No more double faults B-)

Makefile

@@ -25,7 +25,9 @@ ASM_SOURCES := $(shell find $(SOURCE_DIR) -name '*.S')
 OBJECTS := $(patsubst $(SOURCE_DIR)/%, $(BUILD_DIR)/%, $(C_SOURCES:.c=.o) $(ASM_SOURCES:.S=.o))
 DEPENDS := $(patsubst $(SOURCE_DIR)/%, $(BUILD_DIR)/%, $(C_SOURCES:.c=.d))
 
-CFLAGS := -MMD -ffreestanding -O1 -Wall -Wextra -Werror -std=c2x -I$(SOURCE_DIR)/include -no-pie -fstack-protector-strong
+CFLAGS := -MMD -ffreestanding -nostdlib -O1 -Wall -Wextra -Werror -std=c2x -I$(SOURCE_DIR)/include -no-pie -fstack-protector-strong -g3
+CFLAGS += -Wno-unused-function
+CFLAGS += -Wno-unused-variable
 ASFLAGS :=
 
 #$(info C_SOURCES is $(C_SOURCES))
@@ -34,7 +36,7 @@ ASFLAGS :=
 #$(info DEPENDS is $(DEPENDS))
 
 run: myos.iso
-	qemu-system-i386 -cdrom myos.iso
+	qemu-system-i386 -d int -no-reboot -cdrom myos.iso
 
 cross-compiler: cross-compiler-image/Dockerfile
 	podman build cross-compiler-image -t cc-i686

interrupts.h

@@ -1,3 +0,0 @@
-#pragma once
-
-__attribute__((interrupt)) void interrupt_handler_1(struct interrupt_frame* frame);

src/boot.S

@@ -33,7 +33,7 @@ undefined behavior.
 .section .bss
 .align 16
 stack_bottom:
-.skip 16384 # 16 KiB
+.skip 1024 * 16 # 16384, 16 KiB
 stack_top:
 
 /*

src/include/libc.h

@@ -4,7 +4,8 @@
 #include "str.h"
 #include "tty.h"
 
-__attribute__((noreturn)) void panic(struct str s);
+__attribute__((noreturn))
+void panic(struct str s);
 
 void* memmove(void *dest, const void *src, size_t n);
 void* memset(void *s, int c, size_t n);

src/kernel/gdt.c

@@ -32,23 +32,23 @@ struct gdt_table_entry gdt_encode_entry(struct gdt_entry_content content)
     return entry;
 }
 
-void gdt_set(uint16_t size, const struct gdt_table_entry base[], uint32_t offset)
+//void gdt_load(uint16_t size, const struct gdt_table_entry base[], uint32_t offset)
-{
+//{
-    base += offset;
+//    base += offset;
-
+//
-    /* the lgdt instruction requires a packed alignment */
+//    /* the lgdt instruction requires a packed alignment */
-    struct __attribute__((packed)) gdtr  {
+//    struct __attribute__((packed)) gdtr  {
-        uint16_t size;
+//        uint16_t size;
-        uint32_t base;
+//        uint32_t base;
-    } gdt = {
+//    } gdt = {
-        .size = size,
+//        .size = size,
-        .base = (uint32_t)base,
+//        .base = (uint32_t)base,
-    };
+//    };
-
+//
-    __asm__ volatile (
+//    __asm__ volatile (
-        "lgdt %[gdt]"
+//        "lgdt %[gdt]"
-        :
+//        :
-        : [gdt] "m"(gdt)
+//        : [gdt] "m"(gdt)
-    );
+//    );
-}
+//}
 

src/kernel/gdt.h

@@ -1,5 +1,6 @@
 #pragma once
 #include <stdint.h>
+#include "types.h"
 
 #define GDT_LIMIT_MAX 0xFFFFF
 
@@ -48,16 +49,14 @@ enum gdt_access_flag : uint8_t {
 enum gdt_flags : uint8_t {
     GDT_RESERVED             = (1<<0),
     GDT_LONG                 = (1<<1),
-    GDT_SIZE        = (1<<2),
+    GDT_32BIT                = (1<<2),
-    GDT_GRANULARITY = (1<<3),
+    GDT_GRANULARITY_PAGEWISE = (1<<3),
 };
 
