From e87e6e387138a3ad01e0b6523bd648f26fffb040 Mon Sep 17 00:00:00 2001
From: Nathan Singer <nathan.r.singer@gmx.com>
Date: Thu, 29 May 2025 11:33:40 -0400
Subject: [PATCH] adds an idt

---
 kernel/arch/idt/idt.c           | 43 ++++++++++++++++++++++++++
 kernel/arch/idt/idt_src.s       | 54 +++++++++++++++++++++++++++++++++
 kernel/include/kernel/x86/idt.h | 26 ++++++++++++++++
 kernel/kmain.c                  |  2 ++
 4 files changed, 125 insertions(+)
 create mode 100644 kernel/arch/idt/idt.c
 create mode 100644 kernel/arch/idt/idt_src.s
 create mode 100644 kernel/include/kernel/x86/idt.h

diff --git a/kernel/arch/idt/idt.c b/kernel/arch/idt/idt.c
new file mode 100644
index 0000000..5be9f1a
--- /dev/null
+++ b/kernel/arch/idt/idt.c
@@ -0,0 +1,43 @@
+#include <stdbool.h>
+
+#include <kernel/x86/idt.h>
+
+__attribute__((aligned(0x10)))
+static idt_entry_t idt[256];
+
+static idtr_t idtr;
+
+static bool vectors[IDT_MAX_DESCRIPTORS];
+
+void exception_handler()
+{
+	__asm__ volatile ("cli; hlt"); // hangs the computer
+}
+
+void idt_set_descriptor(uint8_t vector, void *isr, uint8_t flags)
+{
+	idt_entry_t* descriptor = &idt[vector];
+
+	descriptor->isr_low = (uint32_t) isr & 0xFFFF;
+	descriptor->kernel_cs = 0x08; // this is whatever the kernel code selector is in the GDT
+	descriptor->attributes = flags;
+	descriptor->isr_high = (uint32_t) isr >> 16;
+	descriptor->reserved = 0;
+}
+
+
+extern void* isr_stub_table[];
+void idt_init(void)
+{
+	idtr.base = (uintptr_t)&idt[0];
+	idtr.limit = (uint16_t) sizeof(idt_entry_t) * IDT_MAX_DESCRIPTORS - 1;
+
+	for (uint8_t vector = 0; vector < 32; vector++) {
+		idt_set_descriptor(vector, isr_stub_table[vector], 0x8E);
+		vectors[vector] = true;
+	}
+
+	__asm__ volatile("lidt %0" : : "m"(idtr)); // load the new IDT
+	__asm__ volatile("sti"); // set the interrupt flag
+}
+
diff --git a/kernel/arch/idt/idt_src.s b/kernel/arch/idt/idt_src.s
new file mode 100644
index 0000000..176a639
--- /dev/null
+++ b/kernel/arch/idt/idt_src.s
@@ -0,0 +1,54 @@
+extern exception_handler
+
+%macro isr_err_stub 1
+isr_stub_%+%1:
+	call exception_handler
+	iret
+%endmacro
+
+%macro isr_no_err_stub 1
+isr_stub_%+%1:
+	call exception_handler
+	iret
+%endmacro
+
+isr_no_err_stub 0
+isr_no_err_stub 1
+isr_no_err_stub 2
+isr_no_err_stub 3
+isr_no_err_stub 4
+isr_no_err_stub 5
+isr_no_err_stub 6
+isr_no_err_stub 7
+isr_err_stub    8
+isr_no_err_stub 9
+isr_err_stub    10
+isr_err_stub    11
+isr_err_stub    12
+isr_err_stub    13
+isr_err_stub    14
+isr_no_err_stub 15
+isr_no_err_stub 16
+isr_err_stub    17
+isr_no_err_stub 18
+isr_no_err_stub 19
+isr_no_err_stub 20
+isr_no_err_stub 21
+isr_no_err_stub 22
+isr_no_err_stub 23
+isr_no_err_stub 24
+isr_no_err_stub 25
+isr_no_err_stub 26
+isr_no_err_stub 27
+isr_no_err_stub 28
+isr_no_err_stub 29
+isr_err_stub    30
+isr_no_err_stub 31
+
+global isr_stub_table
+isr_stub_table:
+%assign i 0
+%rep 32
+	dd isr_stub_%+i
+%assign i i+1
+%endrep
diff --git a/kernel/include/kernel/x86/idt.h b/kernel/include/kernel/x86/idt.h
new file mode 100644
index 0000000..968507d
--- /dev/null
+++ b/kernel/include/kernel/x86/idt.h
@@ -0,0 +1,26 @@
+#include <stdint.h>
+
+#ifndef ARCH_IDT_H
+#define ARCH_IDT_H
+
+#define IDT_MAX_DESCRIPTORS 32 // number of entries in the idt table, 32 i believe
+
+void exception_handler(void);
+
+typedef struct {
+	uint16_t isr_low;	// The lower 16 bits of the ISR's address
+	uint16_t kernel_cs;	// The GDT segment selector that the CPU will load into CS before calling the ISR
+	uint8_t reserved;	// set to zero
+	uint8_t attributes;	// Type and attributes
+	uint16_t isr_high;	// The higher 16 bits of the ISR's address
+} __attribute__((packed)) idt_entry_t;
+
+typedef struct {
+	uint16_t limit;
+	uint32_t base;
+} __attribute__((packed)) idtr_t;
+
+void idt_set_descriptor(uint8_t vector, void* isr, uint8_t flags);
+void idt_init(void);
+
+#endif
diff --git a/kernel/kmain.c b/kernel/kmain.c
index 76f9e77..fc0fe92 100644
--- a/kernel/kmain.c
+++ b/kernel/kmain.c
@@ -4,6 +4,7 @@
 #include <kernel/tty.h>
 #include <kernel/serial.h>
 #include <kernel/x86/gdt.h>
+#include <kernel/x86/idt.h>
 
 #define GDT_SIZE 5
 
@@ -27,6 +28,7 @@ void gdt_init(void)
 void kmain(void)
 {
 	gdt_init();		
+	idt_init();
 
 	terminal_initialize();
 	serial_initialize();