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Challenge yourself


In proc.c, there is an array initcode[] of binary code. What is its relationship with initcode.S?



Testing code: -xc to make the hex dump more readable. Example:
$ od -xc test.txt
0000000 6568 6c6c 0a6f
h e l l o \n
The od Command
od man page
-t is the output format. x is hex. c is chars in default char set.
Note: Each Hexadecimal character represents 4 bits (0 - 15 decimal). A byte is 2 hex.


We want a memory dump of instructions in hex format, and separated by each char.
We have to load initcode, and use it to call system call exec to run init. We cannot directly load init binary as hex dump, otherwise we have to do something similar to what exec does, set up C stacks, parse ELF headers, etc.
So the simply solution is:
1. use objcopy to copy a stripped instruction only data file.
2. Use od to print the data file in hex, separated by byte (char).
3. Append 0x to each char.
4. That is the result if initcode array you see in proc.c.
od - dump files in various formats.


Try to switch the hex dump of initcode to a program you wrote!
I modify initcode.S to use echo:
#include “syscall.h”
# exec(init, argv)
.globl start
la a0, echo
la a1, argv
li a7, SYS_exec
# for(;;) exit();
li a7, SYS_exit
jal exit
# char init[] =/init\0;
.string “/echo\0
# char *argv[] = { init, 0 };
.p2align 2
.long echo
.long 0
After running make, get the hex dump of the binary:
$ od -t xC initcode
0000000 17 05 00 00 13 05 05 02 97 05 00 00 93 85 05 02
0000020 9d 48 73 00 00 00 89 48 73 00 00 00 ef f0 bf ff
0000040 2f 65 63 68 6f 00 00 01 20 00 00 00 00 00 00 00
0000060 00 00 00
Change the array in proc.c:
// a user program that calls exec(“/echo”)
// od -t xC initcode
uchar initcode[] = {
0x17, 0x05, 0x00, 0x00, 0x13, 0x05, 0x05, 0x02,
0x97, 0x05, 0x00, 0x00, 0x93, 0x85, 0x05, 0x02,
0x9d, 0x48, 0x73, 0x00, 0x00, 0x00, 0x89, 0x48,
0x73, 0x00, 0x00, 0x00, 0xef, 0xf0, 0xbf, 0xff,
0x2f, 0x65, 0x63, 0x68, 0x6f, 0x00, 0x00, 0x01,
0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00
Make a special echo:
#include “kernel/types.h"
#include “kernel/stat.h”
#include “user/user.h”
#include “kernel/fcntl.h”
if(open(“console”, O_RDWR) < 0){
mknod(“console”, 1, 1);
open(“console”, O_RDWR);
dup(0); // stdout
dup(0); // stderr
printf(“Mushroom os is the best!\n”);
As a result, your terminal prints the following forever:
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
We hacked the kernel to run our program in the very first user process!