-void gdt_set(uint16_t limit, const struct gdt_table_entry base[], uint32_t offset);
-
 struct gdt_table_entry gdt_encode_entry(struct gdt_entry_content content);
 
 // only compiles with O1 or higher ¯\_(ツ)_/¯, might have to make this a macro
-static inline void gdt_reload_granular(uint32_t code,
+static inline void gdt_flush_granular(uint32_t code,
                                       uint32_t data,
                                       uint32_t extra_segment,
                                       uint32_t general_segment_1,
@@ -88,10 +87,19 @@ static inline void gdt_reload_granular(uint32_t code,
         : "ax" // Clobbered register
     );
 }
-
+static inline void gdt_load(struct gdt_table_entry base[], uint16_t gdt_size, segment_t data, segment_t code)
-static inline void gdt_reload(uint32_t data, uint32_t code)
 {
+    /* the lgdt instruction requires a packed alignment */
+    struct __attribute__((packed)) {
+        uint16_t size;
+        uint32_t base;
+    } gdt = {
+        .size = gdt_size,
+        .base = (uint32_t)base,
+    };
+
     __asm__ volatile (
+		"lgdt %[gdt]\n"
         // Far jump to reload the CS register
         "jmp %[cs], $reload_CS\n"
         "reload_CS:\n"
@@ -103,7 +111,8 @@ static inline void gdt_reload(uint32_t data, uint32_t code)
         "movw %%ax,  %%gs\n"
         "movw %%ax,  %%ss\n"
         : // No output operands
-        : [ds] "i"(data),
+        : [gdt] "m"(gdt),
+		  [ds] "i"(data),
           [cs] "i"(code)
         : "ax" // Clobbered register
     );

src/kernel/idt.c

@@ -3,17 +3,18 @@
 
 #include "idt.h"
 
-struct idt_gate_descriptor idt_encode_descriptor(uint32_t offset, uint16_t segment_selector, uint8_t ist, enum idt_flags type)
+struct idt_gate_descriptor idt_encode_descriptor(void* func, segment_t segment_selector, enum idt_dpl dpl, enum idt_flags type)
 {
     struct idt_gate_descriptor output = {0};
 
+	uint32_t offset = (uint32_t)func;
+
     output.offset_0 = offset & 0xFFFF;
     offset >>= 16;
     output.offset_1 = offset & 0xFFFF;
 
     output.segment_selector = segment_selector;
-    output.ist              = ist;
+    output.flags = type | dpl | IDT_PRESENT;
-    output.flags            = type | IDT_DPL_0 | IDT_PRESENT;
 
     return output;
 }

src/kernel/idt.h

@@ -1,11 +1,13 @@
 #pragma once
 
 #include <stdint.h>
+#include "types.h"
 
 struct __attribute__((packed)) idt_gate_descriptor {
     uint16_t offset_0;
     uint16_t segment_selector;
-    uint8_t ist;
+
+    uint8_t ist; // unused in 32-bit
 
     /* `flags`
        =======
@@ -20,22 +22,26 @@ struct __attribute__((packed)) idt_gate_descriptor {
     uint16_t offset_1;
 };
 
-enum idt_flags : uint8_t {
+_Static_assert(sizeof(struct idt_gate_descriptor) == 8);
-    IDT_GATE_TYPE_TASK_GATE        = 0x5,
-    IDT_GATE_TYPE_INTERUPT_GATE_16 = 0x6,
-    IDT_GATE_TYPE_TRAP_GATE_16     = 0x7,
-    IDT_GATE_TYPE_INTERUPT_GATE_32 = 0xE,
-    IDT_GATE_TYPE_TRAP_GATE_32     = 0xF,
 
-    IDT_DPL_0 = 0 << 5,
+enum idt_flags : uint8_t {
-    IDT_DPL_1 = 1 << 5,
+    IDT_GATE_TYPE_TASK        = 0x5,
-    IDT_DPL_2 = 2 << 5,
+    IDT_GATE_TYPE_INTERRUPT16 = 0x6,
-    IDT_DPL_3 = 3 << 5,
+    IDT_GATE_TYPE_TRAP16      = 0x7,
+    IDT_GATE_TYPE_INTERRUPT32 = 0xE,
+    IDT_GATE_TYPE_TRAP32      = 0xF,
     
