Added Real Mode to Protected Mode switch

Author: scopeInfinity

.gitignore

@@ -1,3 +1,4 @@
 *.vmdk
 *.o
+local_notes/
 build/

Makefile

@@ -23,6 +23,9 @@ app_dashboard = $(BUILD_DIR)/dashboard
 # Parameters
 BT_STAGE2_SECTOR_COUNT = 19 # In Hex
 
+# General Assumptions
+## Integer is 4 bytes
+
 rebuild: clean all
 
 all: images binaries
@@ -93,7 +96,10 @@ debug_stage2_asm: $(bt_stage2_asm_o)
 	xxd $<
 
 qemu: $(image_vmdk) images
-	cpulimit -f -l 10 -- qemu-system-x86_64 -smp 1 -m 1M -hda $<
+	cpulimit -f -l 10 -- qemu-system-x86_64 -smp 1 -m 128M -hda $< -no-shutdown -no-reboot
+
+qemu_debug: $(image_vmdk) images
+	qemu-system-x86_64 -smp 1 -m 128M -hda $< -no-shutdown -no-reboot -d  cpu,exec,in_asm
 
 clean:
 	rm -f $(image_vmdk) $(bt_stage1) $(bt_stage2) $(bt_stage2_c_o) $(bt_stage2_asm_o)

src/app/dashboard.c

@@ -35,8 +35,8 @@ void print_board() {
     print_applications(4,4, (char*)application_list, 5, 15);
 }
 
-int call_main(unsigned int address, int argc, char *argv[]) {
-    return ((int (*) (int, char *[]))address)(argc, argv);
+int call_main(unsigned short cs,unsigned short ip, int argc, char *argv[]) {
+    return ((int (*) (int, char *[]))(unsigned int)ip)(argc, argv);
 }
 
 int run_dashboard(int argc,char **argv) {
@@ -57,7 +57,7 @@ int run_dashboard(int argc,char **argv) {
             continue;
         }
         char *argv[] = {"argv1, argv2", "argv3"};
-        int return_status = call_main(0xC000, 3, argv);
+        int return_status = call_main(0x0C00,0x0000, 3, argv);
 
         print_board();
         set_color_bg(C_DARK_GRAY);

src/bootloader/stage2.asm

@@ -8,6 +8,7 @@
 [BITS 16]
 
 extern entry_stage
+global enter_protected_mode
 
 [SECTION .text]
         CLI
@@ -19,7 +20,31 @@ extern entry_stage
         call entry_stage
         JMP label_exit
 
-label_exit:
+    enter_protected_mode:
+        ; Args: (gdtr_address)
+        ; And never returns.
+
+        push ebp
+        mov ebp, esp
+
+        cli
+
+        ; Load GDT Table
+            ; Workaround for Issue #3
+            mov eax, [esp+8]   ; gdtr_address
+            mov ebx, eax
+            add ebx, 8         ; &gdtr_table
+            mov [eax+2], ebx   ; gdtr.base_address = &gdtr_table
+
+        lgdt [eax]         ; Load GDT Table
+
+        ; Enter Protected mode
+        mov eax, cr0
+        or eax, 0x00000001
+        mov cr0, eax
+        jmp 0x08:long_jump_enter_protected
+
+    label_exit:
         HLT
         JMP label_exit
 
@@ -30,4 +55,36 @@ label_exit:
 
 [SECTION .data]
     bl_stage_2             db      "Bootloader: Stage 2"
-    bl_stage_2_len         equ       ($-bl_stage_2)
+    bl_stage_2_len         equ       ($-bl_stage_2)
+
+
+[BITS 16]
+    long_jump_enter_protected:
+
+        mov ax, 0x10
+        mov ds, ax
+        mov ss, ax
+        mov es, ax
+        mov fs, ax
+        mov gs, ax
+
+        ; Print Done
+        mov ah,0x0F
+        mov ebx,0xb8000
+        mov al,'D'
+        mov [ebx],ax
+
+        add ebx, 2
+        mov al,'o'
+        mov [ebx],ax
+
+        add ebx, 2
+        mov al,'n'
+        mov [ebx],ax
+
+        add ebx, 2
+        mov al,'e'
+        mov [ebx],ax
+
+        ; TODO: Will improve this soon.
+        hlt

src/bootloader/stage2.c

@@ -5,6 +5,80 @@
 
 char message_welcome[] = "C says 'Hello World'";
 char message_dashboard[] = "Opening App 'Dashboard'";
+char message_protected_mode[] = "Enabling Protected Mode...";
+
+#define GDT_TABLE_SIZE 3
+
+struct GDTReference {
+    unsigned short size;
+    unsigned int base_address;
+};
+
+struct GDTEntry {
+    unsigned short limit0;
+    unsigned short base0;
+    unsigned char base1;
+    unsigned char access_byte;
+    unsigned char flags_limit1;
+    unsigned char base2;
+};
+
+// Issue#2: gdt_table must immediately after gdtr.
+struct GDTReference gdtr;
+struct GDTEntry gdt_table[GDT_TABLE_SIZE];
+
+void populate_gct_entry(struct GDTEntry *entry,
+    unsigned int base,
+    unsigned int limit,  // 20 bits
+    unsigned char flags, //  4 bits
+    unsigned char access_byte
+    ) {
+    entry->base0 = (base&0x0000FFFF);
+    entry->base1 = (base&0x00FF0000)>>16;
+    entry->base2 = (base&0xFF000000)>>24;
+    entry->limit0 = (limit&0x0FFFF);
+    entry->flags_limit1 = (flags<<4)|((limit&0xF0000)>>16);
+    entry->access_byte = access_byte;
+}
+
+extern void enter_protected_mode(int gdtr_address);
+
+int populate_gdt_table() {
+    // Assumption DS = 0
+    // Populate simple overlapping code and data segment.
+    populate_gct_entry(
+        &gdt_table[0],
+        0,0,0,0);
+    populate_gct_entry(
+        &gdt_table[1],
+        0x00000000,0x0fffffff,
+        0b00000000,
+        0x9a);
+    populate_gct_entry(
+        &gdt_table[2],
+        0x00000000,0x0fffffff,
+        0b00000000,
+        0x92);
+
+    // Issue#2: For some reason gdtr.base_address assigned is not
+    // being respected later on. So, as a bad workaround hardcoding
+    // base_address as &gdtr+8 in stage2.asm.
+    gdtr.base_address = (int)gdt_table;
+    gdtr.size = (sizeof(gdt_table));
+
+    // Print the table and table addresse.
+    move_xy(6,14);
+    print_int((int)&gdtr);
+    move_xy(6,15);
+    print_int(gdtr.base_address);
+    move_xy(6,16);
+    print_int(gdtr.size);
+    for(int i=0;i<GDT_TABLE_SIZE;i++) {
+        move_xy(6,17+i);
+        print_memory_hex(8*i+(char*)&gdt_table, 8);
+    }
+    return (int)&gdtr;
+}
 
 void entry_stage() {
     move_xy(6, 11);
@@ -13,8 +87,9 @@ void entry_stage() {
     print_line(message_welcome);
     set_color_fg(C_WHITE);
     move_xy(6, 13);
-    print_line(message_dashboard);
-    sleep(100);
-    run_dashboard(0,0);
-    while(1);
+    print_line(message_protected_mode);
+    int gdtr_address = populate_gdt_table();
+    // Note: enter_protected_mode never returns.
+    enter_protected_mode(gdtr_address);
+    // PC should never reach here :)
 }