Comparing C(x86) compile + linking with SIC-XE assembler + linking + loading
main.c contains
#include<stdio.h>
extern int fact(int i);
int main(void){
int i = fact(5);
printf("5 Factorial is %d\n", i);
return 0;
}fact.c contains:
int fact(int i){
if(i == 0)
return 1;
else
return i * fact(i-1);
}Compile with debug information(to see C soruce and assembly interleved) and use objdump to see the generated object code: main.o and fact.o
$ gcc -c -g main.c
$ objdump -dS main.o
main.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main>:
#include<stdio.h>
extern int fact(int i);
int main(void){
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 10 sub $0x10,%rsp
int i = fact(5);
8: bf 05 00 00 00 mov $0x5,%edi
d: e8 00 00 00 00 callq 12 <main+0x12>
12: 89 45 fc mov %eax,-0x4(%rbp)
printf("5 Factorial is %d\n", i);
15: 8b 45 fc mov -0x4(%rbp),%eax
18: 89 c6 mov %eax,%esi
1a: bf 00 00 00 00 mov $0x0,%edi
1f: b8 00 00 00 00 mov $0x0,%eax
24: e8 00 00 00 00 callq 29 <main+0x29>
return 0;
29: b8 00 00 00 00 mov $0x0,%eax
}
2e: c9 leaveq
2f: c3 retq
$ gcc -c -g fact.c
$ objdump -dS fact.o
fact.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <fact>:
int fact(int i){
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 10 sub $0x10,%rsp
8: 89 7d fc mov %edi,-0x4(%rbp)
if(i == 0)
b: 83 7d fc 00 cmpl $0x0,-0x4(%rbp)
f: 75 07 jne 18 <fact+0x18>
return 1;
11: b8 01 00 00 00 mov $0x1,%eax
16: eb 11 jmp 29 <fact+0x29>
else
return i * fact(i-1);
18: 8b 45 fc mov -0x4(%rbp),%eax
1b: 83 e8 01 sub $0x1,%eax
1e: 89 c7 mov %eax,%edi
20: e8 00 00 00 00 callq 25 <fact+0x25>
25: 0f af 45 fc imul -0x4(%rbp),%eax
}
29: c9 leaveq
2a: c3 retq Lets invoke the linker to create the executable program, and use objdump again to see the object code from main and fact inside the executable.
$ gcc main.o fact.o
a.out: file format elf64-x86-64
Disassembly of section .init:
00000000004003c8 <_init>:
4003c8: 48 83 ec 08 sub $0x8,%rsp
4003cc: 48 8b 05 25 0c 20 00 mov 0x200c25(%rip),%rax
4003d3: 48 85 c0 test %rax,%rax
4003d6: 74 05 je 4003dd <_init+0x15>
.
.
.
40051b: 48 89 e5 mov %rsp,%rbp
40051e: ff d0 callq *%rax
400520: 5d pop %rbp
400521: e9 7a ff ff ff jmpq 4004a0 <register_tm_clones>
0000000000400526 <main>: # main function
400526: 55 push %rbp
400527: 48 89 e5 mov %rsp,%rbp
40052a: 48 83 ec 10 sub $0x10,%rsp
40052e: bf 05 00 00 00 mov $0x5,%edi
400533: e8 1e 00 00 00 callq 400556 <fact>
400538: 89 45 fc mov %eax,-0x4(%rbp)
40053b: 8b 45 fc mov -0x4(%rbp),%eax
40053e: 89 c6 mov %eax,%esi
400540: bf 14 06 40 00 mov $0x400614,%edi
400545: b8 00 00 00 00 mov $0x0,%eax
40054a: e8 b1 fe ff ff callq 400400 <printf@plt>
40054f: b8 00 00 00 00 mov $0x0,%eax
400554: c9 leaveq
400555: c3 retq
0000000000400556 <fact>: # fact function
400556: 55 push %rbp
400557: 48 89 e5 mov %rsp,%rbp
40055a: 48 83 ec 10 sub $0x10,%rsp
40055e: 89 7d fc mov %edi,-0x4(%rbp)
400561: 83 7d fc 00 cmpl $0x0,-0x4(%rbp)
400565: 75 07 jne 40056e <fact+0x18>
400567: b8 01 00 00 00 mov $0x1,%eax
40056c: eb 11 jmp 40057f <fact+0x29>
40056e: 8b 45 fc mov -0x4(%rbp),%eax
400571: 83 e8 01 sub $0x1,%eax
400574: 89 c7 mov %eax,%edi
400576: e8 db ff ff ff callq 400556 <fact>
40057b: 0f af 45 fc imul -0x4(%rbp),%eax
40057f: c9 leaveq
400580: c3 retq
400581: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
400588: 00 00 00
40058b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
0000000000400590 <__libc_csu_init>:
400590: 41 57 push %r15
400592: 41 56 push %r14
400594: 41 89 ff mov %edi,%r15d
.