     IDT_PRESENT = 1 << 7,
 };
 
-struct idt_gate_descriptor idt_encode_descriptor(uint32_t offset, uint16_t segment_selector, uint8_t ist, enum idt_flags type);
+enum idt_dpl : uint8_t {
+    IDT_DPL_0 = 0 << 5,
+    IDT_DPL_1 = 1 << 5,
+    IDT_DPL_2 = 2 << 5,
+    IDT_DPL_3 = 3 << 5,
+};
+
+struct idt_gate_descriptor idt_encode_descriptor(void* func, segment_t segment_selector, enum idt_dpl dpl, enum idt_flags type);
 
 void idt_load(struct idt_gate_descriptor table[], uint16_t size);
 

src/kernel/interrupts.c

@@ -1,11 +1,10 @@
 #include <stdint.h>
 #include "libc.h"
+#include "tty.h"
+#include "interrupts.h"
+#include "kernel_state.h"
 
-#ifdef __x86_64__
+#define EXCEPTION_DEPTH_MAX 5
-typedef unsigned long long int uword_t;
-#else
-typedef unsigned int uword_t;
-#endif
 
 struct __attribute__((packed)) interrupt_frame {
     uword_t ip;
@@ -15,7 +14,131 @@ struct __attribute__((packed)) interrupt_frame {
     uword_t ss;
 };
 
-__attribute__((interrupt)) void interrupt_handler_1(struct interrupt_frame* frame)
+static void print_interrupt_frame(struct interrupt_frame* f)
+{
+    printf(str_attach(
+           "Interrupt frame:\n"
+           "================\n"
+           "ip:    {x32}\n"
+           "cs:    {x32}\n"
+           "flags: {x32}\n"
+           "sp:    {x32}\n"
+           "ss:    {x32}\n"),
+           f->ip,
+           f->cs,
+           f->flags,
+           f->sp,
+           f->ss);
+}
+
+/**
+ * Exceptions
+ * ==========
+ */
+__attribute__((interrupt, noreturn))
+void exception_handler_general_protection_fault(struct interrupt_frame* frame, int err)
+{
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    //terminal_clear();
+    terminal_set_color(VGA_COLOR_WHITE, VGA_COLOR_RED);
+    printf(str_attach(
+        "general protection fault by segment selector {str} :(\n"),
+        gdt_segment_index_str[err/8]);
+    if (frame != NULL) {
+        print_interrupt_frame(frame);
+    }
+    __asm__ volatile("cli; hlt");
+    __builtin_unreachable();
+}
+
+__attribute__((interrupt, noreturn))
+void exception_handler_double_fault(struct interrupt_frame* frame)
 {
     (void)frame;
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    panic(str_attach("double fault :("));
 }
+
+__attribute__((interrupt, noreturn))
+void exception_handler_div_by_zero(struct interrupt_frame* frame)
+{
+    (void)frame;
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    panic(str_attach("div by zero occured :("));
+}
+
+__attribute__((interrupt, noreturn))
+void exception_handler_page_fault(struct interrupt_frame* frame, int err)
+{
+    (void)frame;
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    terminal_set_color(VGA_COLOR_WHITE, VGA_COLOR_RED);
+    printf(str_attach(
+        "page fault :(, err: 0x{x32}\n"),
+        err,
+        gdt_segment_index_str[err/8]);
+    __asm__ volatile("cli; hlt");
+    __builtin_unreachable();
+}
+
+__attribute__((interrupt, noreturn))
+void exception_default(struct interrupt_frame* frame)
+{
+    (void)frame;
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    panic(str_attach("non-implemented exception occured"));
+    
+    kernel.nested_exception_counter = 0;
+}
+
+/**
+ * Interrupts
+ * ==========
+ */
+__attribute__((interrupt, noreturn))
+void interrupt_default(struct interrupt_frame* frame)
+{
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    (void)frame;
+    panic(str_attach("non-implemented interrupt invoked"));
+    
+    kernel.nested_exception_counter = 0;
+}
+
+__attribute__((interrupt))
+void interrupt_handler_1(struct interrupt_frame* frame)
+{
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    (void)frame;
+    printf(str_attach("interrupt_handler_1 called!\n"));
+
+    kernel.nested_exception_counter = 0;
+}
+
+__attribute__((interrupt))
+void interrupt_handler_userspace_exit(struct interrupt_frame* frame)
+{
+    if (kernel.nested_exception_counter++ > EXCEPTION_DEPTH_MAX) {
+        panic(str_attach("fatal: too many nested exceptions\n"));
+    }
+    (void)frame;
+    printf(str_attach("interrupt_handler_userspace_exit called!\n"));
+
+    kernel.nested_exception_counter = 0;
+}
+
+

