DS & ES registers are used from our kernerl. => By entering/leaving the kernel, saving/restoring of these registers are required.

arch/x86/include/asm/stddef.h

@@ -83,9 +83,14 @@ typedef unsigned int wint_t;
 /** @brief This defines what the stack looks like after an ISR was called.
  *
  * All the interrupt handler routines use this type for their only parameter.
+ * Note: Our kernel doesn't use the GS and FS registers.
  */
 struct state {
 #ifdef CONFIG_X86_32
+	// ds register
+	uint32_t ds;
+	// es register
+	uint32_t es;
 	/// EDI register
 	uint32_t edi;
 	/// ESI register

arch/x86/kernel/entry32.asm

@@ -438,8 +438,6 @@ extern syscall_handler
 ; used to realize system calls
 isrsyscall:
     push ds
-    push fs
-    push gs
     push es
     push ebp
     push edi
@@ -448,6 +446,10 @@ isrsyscall:
     push ecx
     push ebx
     push eax
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov eax, [esp]
     call syscall_handler
     add esp, 4 ; eax contains the return value
 	       ; => we did not restore eax
@@ -458,8 +460,6 @@ isrsyscall:
     pop edi
     pop ebp
     pop es
-    pop gs
-    pop fs
     pop ds
     iret
 
@@ -758,6 +758,7 @@ ALIGN 4
 switch_context:
     ; create on the stack a pseudo interrupt
     ; afterwards, we switch to the task with iret
+    ; we already in kernel space => no pushing of SS required
     mov eax, [esp+4]            ; on the stack is already the address to store the old esp
     pushf                       ; EFLAGS
     push DWORD 0x8              ; CS
@@ -765,6 +766,8 @@ switch_context:
     push DWORD 0x0              ; Interrupt number
     push DWORD 0x00edbabe       ; Error code
     pusha                       ; Registers...
+    push es
+    push ds
 
     jmp common_switch
 
@@ -775,6 +778,11 @@ rollback:
 ALIGN 4
 common_stub:
     pusha
+    push es
+    push ds
+    mov ax, 0x10
+    mov es, ax
+    mov ds, ax
 
     ; use the same handler for interrupts and exceptions
     push esp
@@ -798,6 +806,8 @@ common_switch:
     call finish_task_switch
 
 no_context_switch:
+    pop ds
+    pop es
     popa
     add esp, 8
     iret

arch/x86/kernel/gdt.c

@@ -99,6 +99,7 @@ int arch_fork(task_t* task)
 	state->int_no = 0xB16B00B5;
 	state->error =  0xC03DB4B3;
 	state->cs = 0x08;
+	state->ds = state->es = 0x10;
 	// store the current EFLAGS
 	asm volatile ("pushf; pop %0" : "=m"(state->eflags));
 	// enable interrupts
@@ -178,6 +179,7 @@ int create_default_frame(task_t* task, entry_point_t ep, void* arg)
 	stptr->cs = 0x08;
 #ifdef CONFIG_X86_32
 	stptr->eflags = 0x1202;
+	stptr->ds = stptr->es = 0x10;
 	// the creation of a kernel tasks didn't change the IOPL level
 	// => useresp & ss is not required
 #else

arch/x86/mm/page32.c

@@ -682,8 +682,8 @@ static void pagefault_handler(struct state *s)
 
 default_handler:
 	kprintf("PAGE FAULT: Task %u got page fault at %p (irq %d, cs:eip 0x%x:0x%x)\n", task->id, viraddr, s->int_no, s->cs, s->eip);
-	kprintf("Register state: eax = 0x%x, ebx = 0x%x, ecx = 0x%x, edx = 0x%x, edi = 0x%x, esi = 0x%x, ebp = 0x%x, esp = 0x%x\n", 
-		s->eax, s->ebx, s->ecx, s->edx, s->edi, s->esi, s->ebp, s->esp);
+	kprintf("Register state: eax = 0x%x, ebx = 0x%x, ecx = 0x%x, edx = 0x%x, edi = 0x%x, esi = 0x%x, ebp = 0x%x, esp = 0x%xi, ds = 0x%x, es = 0x%x\n", 
+		s->eax, s->ebx, s->ecx, s->edx, s->edi, s->esi, s->ebp, s->esp, s->ds, s->es);
 
 	irq_enable();
 	abort();