ra – ret1
ret2
Fib(1)
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30. Virtual / Physical memory
User memory space
OS memory space
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31. (virtual memory - user)
0x7FFF EFFF
Stack
Heap
BSS
Text: instructions
Data: variables with initial value
BSS: variables without initial value
HEAP: for malloc/free
STACK: for function call
Global Data
Data
Text 0x
Layout of Memory
(virtual memory - user)
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32. Layout of Memory ( OS memory space) Reserved for kernel 800000016
Stack segment
Dynamic data
Data segment
Static data
Text segment
Reserved
For Interrupt vector
Firmware
Layout of Memory
( OS memory space)
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33. Virtual Memory / Physical Memory
Why Virtual Memory
Limited physical memory size
64MB to 1GB
Unlimited virtual memory size
Each process may have 2GB
Many processes in the system
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34. Virtual Memory/Physical Memory
Physical memory as cache of virtual memory (disk)
Physical memory and virtual memory broke into fixed size pages;
Each physical page holds a virtual page (may come from different processes)
Only the active pages of each process reside in physical memory, physical memory works as cache of virtual memory (disk)
Other pages stay on disk
P1: pagek
P2: pagen
Pn: pagem
Physical pagei
Page table
Physical address
Virtual address v
rwx
Physical page
Start address
Virtual page
Disk address
Present bit
Protection bits
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35. Virtual and Physical Memory
Process 1
Process 2 OS
Page 0
U1/P0
U2/P0 OS
U1/P1
U2/P1
Page 1
U1/P0
U1/P2
U2/P2
Page 2
U2/P3
U1/P3
U2/P3
Page 3
U1/P3
U1/P4
U2/P4
Page 4
U1/P7
U1/P5
U2/P5
Page 5
U1/P6
U1/P6
U2/P6
Page 6
U2/P1
U1/P7
U2/P7
Page 7
On Disk
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36. Virtual Memory / Physical Memory
Why Segmentation Fault ?
main() {
int *p;
*p=12; }
Invalid pointer – p points to arbitrary address (address 0?)
Page protection will assign “readable/executable” to the pages in this section
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37. Physical / Virtual Memory
#include <malloc.h>
main() {
int *p;
p=(int *)malloc(sizeof(int));
*p=12; }
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