feat: Added nanoprintf, kprintf and GDT

kernel/GNUmakefile

@@ -3,7 +3,6 @@ MAKEFLAGS += -rR
 .SUFFIXES:
 
 SRCDIR := src
-INCDIR := src
 OBJDIR := build
 BINDIR := bin
 
@@ -18,7 +17,7 @@ CFLAGS := -g -O2 -pipe -Wall -Wextra -Werror -std=gnu11 -ffreestanding \
           -ffunction-sections -fdata-sections -m64 -march=x86-64 \
           -mno-80387 -mno-mmx -mno-sse -mno-sse2 -mno-red-zone \
           -mcmodel=kernel -Wno-unused-variable
-CPPFLAGS := -I$(INCDIR) -I$(SRCDIR) -MMD -MP -DLIMINE_API_REVISION=3
+CPPFLAGS := -I../external -I$(SRCDIR) -MMD -MP -DLIMINE_API_REVISION=3
 NASMFLAGS := -F dwarf -g
 LDFLAGS := -nostdlib -static -z max-page-size=0x1000 -Wl,--gc-sections \
            -T linker.ld -Wl,-m,elf_x86_64

kernel/src/arch/gdt.c

@@ -0,0 +1,39 @@
+#include <arch/gdt.h>
+
+gdt_entry_t gdt[5];
+gdt_ptr_t gdt_ptr;
+
+void gdt_init()
+{
+    gdt[0] = (gdt_entry_t){0, 0, 0, 0x00, 0x00, 0};                               // Null descriptor
+    gdt[1] = (gdt_entry_t){0, 0, 0, GDT_KERNEL_CODE, GDT_GRANULARITY_FLAT, 0};    // Kernel code segment
+    gdt[2] = (gdt_entry_t){0, 0, 0, GDT_KERNEL_DATA, GDT_GRANULARITY_FLAT, 0};    // Kernel data segment
+    gdt[3] = (gdt_entry_t){0, 0, 0, GDT_USER_CODE, GDT_GRANULARITY_LONG_MODE, 0}; // User code segment
+    gdt[4] = (gdt_entry_t){0, 0, 0, GDT_USER_DATA, 0x00, 0};                      // User data segment
+
+    gdt_ptr.limit = (uint16_t)(sizeof(gdt) - 1);
+    gdt_ptr.base = (uint64_t)&gdt;
+
+    gdt_flush(gdt_ptr);
+}
+
+void gdt_flush(gdt_ptr_t gdt_ptr)
+{
+    __asm__ volatile(
+        "mov %0, %%rdi\n"
+        "lgdt (%%rdi)\n"
+        "push $0x8\n"
+        "lea 1f(%%rip), %%rax\n"
+        "push %%rax\n"
+        "lretq\n"
+        "1:\n"
+        "mov $0x10, %%ax\n"
+        "mov %%ax, %%es\n"
+        "mov %%ax, %%ss\n"
+        "mov %%ax, %%gs\n"
+        "mov %%ax, %%ds\n"
+        "mov %%ax, %%fs\n"
+        :
+        : "r"(&gdt_ptr)
+        : "memory");
+}

