Initialization of GDT

Author: AvihooI

.vscode/settings.json

@@ -1,5 +1,8 @@
 {
     "files.associations": {
-        "terminal.h": "c"
+        "terminal.h": "c",
+        "idt.h": "c",
+        "gdt.h": "c",
+        "types.h": "c"
     }
 }

Makefile

@@ -10,7 +10,10 @@ TARGET := testos.bin
 ALL_OBJ := environment.o \
 		boot.o \
 		kernel.o \
-		string.o
+		string.o \
+		idt.o \
+		gdt.o \
+		load_gdt.o
 ALL_DEP := $(patsubst %.o,.%.d,$(ALL_OBJ))
 
 #Assembly configuration
@@ -25,7 +28,7 @@ CFLAGS := -std=gnu99 -m32 -ffreestanding -O2 -Wall -Wextra
 LD := ld
 LD_FLAGS := -m elf_i386 -T linker.ld
 
-#all rules
+#All rules
 .PHONY : all testos clean
 all : $(TARGET)
 testos : $(TARGET)

boot.asm

@@ -17,3 +17,4 @@ start:
     extern environment
     jmp environment
     hlt
+

environment.asm

@@ -12,8 +12,3 @@ align 4
 stack_bottom:
 resb 16384
 stack_top:
-
-section .text
-resb 16384
-
-

gdt.c

@@ -0,0 +1,42 @@
+#include "gdt.h"
+
+struct gdt_entry gdt_entry(dword base, dword limit, byte flags, byte type_s_dpl_p)
+{
+    struct gdt_entry result;
+
+    result.base_lo = base_lo(base);
+    result.base_mid = base_mid(base);
+    result.base_hi = base_hi(base);
+    result.limit_lo = limit_lo(limit);
+    result.limit_hi_flags = limit_hi_flags(limit, flags);
+    result.type_s_dpl_p = type_s_dpl_p;
+
+    return result;
+}
+
+void gdt_init()
+{
+    qword* gdt_d;
+    byte flags_;
+    flags_ = flags(0,0,1,1);
+
+    /*
+        we need this since the two first entries (not actually entries)
+        contain the size of the table and the address to the table
+    */
+    gdt_d = (qword*)gdt; 
+    gdt_d[0] = 6*8; //size of the table
+    gdt_d[1] = (qword)&gdt; //address of the table
+
+    //ring 0 data segment
+    gdt[2] = gdt_entry(0, 4*1024*1024,flags_, type_s_dpl_p(type(0, 0, 1, 0), 0, 0, 1));
+    //ring 0 code segment
+    gdt[3] = gdt_entry(0, 4*1024*1024,flags_, type_s_dpl_p(type(1, 0, 1, 0), 0, 0, 1));
+    //ring 3 data segment
+    gdt[4] = gdt_entry(0, 4*1024*1024,flags_, type_s_dpl_p(type(0, 0, 1, 0), 0, 3, 1));
+    //ring 3 code segment
+    gdt[5] = gdt_entry(0, 4*1024*1024,flags_, type_s_dpl_p(type(1, 0, 1, 0), 0, 3, 1));
+
+    //load the gdt table
+    load_gdt((dword)gdt, 6*8);
+}