src/kernel/interrupts.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include "types.h"
+
+struct __attribute__((packed)) interrupt_frame {
+    uword_t ip;
+    uword_t cs;
+    uword_t flags;
+    uword_t sp;
+    uword_t ss;
+};
+
+__attribute__((interrupt, noreturn))
+void exception_handler_div_by_zero(struct interrupt_frame* frame);
+
+__attribute__((interrupt, noreturn))
+void exception_handler_general_protection_fault(struct interrupt_frame* frame, int err);
+
+__attribute__((interrupt, noreturn))
+void exception_handler_double_fault(struct interrupt_frame* frame);
+
+__attribute__((interrupt, noreturn))
+void exception_default(struct interrupt_frame* frame);
+
+__attribute__((interrupt, noreturn))
+void interrupt_default(struct interrupt_frame* frame);
+
+__attribute__((interrupt))
+void interrupt_handler_1(struct interrupt_frame* frame);
+
+__attribute__((interrupt))
+void interrupt_handler_userspace_exit(struct interrupt_frame* frame);
+

src/kernel/kernel.c

@@ -5,34 +5,51 @@
 #include "gdt.h"
 #include "tss.h"
 #include "idt.h"
+#include "interrupts.h"
+#include "types.h"
+#include "kernel_state.h"
 
 // Future user-space
 #include "libc.h"
 #include "tty.h"
+#include "str.h"
 
-// TOOD: clean up this
+static void pic_disable()
-typedef void(*func_t)(void*);
-
-static void ring3_mode(uint32_t, uint32_t, uint32_t, uint32_t, func_t);
-static void user_mode_code(void*);
-
-void gdt_setup_flat_model()
 {
+    __asm__ volatile ("outb %0, %1"
+            :
+            : "a"((uint8_t)0xff), "Nd"((uint16_t)0x21)
+            : "memory");
+    __asm__ volatile ("outb %0, %1"
+            :
+            : "a"((uint8_t)0xff), "Nd"((uint16_t)0xA1)
+            : "memory");
 }
 
-/* copied and only slightly modified from an osdev wiki article with poor
-   taste, TODO: change
-   ----------------------------------------------*/
 static void user_mode_code(void*)
 {
-    printf(str_attach("hello world :)\n"));
+    printf(str_attach("hello from user-space before interrupt :)\n"));
+    __asm__ volatile ("int $0x80");
+    __asm__ volatile ("int $0x81");
+    //while (1) /* busy loop */;
+
+#if 0
+    printf(str_attach("hello from user-space before exception :)\n"));
+    /* test division by 0 exception */
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wdiv-by-zero"
+    volatile int a = 5/0;
+    (void)a;
+    #pragma GCC diagnostic pop
+
+    // should not happen
+    printf(str_attach("hello from userspace after interrupt and exception!\n"));
+#endif
+    
 }
 
-static void ring3_mode(uint32_t udata_offset, uint32_t udata_rpl, uint32_t ucode_offset, uint32_t ucode_rpl, func_t callback)
+static void ring3_mode(segment_t udata_segment, segment_t ucode_segment, func_t callback)
 {
-    const uint32_t udata = udata_offset | udata_rpl;
-    const uint32_t ucode = ucode_offset | ucode_rpl;
-
     __asm__ volatile (
         "mov %[udata], %%ax\n"
         "mov %%ax, %%ds\n"
@@ -42,15 +59,15 @@ static void ring3_mode(uint32_t udata_offset, uint32_t udata_rpl, uint32_t ucode
         "push %%ax\n"
 
