Update post of cs350's lab.

_posts/2022-02-22-cs350-labs.md

@@ -30,6 +30,7 @@ Remember, it's for helping in learning. DON'T COPY & PASTE CODE!
 ## Lab6-7-Scheduling
 
 ### First user process in xv6
+#### Kernel works
 In xv6, as the same as conventional linux OS, the very first user level process is **init**.
 Before **init**'s running, all the OS bootstraps are happened in a high privileged mode(kernel level).
 
@@ -112,10 +113,166 @@ userinit(void)
   p->state = RUNNABLE;
 }
 ~~~  
+#### Where the user-level code was integrated?
 If you search the keyword "_binary_initcode_start" in the source code, you can't find any references.
 The clue comes from the *Makefile*.
   
+In the makefile, **initcode** is a prerequisites to compile the kernel image.
+**Step 1**: Before kernel was compiled, *initcode.S* was first compiled to a runnable binary *initcode*.
+This binary was very odd because it was not supposed to let any other OS to run it.
+*Initcode.s* was first compiled without any standard including, and generating the intermediate file *initcode.o*.
+  
+**Step 2**: *Initcode.o* then linked to *Initcode.out* with two uncommon settings. 
+First it specify the entry of this binary file as when "start" symbol points to.
+This "start" symbol was declared in the assembly code.
+Second it specify a absolute address(0) for the text segments. 
+By doing this, text segments will be placed at the start of the binary file (except the header of the ELF)[^ldman].
+  
+**Step 3**: *Initcode.out* is already a minimized binary but it's not enough.
+That's why when using **objcopy** to copy it to the file *initcode*, it further strip all headers and debug information[^objcopyman].
+At this point, we have a minimal binary file *initcode*.
+From the first byte of this file, it's only includes runnable instructions.
+And the size of the file is only 44 bytes.
+~~~bash
+initcode: initcode.S
+	$(CC) $(CFLAGS) -nostdinc -I. -c initcode.S                         # Step 1
+	$(LD) $(LDFLAGS) -N -e start -Ttext 0 -o initcode.out initcode.o    # Step 2
+	$(OBJCOPY) -S -O binary initcode.out initcode                       # Step 3
+	$(OBJDUMP) -S initcode.o > initcode.asm
+~~~
+  
+This binary later were appended to the kernel using following commands. 
+And during this appending, 3 symbols were generated and added to the symbol table of the *kernel*[^ldman].
+**"_binary_initcode_start"** contains the address of where the initcode segment was appended to.
+**"_binary_initcode_end"** contains the address of where the initcode segment was ended at.
+**"_binary_initcode_size"** is a \*ABS\* type symbol with value 0x2C(45) that specify the size of the initcode segment is 45 bytes.
+~~~bash
+kernel: $(OBJS) entry.o entryother initcode kernel.ld
+	$(LD) $(LDFLAGS) -T kernel.ld -o kernel entry.o $(OBJS) -b binary initcode entryother # <- This Line
+	$(OBJDUMP) -S kernel > kernel.asm
+	$(OBJDUMP) -t kernel | sed '1,/SYMBOL TABLE/d; s/ .* / /; /^$$/d' > kernel.sym
+~~~
+**In short summary**,
+using objdump, we can verify that source code *initcode.S* has been compiled and loaded into the kernel.
+Also the segment of initcode's instructions was located by the pointer "_binary_initcode_start".
+That's explain when calling ***inituvm(p->pgdir, _binary_initcode_start, (int)_binary_initcode_size);***, 
+functionalities implemented in initcode.S will be loaded into the runtime of the first process within xv6.
+~~~bash
+# Header of the file kernel
+kernel:     file format elf32-i386
+kernel
+architecture: i386, flags 0x00000112:
+EXEC_P, HAS_SYMS, D_PAGED
+start address 0x0010000c
+
+Program Header:
+    LOAD off    0x00001000 vaddr 0x80100000 paddr 0x00100000 align 2**12
+         filesz 0x00008c6a memsz 0x00008c6a flags r-x
+...
+Sections:
+Idx Name          Size      VMA       LMA       File off  Algn
+  0 .text         00008586  80100000  00100000  00001000  2**2
+                  CONTENTS, ALLOC, LOAD, READONLY, CODE
+...
+SYMBOL TABLE:
+...
+8010b50c g       .data	00000000 _binary_initcode_end
+...
+8010b4e0 g       .data	00000000 _binary_initcode_start
+...
+0000002c g       *ABS*	00000000 _binary_initcode_size
+...
+~~~
+
+#### User-level code
+
+Take a look of content in the *initcode.S*, you will find the code can explain itself well.
+There are no other jobs but just calling system call **exec** to run a user-level binary **"init"**.
+  
+*Initcode.S*:
+~~~bash
+# Initial process execs /init.
+
+#include "syscall.h"
+#include "traps.h"
 
 
+# exec(init, argv)
+.globl start
+start:
+  pushl $argv
+  pushl $init
+  pushl $0  // where caller pc would be
+  movl $SYS_exec, %eax
+  int $T_SYSCALL
 
+# for(;;) exit();
+exit:
+  movl $SYS_exit, %eax
+  int $T_SYSCALL
+  jmp exit
 
+# char init[] = "/init\0";
+init:
+  .string "/init\0"
+
+# char *argv[] = { init, 0 };
+.p2align 2
+argv:
+  .long init
+  .long 0
+~~~
+
+The **"init"** mentioned above is not a pure user-level binary executable that compiled from the source code *init.c*.
+Within *init.c*, a file named *console* will be created at the runtime for saving standard outputs and errors.
+Then it will forked a child process(the second user process), and let it run program **"sh"**.
+  
+**"sh"** is the xv6's default shell, a user-level program that generated from source *sh.c*.
+After the shell boots up, you can interactive with the xv6. 
+This's how first process (and second process) was started in the xv6.
+  
+*init.c*:
+~~~c
+// init: The initial user-level program
+
+#include "types.h"
+#include "stat.h"
+#include "user.h"
+#include "fcntl.h"
+
+char *argv[] = { "sh", 0 };
+
+int
+main(void)
+{
+  int pid, wpid;
+
+  if(open("console", O_RDWR) < 0){
+    mknod("console", 1, 1);
+    open("console", O_RDWR);
+  }
+  dup(0);  // stdout
+  dup(0);  // stderr
+
+  for(;;){
+    printf(1, "init: starting sh\n");
+    pid = fork();
+    if(pid < 0){
+      printf(1, "init: fork failed\n");
+      exit();
+    }
+    if(pid == 0){
+      exec("sh", argv);
+      printf(1, "init: exec sh failed\n");
+      exit();
+    }
+    while((wpid=wait()) >= 0 && wpid != pid)
+      printf(1, "zombie!\n");
+  }
+}
+~~~
+
+[^ldman]: [ld\(1\) - Linux man page](https://linux.die.net/man/1/ld)
+[^objcopyman]: [3 objcopy - binutils mannual](https://sourceware.org/binutils/docs/binutils/objcopy.html)
+
+### Xv6's round robin schduler