* Implemented GDT

* Incremented kernel version
* Updated README.md
* Updated Makefile

Author: Arun007coder

Makefile

@@ -19,7 +19,7 @@ AS=$(TOOLDIR)/i686-elf-as
 ASFLAGS=
 
 NASM=nasm
-NASMFLAGS=
+NASMFLAGS=-f elf32 -O0 -w+zeroing
 
 QEMU=qemu-system-i386
 QEMUFLAGS=
@@ -32,6 +32,8 @@ ISOFILE=$(PROJECT).iso
 OBJECTS= 	$(SRCDIR)/boot/boot.o \
 			$(SRCDIR)/drivers/io/ports.o \
 			$(SRCDIR)/drivers/power/reboot.o \
+			$(SRCDIR)/drivers/cpu/gdt.o \
+			$(SRCDIR)/drivers/cpu/gdt_helper.o \
 			$(SRCDIR)/common/libs/string.o \
 			$(SRCDIR)/common/libs/printf.o \
 			$(SRCDIR)/drivers/video/vga_text.o \

docs/README.md

@@ -2,7 +2,7 @@
 
 A new rewrite of SectorOS_RE2.
 
-The kernel is currently v0.23.03.4ALPR
+The kernel is currently v0.23.03.5ALPR
 
 ## Build
 

docs/changelog.txt

@@ -40,4 +40,10 @@
 * Updated README.md
 * Updated system.h
 * Updated string.h
+* Updated Makefile
+
+27-03-2023: 08:11 PM IST v0.23.03.5ALPR
+* Implemented GDT
+* Incremented kernel version
+* Updated README.md
 * Updated Makefile

src/drivers/cpu/gdt.c

@@ -0,0 +1,37 @@
+#include "gdt.h"
+
+gdt_entry_t entries[GDT_MAX_ENTRIES];
+gdt_ptr_t ptr;
+
+void init_gdt()
+{
+    printf("[GDT] Configuring GDT pointer...\n");
+    ptr.limit = (sizeof(gdt_entry_t) * GDT_MAX_ENTRIES) - 1;
+    ptr.base  = (uint32_t)&entries;
+
+    printf("[GDT] Adding entries...\n");
+
+    gdt_setGate(0, 0, 0, 0, 0);
+    gdt_setGate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+    gdt_setGate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+    gdt_setGate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF);
+    gdt_setGate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF);
+
+    printf("[GDT] Flushing GDT entries...\n");
+
+    flush_gdt((uint32_t)&ptr);
+
+    printf("[GDT] Initialized successfully\n");
+}
+
+void gdt_setGate(int32_t index, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran)
+{
+    entries[index].base_low    = (base & 0xFFFF);
+    entries[index].base_middle = (base >> 16) & 0xFF;
+    entries[index].base_high   = (base >> 24) & 0xFF;
+    entries[index].limit_low   = (limit & 0xFFFF);
+    entries[index].granularity = (limit >> 16) & 0x0F;
+    entries[index].granularity |= gran & 0xF0;
+    entries[index].access      = access;
+    printf("[GDT] New entry %d. Base: 0x%06x, Limit: 0x%06x, Access: 0x%02x, Gran: 0x%02x\n", index, base, limit, access, gran);
+}

src/drivers/cpu/gdt_helper.asm

@@ -0,0 +1,15 @@
+[GLOBAL flush_gdt]
+
+flush_gdt:
+    mov eax, [esp+4]
+    lgdt [eax]
+
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+    jmp 0x08:.flush
+.flush:
+    ret

src/include/gdt.h

@@ -0,0 +1,35 @@
+#ifndef __GDT_H__
+#define __GDT_H__
+
+#include "system.h"
+#include "printf.h"
+#include "string.h"
+
+extern void flush_gdt(uint32_t gdt_ptr);
+
+typedef struct gdt_entry
+{
+    uint16_t limit_low;
+    uint16_t base_low;
+    uint8_t  base_middle;
+    uint8_t  access;
+    uint8_t  granularity;
+    uint8_t  base_high;
+}__attribute__((packed))gdt_entry_t;
+
+typedef struct gdt_ptr
+{
+    uint16_t limit;
+    uint32_t base;
+}__attribute__((packed))gdt_ptr_t;
+
+#define GDT_MAX_ENTRIES 5
+#define GDT_GET_USER_SELECTOR(idx) (idx | 0x03)
+
+extern gdt_entry_t entries[GDT_MAX_ENTRIES];
+
+void init_gdt();
+
+void gdt_setGate(int32_t num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran);
+
+#endif /*__GDT_H__*/

src/include/system.h

@@ -26,7 +26,7 @@ void kernel_panic(char* message);
 * RELN: index of current release in the current month
 * STATUS: [PR]:Prerelease, [AL]:alpha, [NR]:Normal release
 */
-#define KERNEL_VERSION "0.23.03.4ALPR"
+#define KERNEL_VERSION "0.23.03.5ALPR"
 
 #define KB 1024
 #define MB (1024*KB)

src/kernel/kernel.c

@@ -3,6 +3,7 @@
 #include "serial.h"
 #include "vga_text.h"
 #include "printf.h"
+#include "gdt.h"
 
 void kernelmain(const multiboot_info_t* info, uint32_t multiboot_magic)
 {
@@ -12,6 +13,8 @@ void kernelmain(const multiboot_info_t* info, uint32_t multiboot_magic)
     text_chcolor(VGA_LIGHT_GREEN, VGA_BLACK);
     text_clear();
 
+    init_gdt();
+
     if(multiboot_magic != 0x2BADB002)
     {
         serial_puts("multiboot bootloader magic is invalid\n");