         "mov %%esp, %%eax\n"
-        "push %%eax\n"       // current esp
+        "push %%eax\n"       // esp
         "pushf\n"            // eflags
         "push %[ucode]\n"
         "push %[callback]\n" // instruction address to return to
         "iret"
         :
-        : [udata]    "m"(udata),
+        : [udata]    "i"(udata_segment),
-          [ucode]    "m"(ucode),
+          [ucode]    "i"(ucode_segment),
-          [callback] "m"(callback)
+          [callback] "i"(callback)
         : "eax"
     );
 }
@@ -65,25 +82,12 @@ void kernel_main(void)
 {
     __asm__ volatile("cli");
 
-    static struct tss tss = {0};
+    _Static_assert(sizeof(kernel.gdt) == 0x30);
 
-    enum segment_index : size_t {
+    kernel.gdt[SEGMENT_NULL] = gdt_encode_entry((struct gdt_entry_content){0});
-        null_segment        = 0,
-        kernel_code_segment = 1,
-        kernel_data_segment = 2,
-        user_code_segment   = 3,
-        user_data_segment   = 4,
-        task_state_segment  = 5,
-        segment_count,
-    };
-
-    static struct gdt_table_entry gdt[segment_count];
-    _Static_assert(sizeof(gdt) == 0x30);
-
-    gdt[null_segment] = gdt_encode_entry((struct gdt_entry_content){0});
 
     /* kernel */
-    gdt[kernel_code_segment] = gdt_encode_entry((struct gdt_entry_content){
+    kernel.gdt[SEGMENT_KERNEL_CODE] = gdt_encode_entry((struct gdt_entry_content){
                      .base = 0,
                      .limit = GDT_LIMIT_MAX,
                      .access_byte = GDT_ACCESS_RW
@@ -91,21 +95,21 @@ void kernel_main(void)
                                   | GDT_ACCESS_DESCRIPTOR
                                   | GDT_ACCESS_DPL_0
                                   | GDT_ACCESS_PRESENT,
-                     .flags = GDT_SIZE
+                     .flags = GDT_32BIT
-                            | GDT_GRANULARITY});
+                            | GDT_GRANULARITY_PAGEWISE});
 
-    gdt[kernel_data_segment] = gdt_encode_entry((struct gdt_entry_content){
+    kernel.gdt[SEGMENT_KERNEL_DATA] = gdt_encode_entry((struct gdt_entry_content){
                      .base = 0,
                      .limit = GDT_LIMIT_MAX,
                      .access_byte = GDT_ACCESS_RW
                                   | GDT_ACCESS_DESCRIPTOR
                                   | GDT_ACCESS_DPL_0
                                   | GDT_ACCESS_PRESENT,
-                     .flags = GDT_SIZE
+                     .flags = GDT_32BIT
-                            | GDT_GRANULARITY});
+                            | GDT_GRANULARITY_PAGEWISE});
 
     /* user */
-    gdt[user_code_segment] = gdt_encode_entry((struct gdt_entry_content) {
+    kernel.gdt[SEGMENT_USER_CODE] = gdt_encode_entry((struct gdt_entry_content) {
                      .base = 0,
                      .limit = GDT_LIMIT_MAX,
                      .access_byte = GDT_ACCESS_RW
@@ -113,54 +117,111 @@ void kernel_main(void)
                                   | GDT_ACCESS_DESCRIPTOR
                                   | GDT_ACCESS_DPL_3
                                   | GDT_ACCESS_PRESENT,
-                     .flags = GDT_SIZE
+                     .flags = GDT_32BIT
-                            | GDT_GRANULARITY});
+                            | GDT_GRANULARITY_PAGEWISE});
 
-    gdt[user_data_segment] = gdt_encode_entry((struct gdt_entry_content) {
+    kernel.gdt[SEGMENT_USER_DATA] = gdt_encode_entry((struct gdt_entry_content) {
                      .base = 0,
                      .limit = GDT_LIMIT_MAX,
                      .access_byte = GDT_ACCESS_RW
                                   | GDT_ACCESS_DESCRIPTOR
                                   | GDT_ACCESS_DPL_3
                                   | GDT_ACCESS_PRESENT,
-                     .flags = GDT_SIZE
+                     .flags = GDT_32BIT
-                            | GDT_GRANULARITY});
+                            | GDT_GRANULARITY_PAGEWISE});
 