.
.
0000000000400604 <_fini>:
400604: 48 83 ec 08 sub $0x8,%rsp
400608: 48 83 c4 08 add $0x8,%rsp
40060c: c3 retq To see the symbol table we will invoke nm a.out (GNU nm lists the symbols from object files objfile)
$ nm a.out
0000000000601038 B __bss_start
0000000000601038 b completed.7585
0000000000601028 D __data_start
0000000000601028 W data_start
0000000000400460 t deregister_tm_clones
00000000004004e0 t __do_global_dtors_aux
0000000000600e18 t __do_global_dtors_aux_fini_array_entry
0000000000601030 D __dso_handle
0000000000600e28 d _DYNAMIC
0000000000601038 D _edata
0000000000601040 B _end
0000000000400556 T fact
0000000000400604 T _fini
0000000000400500 t frame_dummy
0000000000600e10 t __frame_dummy_init_array_entry
0000000000400778 r __FRAME_END__
0000000000601000 d _GLOBAL_OFFSET_TABLE_
w __gmon_start__
0000000000400628 r __GNU_EH_FRAME_HDR
00000000004003c8 T _init
0000000000600e18 t __init_array_end
0000000000600e10 t __init_array_start
0000000000400610 R _IO_stdin_used
w _ITM_deregisterTMCloneTable
w _ITM_registerTMCloneTable
0000000000600e20 d __JCR_END__
0000000000600e20 d __JCR_LIST__
w _Jv_RegisterClasses
0000000000400600 T __libc_csu_fini
0000000000400590 T __libc_csu_init
U __libc_start_main@@GLIBC_2.2.5
0000000000400526 T main
U printf@@GLIBC_2.2.5
00000000004004a0 t register_tm_clones
0000000000400430 T _start
0000000000601038 D __TMC_END__Here you will note that main is at location 400526 and fact is at location 400556. The T in the second column means they are in the text section of our program. Which matches with our object dump.
main.asm contains,
PROG START 0
EXTDEF THREE
EXTREF ONE,TWO,FUNC
FIRST +LDA ONE
LOOP +STA TWO+3
+J LOOP
+JSUB FUNC
THREE WORD TWO-ONE
END FIRST
func.asm contains,
FUNC START 0
EXTDEF ONE,TWO
EXTREF THREE
LOOP +STA THREE
J LOOP
RSUB
ONE WORD 8
TWO RESW 3
FOUR WORD ONE
END LOOP
Compile the assembler first, and then feed the asm files in the assembler.
$ javac Assembler/Main.java
$ java Assembler.Main main.asmIt will generate object code main.o
Reading from File : main.asm
*********** PASS 1 ***********
> Generated Intermediate File : main.int
00000 PROG START 0
00000 EXTDEF THREE
00000 EXTREF ONE,TWO,FUNC
00000 FIRST +LDA ONE
00004 LOOP +STA TWO+3
00008 +J LOOP
0000C +JSUB FUNC
00010 THREE WORD TWO-ONE
00013 END FIRST
> Symbol Table
Symbol Value rflag iflag mflag
FIRS 00000 1 1 0
LOOP 00004 1 1 0
PROG 00000 1 1 0
THRE 00010 1 1 0
> Literal Table
Literal Value length address
(Literal Table is Empty)
Continue to see the output of Pass 2.