kernel/src/arch/gdt.h

@@ -0,0 +1,52 @@
+#ifndef GDT_H
+#define GDT_H
+
+#include <stdint.h>
+
+// GDT Access Flags
+#define GDT_ACCESS_PRESENT 0x80    // Segment is present
+#define GDT_ACCESS_RING0 0x00      // Privilege Level 0 (Kernel)
+#define GDT_ACCESS_RING3 0x60      // Privilege Level 3 (User)
+#define GDT_ACCESS_SYSTEM 0x00     // System segment (e.g., TSS)
+#define GDT_ACCESS_CODE 0x18       // Code segment (Executable)
+#define GDT_ACCESS_DATA 0x10       // Data segment (Readable/Writable)
+#define GDT_ACCESS_DIRECTION 0x04  // Direction bit (0 = Grows up, 1 = Grows down)
+#define GDT_ACCESS_EXECUTABLE 0x08 // Executable segment
+#define GDT_ACCESS_RW 0x02         // Readable for code, Writable for data
+#define GDT_ACCESS_ACCESSED 0x01   // CPU sets this when accessed
+
+// Common Access Flags
+#define GDT_KERNEL_CODE (GDT_ACCESS_PRESENT | GDT_ACCESS_RING0 | GDT_ACCESS_CODE | GDT_ACCESS_EXECUTABLE | GDT_ACCESS_RW)
+#define GDT_KERNEL_DATA (GDT_ACCESS_PRESENT | GDT_ACCESS_RING0 | GDT_ACCESS_DATA | GDT_ACCESS_RW)
+#define GDT_USER_CODE (GDT_ACCESS_PRESENT | GDT_ACCESS_RING3 | GDT_ACCESS_CODE | GDT_ACCESS_EXECUTABLE | GDT_ACCESS_RW)
+#define GDT_USER_DATA (GDT_ACCESS_PRESENT | GDT_ACCESS_RING3 | GDT_ACCESS_DATA | GDT_ACCESS_RW)
+#define GDT_TSS 0xE9
+
+// Granularity Flags
+#define GDT_GRANULARITY_4K 0x80
+#define GDT_GRANULARITY_32B 0x40
+#define GDT_GRANULARITY_LONG_MODE 0x20
+#define GDT_GRANULARITY_FLAT (GDT_GRANULARITY_4K | GDT_GRANULARITY_LONG_MODE)
+
+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;
+    uint64_t base;
+} __attribute__((packed)) gdt_ptr_t;
+
+extern gdt_ptr_t gdt_ptr;
+
+void gdt_init();
+void gdt_flush(gdt_ptr_t gdt_ptr);
+
+#endif // GDT_H

kernel/src/emk.c

@@ -3,6 +3,9 @@
 #include <arch/cpu.h>
 #include <arch/io.h>
 #include <dev/serial.h>
+#include <util/kprintf.h>
+#include <util/log.h>
+#include <arch/gdt.h>
 
 __attribute__((used, section(".limine_requests"))) static volatile LIMINE_BASE_REVISION(3);
 __attribute__((used, section(".limine_requests_start"))) static volatile LIMINE_REQUESTS_START_MARKER;
@@ -18,10 +21,11 @@ void emk_entry(void)
 
     if (!LIMINE_BASE_REVISION_SUPPORTED)
     {
-        serial_write(COM1, (uint8_t *)"ERROR: Limine base revision is not supported\n", 45);
+        early("ERROR: Limine base revision is not supported\n");
         hcf();
     }
 
-    serial_write(COM1, (uint8_t *)"Hello, World!\n", 14);
+    gdt_init();
+    early("Initialized GDT");
     hlt();
 }

kernel/src/util/kprintf.c

@@ -0,0 +1,28 @@
+#include <dev/serial.h>
+#include <util/kprintf.h>
+
+#define NANOPRINTF_USE_FIELD_WIDTH_FORMAT_SPECIFIERS 1
+#define NANOPRINTF_USE_PRECISION_FORMAT_SPECIFIERS 1
+#define NANOPRINTF_USE_LARGE_FORMAT_SPECIFIERS 1
+#define NANOPRINTF_USE_SMALL_FORMAT_SPECIFIERS 1
+#define NANOPRINTF_USE_FLOAT_FORMAT_SPECIFIERS 0
+#define NANOPRINTF_USE_BINARY_FORMAT_SPECIFIERS 1
+#define NANOPRINTF_USE_WRITEBACK_FORMAT_SPECIFIERS 0
+#define NANOPRINTF_IMPLEMENTATION
+#include <nanoprintf.h>
+
+int kprintf(const char *fmt, ...)
+{
+    va_list args;
+    va_start(args, fmt);
+    char buffer[1024];
+    int length = npf_vsnprintf(buffer, sizeof(buffer), fmt, args);
+
+    if (length >= 0 && length < (int)sizeof(buffer))
+    {
+        serial_write(COM1, (uint8_t *)buffer, length);
+    }
+
+    va_end(args);
+    return length;
+}

kernel/src/util/kprintf.h

@@ -0,0 +1,7 @@
+#ifndef KPRINTF_H
+#define KPRINTF_H
+
+/* Minimal kprintf using nanoprintf, see external/nanoprintf.h */
+int kprintf(const char *fmt, ...);
+
+#endif // KPRINTF_H

kernel/src/util/log.h

@@ -0,0 +1,8 @@
+#ifndef LOG_H
+#define LOG_H
+
+#include <util/kprintf.h>
+
+#define early(fmt, ...) kprintf("early: " fmt "\n", ##__VA_ARGS__)
+
+#endif // LOG_H