     /* tss */
-    gdt[task_state_segment] = gdt_encode_entry((struct gdt_entry_content) {
+    kernel.gdt[SEGMENT_TASK_STATE] = gdt_encode_entry((struct gdt_entry_content) {
-                     .base = (uint32_t)&tss,
+                     .base = (uint32_t)&kernel.tss,
-                     .limit = sizeof(tss)-1,
+                     .limit = sizeof(kernel.tss)-1,
                      .access_byte = GDT_ACCESS_ACCESSED
                                   | GDT_ACCESS_EXEC
                                   | GDT_ACCESS_DPL_0
                                   | GDT_ACCESS_PRESENT,
                      .flags = 0});
 
-    gdt_set(sizeof gdt, gdt, 0);
-
-    gdt_reload(kernel_data_segment * sizeof (struct gdt_table_entry),
-               kernel_code_segment * sizeof (struct gdt_table_entry));
-
     /* Setup the TSS */
-    tss.ss0 = kernel_data_segment * sizeof (struct gdt_table_entry);
+    memset(&kernel.tss, 0, sizeof kernel.tss);
-    tss.esp0 = 0; /* TODO: set kernel stack pointer */
+    kernel.tss.ss0 = segment(SEGMENT_KERNEL_DATA, SEGMENT_GDT, 0);
-
+#if 1
-    tss_load(task_state_segment * sizeof (struct gdt_table_entry));
+    static uint8_t kernel_stack[1024];
+    kernel.tss.esp0 = (uint32_t)kernel_stack;
+#else
+    kernel.tss.esp0 = 0;
+#endif
 
+#if 1
     /* Setup the IDT */
-	//static struct idt_gate_descriptor idt[256] = {0};
+    for (size_t i = 0; i < 32; i++) {
+        kernel.idt[i] = idt_encode_descriptor(
+               exception_default,
+               segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+               IDT_DPL_3,
+               IDT_GATE_TYPE_TRAP32);
+    }
+    for (size_t i = 32; i < IDT_COUNT; i++) {
+        kernel.idt[i] = idt_encode_descriptor(
+               interrupt_default,
+               segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+               IDT_DPL_3,
+               IDT_GATE_TYPE_INTERRUPT32);
+    }
 
-	//struct idt_gate_descriptor idt_encode_descriptor(uint32_t offset, uint16_t segment_selector, uint8_t ist, enum idt_flags type);
+    kernel.idt[0x0] = idt_encode_descriptor(
-	//idt[80] = idt_encode_descriptor(interrupt_handler_1, kernel_code_segment, 0, IDT_GATE_TYPE_INTERUPT_GATE_32);
+            exception_handler_div_by_zero,
+            segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+            IDT_DPL_3,
+            IDT_GATE_TYPE_TRAP32
+    );
 
-	//idt_load(idt, sizeof idt);
+    kernel.idt[0x8] = idt_encode_descriptor(
+            exception_handler_double_fault,
+            segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+            IDT_DPL_3,
+            IDT_GATE_TYPE_TRAP32
+    );
+
+    kernel.idt[0xD] = idt_encode_descriptor(
+            exception_handler_general_protection_fault,
+            segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+            IDT_DPL_3,
+            IDT_GATE_TYPE_TRAP32
+    );
+
+    // Interrupts
+    kernel.idt[0x80] = idt_encode_descriptor(
+            interrupt_handler_1,
+            segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+            IDT_DPL_3,
+            IDT_GATE_TYPE_INTERRUPT32
+    );
+
+    kernel.idt[0x81] = idt_encode_descriptor(
+            interrupt_handler_userspace_exit,
+            segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0),
+            IDT_DPL_3,
+            IDT_GATE_TYPE_INTERRUPT32
+    );
+#endif
+
+    /* Flush the gdt and tss */
+    gdt_load(kernel.gdt,
+             sizeof kernel.gdt,
+             segment(SEGMENT_KERNEL_DATA, SEGMENT_GDT, 0),
+             segment(SEGMENT_KERNEL_CODE, SEGMENT_GDT, 0));
+    tss_load(segment(SEGMENT_TASK_STATE, SEGMENT_GDT, 0));
+    idt_load(kernel.idt, sizeof kernel.idt);
+
+    /* enable interrupts */
+    __asm__ volatile("sti");
 