*********** PASS 2 ***********
> Adding object code to Intermediate File : main.txt
00000 PROG START 0
00000 EXTDEF THREE
00000 EXTREF ONE,TWO,FUNC
00000 FIRST +LDA ONE 03100000
00004 LOOP +STA TWO+3 0F100003
00008 +J LOOP 3F100004
0000C +JSUB FUNC 4B100000
00010 THREE WORD TWO-ONE 000000
00013 END FIRST
Continue to see the Updated Symbol table and Object Code
> Updated Symbol Table
Symbol Value rflag iflag mflag
FIRS 00000 1 1 0
FUNC 00000 0 0 0
LOOP 00004 1 1 0
ONE 00000 0 0 0
PROG 00000 1 1 0
THRE 00010 1 1 0
TWO 00000 0 0 0
> Generated Object Code : main.o
H^PROG^000000^000013
D^THRE^000010
R^ONE ^TWO ^FUNC
T^000000^13^03100000^0F100003^3F100004^4B100000^000000
M^000001^05^+ONE
M^000005^05^+TWO
M^000009^05^+PROG
M^00000D^05^+FUNC
M^000010^06^-ONE
M^000010^06^+TWO
E^000000Feed func.asm to the assembler, and the assembler will generate func.o
Reading from File : func.asm
*********** PASS 1 ***********
> Generated Intermediate File : func.int
00000 FUNC START 0
00000 EXTDEF ONE,TWO
00000 EXTREF THREE
00000 LOOP +STA THREE
00004 J LOOP
00007 RSUB
0000A ONE WORD 8
0000D TWO RESW 3
00016 FOUR WORD ONE
00019 END LOOP
> Symbol Table
Symbol Value rflag iflag mflag
FOUR 00016 1 1 0
FUNC 00000 1 1 0
LOOP 00000 1 1 0
ONE 0000A 1 1 0
TWO 0000D 1 1 0
> Literal Table
Literal Value length address
(Literal Table is Empty)
Continue to see the output of Pass 2.
*********** PASS 2 ***********
> Adding object code to Intermediate File : func.txt
00000 FUNC START 0
00000 EXTDEF ONE,TWO
00000 EXTREF THREE
00000 LOOP +STA THREE 0F100000
00004 J LOOP 3F2FF9
00007 RSUB 4F0000
0000A ONE WORD 8 000008
0000D TWO RESW 3
00016 FOUR WORD ONE 00000A
00019 END LOOP
Continue to see the Updated Symbol table and Object Code
> Updated Symbol Table
Symbol Value rflag iflag mflag
FOUR 00016 1 1 0
FUNC 00000 1 1 0
LOOP 00000 1 1 0
ONE 0000A 1 1 0
THRE 00000 0 0 0
TWO 0000D 1 1 0
> Generated Object Code : func.o
H^FUNC^000000^000019
D^ONE ^00000A^TWO ^00000D
R^THRE
T^000000^0D^0F100000^3F2FF9^4F0000^000008
T^000016^03^00000A
M^000001^05^+THRE
M^000016^06^+FUNC
E^000000We have our linker and loader invoked together. We will see the machine code laied out in the memory. The excution will start from 3860. The symbol table is added at the end.
$ java LinkerLoader.Main main.o func.o
Memory Layout :
0 1 2 3 4 5 6 7 8 9 A B C D E F
386X 03 10 38 7d 0F 10 38 83 3F 10 38 64 4B 10 38 73
387X 10 03 84 0F 10 38 70 3F 2F F9 4F 00 00 00 00 08
388X ?? ?? ?? ?? ?? ?? ?? ?? ?? 00 38 7d ?? ?? ?? ??
Execution Address : 3860
External Symbol Table :
CSECT SYMBOL ADDR CSADDR LDADDR LENGTH
FUNC 3873 19
PROG 3860 13
THRE 10 3870
ONE a 387d
TWO d 3880