gdt.h

@@ -0,0 +1,151 @@
+/*
+This header file declares all the necessary bits to initialize and load the GDT (global decriptor table)
+*/
+
+#ifndef __gdt_h__
+#define __gdt_h__
+
+#include "types.h"
+
+enum gdt_entry_type
+{
+    ACCESSED    = 1 << 0,
+    WR          = 1 << 1,
+    EC          = 1 << 2,
+    CS          = 1 << 3
+};
+
+/*
+    GDT entry structure (bit representation):
+    0-15    : Segment limit - low portion       | WORD 
+    16-31   : Base address - low portion        | WORD
+    32-39   : Base address - mid portion        | BYTE
+    40-43   : Type (Page 2758 in IA-32 manual)  | BYTE 
+    44      : S (system/cs-ds)                  | "  "
+    45-6    : DPL (privilege ring 0 to 3)       | "  "
+    47      : P (present in memory)             | "  "
+    48-51   : Segment limit - high portion      | BYTE
+    52      : AVL (available bit)               | "  " (flags)
+    53      : L (32/64 bit segment)             | "  " (flags)
+    54      : D/B (16 bit/32 bit)               | "  " (flags)
+    55      : G (granularity)                   | "  " (flags)
+    56-63   : Base address - high portion       | BYTE
+*/
+
+/*
+    base structure:
+    0x0000FFFF low
+    0x00FF0000 mid
+    0xFF000000 high
+*/
+
+/*
+    limit structure:
+    0x0000FFFF low
+    0x000F0000 high
+*/
+
+struct gdt_entry
+{
+    word limit_lo;
+    word base_lo;
+    byte base_mid;
+    byte type_s_dpl_p;
+    byte limit_hi_flags;
+    byte base_hi;
+} __attribute__((packed));
+
+inline word limit_lo(dword limit)
+{
+    word result;
+
+    result = limit & 0x0000FFFF; 
+
+    return result;
+}
+
+inline word base_lo(dword base)
+{
+    word result;
+
+    result = base & 0x0000FFFF;
+
+    return result;
+}
+
+inline byte base_mid(dword base)
+{
+    byte result;
+
+    result = (base & 0x00FF0000) >> 4*4;
+
+    return result;
+}
+
+inline byte limit_hi_flags(dword limit, byte flags)
+{
+    byte limit_hi, result;
+
+    //take bits 16-19 from limit and put them in limit_hi
+    limit_hi = (limit & 0x000F0000) >> (4*4);
+
+    //or limit_hi and flags together
+    result = limit_hi | (flags << 4);
+
+    return result;
+}
+
+inline byte base_hi(dword base)
+{
+    byte result;
+
+    result = (base & 0xFF000000) >> (4*6);
+
+    return result;
+}
+
+inline byte flags(byte avl, byte l, byte db, byte g)
+{
+    byte result;
+
+    result = ((avl & 1) << 0) |
+             ((l & 1)   << 2) |
+             ((db & 1)  << 3) |
+             ((g & 1)   << 4);
+
+    return result;
+}
+
+inline byte type(byte ds_cs, byte ec, byte rw, byte a)
+{
+    byte result;
+
+    result = ((a & 1)       << 0) |
+             ((rw & 1)      << 1) |
+             ((ec & 1)      << 2) |
+             ((ds_cs & 1)   << 3);
+
+    return result;
+}
+
+inline byte type_s_dpl_p(byte type, byte s, byte dpl, byte p)
+{
+    byte result;
+
+    result = (type                )|
+             ((s & 1)         << 4)|
+             ((dpl & (1|2))   << 5)|
+             ((p & 1)         << 7);
+
+    return result;
+}
+
+struct gdt_entry gdt_entry(dword base, dword limit, byte flags, byte type_s_dpl_p);
+
+extern void load_gdt(dword gdt_ptr, word table_size);
+
+void gdt_init();
+
+struct gdt_entry gdt[6];
+
+#endif

gdt.s

@@ -0,0 +1 @@
+	.file	"gdt.c"

idt.c

@@ -0,0 +1 @@
+#include "idt.h"

idt.h

@@ -0,0 +1,6 @@
+#ifndef __idt_h__
+#define __idt_h__
+
+#include "types.h"
+
+#endif

kernel.c

@@ -1,12 +1,16 @@
 #include "terminal.h"
+#include "gdt.h"
 
 void kernel_main()
 {
     terminal_clear(vga_entry(' ', vga_entry_color(VGA_COLOR_GRAY, VGA_COLOR_BLUE)));
-    terminal_write_str("Hello kernel world! 1\n");
-
-
-
+    terminal_write_str("TestOS V 0.0000001\n");
+    gdt_init();
 
     return;
-} 
+}
+
+void* do_nothing(void* ptr)
+{
+    return ptr + 1;
+}