     /* Finally go to ring 3 */
-    ring3_mode(user_data_segment * sizeof (struct gdt_table_entry), 3,
+    printf(str_attach("hello from kernel space!\n"));
-               user_code_segment * sizeof (struct gdt_table_entry), 3,
+
+    pic_disable(); // temporary duct-tape 
+
+    ring3_mode(segment(SEGMENT_USER_DATA, SEGMENT_GDT, 3),
+               segment(SEGMENT_USER_CODE, SEGMENT_GDT, 3),
                user_mode_code);
-
-
-    __asm__ volatile("sti");
 }
 

src/kernel/kernel_state.c

@@ -0,0 +1,18 @@
+
+#include "str.h"
+
+#include "kernel_state.h"
+
+const struct str gdt_segment_index_str[SEGMENT_COUNT] = {
+    [SEGMENT_NULL       ] = str_attach("SEGMENT_NULL"),
+    [SEGMENT_KERNEL_CODE] = str_attach("SEGMENT_KERNEL_CODE"),
+    [SEGMENT_KERNEL_DATA] = str_attach("SEGMENT_KERNEL_DATA"),
+    [SEGMENT_USER_CODE  ] = str_attach("SEGMENT_USER_CODE"),
+    [SEGMENT_USER_DATA  ] = str_attach("SEGMENT_USER_DATA"),
+    [SEGMENT_TASK_STATE ] = str_attach("SEGMENT_TASK_STATE"),
+};
+
+const struct str idt_index_str[IDT_COUNT] = {
+};
+
+struct kernel_state kernel = { 0 };

src/kernel/kernel_state.h

@@ -0,0 +1,34 @@
+
+#pragma once
+
+#include "str.h"
+#include "tss.h"
+#include "idt.h"
+#include "gdt.h"
+
+enum gdt_segment_index : size_t {
+    SEGMENT_NULL,
+    SEGMENT_KERNEL_CODE,
+    SEGMENT_KERNEL_DATA,
+    SEGMENT_USER_CODE,
+    SEGMENT_USER_DATA,
+    SEGMENT_TASK_STATE,
+    SEGMENT_COUNT,
+};
+extern const struct str gdt_segment_index_str[SEGMENT_COUNT]; // reverse lookup enum -> str
+
+enum idt_index : size_t {
+    IDT_COUNT = 256
+};
+extern const struct str idt_index_str[IDT_COUNT]; // reverse lookup enum -> str
+
+constexpr size_t BACKTRACE_MAX = 256;
+
+struct kernel_state {
+    struct tss tss;
+    struct gdt_table_entry gdt[SEGMENT_COUNT];
+    struct idt_gate_descriptor idt[IDT_COUNT];
+	int nested_exception_counter;
+};
+
+extern struct kernel_state kernel;

src/kernel/types.h

@@ -0,0 +1,27 @@
+
+#pragma once
+
+
+typedef void(*func_t)(void*);
+
+/* define uword_t per the instruction set manual */
+#ifdef __x86_64__
+typedef unsigned long long int uword_t;
+#else
+typedef unsigned int uword_t;
+#endif
+
+/* segment selectors identifies segments either in the gdt or the ldt */
+typedef uint16_t segment_t;
+
+enum segment_ti : uint16_t {
+	SEGMENT_GDT = 0,
+	SEGMENT_LDT = 1,
+};
+
+static inline segment_t segment(uint16_t index, enum segment_ti ti, uint16_t rpl)
+{
+	return (index << 3) | ((ti & 0xF) << 1) | (rpl & 0xFF);
+}
+
+

src/lib/libc.c

@@ -15,10 +15,10 @@
 __attribute__((noreturn))
 void panic(struct str s)
 {
-    terminal_clear();
+    //terminal_clear();
     terminal_set_color(VGA_COLOR_WHITE, VGA_COLOR_RED);
     terminal_write(s);
-    __asm__ volatile("hlt");
+    __asm__ volatile("cli; hlt");
     __builtin_unreachable();
 }
 

src/lib/printf.c

@@ -31,12 +31,16 @@ static inline int ps_get(struct printf_state* s)
     return s->str.data[s->i++];
 }
 
