Fix mistakes in "Processes and Threads" (#122)

* fix typo in Scheduler

* define segment selector values in GDT

* use proper macro for segment selector in "Processes and Threads"

* fix missing KERNEL_DS instead of KERNEL_SS in "Processes and Threads"

Author: sudw1n

02_Architecture/04_GDT.md

@@ -183,6 +183,16 @@ A simple example is outline just below, for a simple 64-bit long mode setup we'd
 - Selector 0x18: user code (64-bit, ring 3)
 - Selector 0x20: user data (64-bit)
 
+For future reference, we should define macros for these selectors. So let's assume we have the following:
+
+```c
+#define NULL_SELECTOR 0x00
+#define KERNEL_CS 0x08
+#define KERNEL_DS 0x10
+#define USER_CS 0x18
+#define USER_DS 0x20
+```
+
 To create a GDT populated with these entries we'd do something like the following:
 
 ```c

05_Scheduling/02_Scheduler.md

@@ -184,7 +184,7 @@ For the purpose of our example the scheduler will only have three states for now
 
 * READY: The process is in the queue and waiting to be scheduled.
 * RUNNING: The process is currently running on the cpu.
-* DEAD: The process has finished running and should not be scheduled. It's resources can also be cleaned up.
+* DEAD: The process has finished running and should not be scheduled. Its resources can also be cleaned up.
 
 We'll modify our selection algorithm to take these new states into account:
 

05_Scheduling/03_Processes_And_Threads.md

@@ -72,10 +72,10 @@ process_t* create_process(char* name, void(*function)(void*), void* arg) {
     strncpy(process->name, name, NAME_MAX_LEN);
     process->pid = next_free_pid++;
     process->process_status = READY;
-    process->context.iret_ss = KERNEL_SS;
+    process->context.iret_ss = KERNEL_DS; // from the GDT chapter
     process->context.iret_rsp = alloc_stack();
     process->context.iret_flags = 0x202;
-    process->context.iret_cs = KERNEL_CS;
+    process->context.iret_cs = KERNEL_CS; // from the GDT chapter
     process->context.iret_rip = (uint64_t)function;
     process->context.rdi = (uint64_t)arg;
     process->context.rbp = 0;
@@ -237,10 +237,10 @@ thread_t* add_thread(process_t* proc, char* name, void(*function)(void*), void*
     thread->tid = next_thread_id++;
     thread->status = READY;
     thread->next = NULL:
-    thread->context.iret_ss = KERNEL_SS;
+    thread->context.iret_ss = KERNEL_DS; // from the GDT chapter
     thread->context.iret_rsp = alloc_stack();
     thread->context.iret_flags = 0x202;
-    thread->context.iret_cs = KERNEL_CS;
+    thread->context.iret_cs = KERNEL_CS; // from the GDT chapter
     thread->context.iret_rip = (uint64_t)function;
     thread->context.rdi = (uint64_t)arg;
     thread->context.rbp = 0;