-static int print_long(unsigned long n, struct str alphabet, bool is_signed)
+static int print_long(unsigned long n, struct str alphabet, bool is_signed, unsigned int padding, char pad_char)
 {
-    constexpr size_t BUF_SZ = 21;
+    constexpr size_t BUF_SZ = 128;
     char buf[BUF_SZ];
     size_t i = 0;
 
+    if (padding > BUF_SZ) {
+        return -1;
+    }
+
     if (n == 0) {
         terminal_putchar('0');
         return 1;
@@ -59,32 +63,55 @@ static int print_long(unsigned long n, struct str alphabet, bool is_signed)
         buf[BUF_SZ-i] = alphabet.data[n % alphabet.len];
         n /= alphabet.len;
     }
+    while (i < padding) {
+        i += 1;
+        buf[BUF_SZ-i] = pad_char;
+    }
 
     terminal_write((struct str){.data = &buf[BUF_SZ-i], .len = i});
 
     return i;
 }   
 
-static int print_i32(struct printf_state* s)
+static int print_i32(struct printf_state* s, int padding, char pad_char)
 {
+    padding  = padding ? padding : 0;
+    pad_char = pad_char ? pad_char : ' ';
     long n = va_arg(s->ap, int32_t);
-    return print_long(n, str_attach("0123456789"), true);
+    return print_long(n, str_attach("0123456789"), true, padding, pad_char);
 }
 
-static int print_u32(struct printf_state* s)
+static int print_u32(struct printf_state* s, int padding, char pad_char)
 {
+    padding  = padding ? padding : 0;
+    pad_char = pad_char ? pad_char : ' ';
     unsigned long n = va_arg(s->ap, uint32_t);
-    return print_long(n, str_attach("0123456789"), false);
+    return print_long(n, str_attach("0123456789"), false, padding, pad_char);
 }
 
-static int print_x32(struct printf_state* s)
+static int print_x32(struct printf_state* s, int padding, char pad_char)
 {
+    padding = padding ? padding : 0;
+    pad_char = pad_char ? pad_char : '0';
     unsigned long n = va_arg(s->ap, uint32_t);
-    return print_long(n, str_attach("0123456789abcdefgh"), false);
+    struct str alphabet = str_attach("0123456789abcdef");
+    return print_long(n, alphabet, false, padding, pad_char);
 }
 
-static int print_str(struct printf_state* s)
+static int print_b32(struct printf_state* s, int padding, char pad_char)
 {
+    padding = padding ? padding : 32;
+    pad_char = pad_char ? pad_char : '0';
+    unsigned long n = va_arg(s->ap, uint32_t);
+    struct str alphabet = str_attach("01");
+    return print_long(n, alphabet, false, padding, pad_char);
+}
+
+static int print_str(struct printf_state* s, int padding, char pad_char)
+{
+    // TODO: implement padding
+    (void)padding;
+    (void)pad_char;
     const struct str str = va_arg(s->ap, struct str);
     for (size_t i=0; i < str.len; i++) {
         terminal_putchar(str.data[i]);
@@ -97,6 +124,8 @@ static int parse_format_cmd(struct printf_state* s)
 {
     int c;
     uint32_t cmd = 0;
+    int pad = 0;
+    char pad_char = '\0';
 
     constexpr uint32_t CHAR_MASK = (1<<CHAR_BIT)-1;
 
@@ -112,18 +141,18 @@ static int parse_format_cmd(struct printf_state* s)
         // 16 bit types are promoted to 32 bit by va_arg(...) anyways
         case 'i16':
         case 'i32':
-			return print_i32(s);
+            return print_i32(s, pad, pad_char);
 
         case 'u16':
         case 'u32':
-			return print_u32(s);
+            return print_u32(s, pad, pad_char);
 
         case 'x16':
         case 'x32':
-			return print_x32(s);
+            return print_x32(s, pad, pad_char);
 
         case 'str':
-			return print_str(s);
+            return print_str(s, pad, pad_char);
 
         default:
             return -1;