From bd60d787b3691eaf04ef1d7e7f779466c62818f6 Mon Sep 17 00:00:00 2001
From: Marvin Borner
Date: Sun, 15 Sep 2019 00:03:16 +0200
Subject: Began global descriptor table

---
 src/gdt/gdt.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 src/gdt/gdt.h |  6 ++++++
 2 files changed, 57 insertions(+)
 create mode 100644 src/gdt/gdt.c
 create mode 100644 src/gdt/gdt.h

(limited to 'src')

diff --git a/src/gdt/gdt.c b/src/gdt/gdt.c
new file mode 100644
index 0000000..813b432
--- /dev/null
+++ b/src/gdt/gdt.c
@@ -0,0 +1,51 @@
+struct gdt_entry {
+    unsigned short limit_low;
+    unsigned short base_low;
+    unsigned char base_middle;
+    unsigned char access;
+    unsigned char granularity;
+    unsigned char base_high;
+} __attribute__((packed));
+
+struct gdt_ptr {
+    unsigned short limit;
+    unsigned int base;
+} __attribute__((packed));
+
+struct gdt_entry gdt[3];
+struct gdt_ptr gp;
+
+extern void gdt_flush();
+
+void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran) {
+    // Set descriptor base address
+    gdt[num].base_low = (base & 0xFFFF);
+    gdt[num].base_middle = (base >> 16) & 0xFF;
+    gdt[num].base_high = (base >> 24) & 0xFF;
+
+    // Set descriptor limits
+    gdt[num].limit_low = (limit & 0xFFFF);
+    gdt[num].granularity = ((limit >> 16) & 0x0F);
+
+    // Set granularity and access flags
+    gdt[num].granularity |= (gran & 0xF0);
+    gdt[num].access = access;
+}
+
+void gdt_install() {
+    // Set GDT pointer and limit
+    gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
+    gp.base = &gdt;
+
+    // NULL descriptor
+    gdt_set_gate(0, 0, 0, 0, 0);
+
+    // Code segment
+    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+
+    // Data segment
+    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+
+    // Remove old GDT and install the new changes!
+    gdt_flush();
+}
\ No newline at end of file
diff --git a/src/gdt/gdt.h b/src/gdt/gdt.h
new file mode 100644
index 0000000..46d80e2
--- /dev/null
+++ b/src/gdt/gdt.h
@@ -0,0 +1,6 @@
+#ifndef MELVIX_GDT_H
+#define MELVIX_GDT_H
+
+void gdt_install();
+
+#endif
-- 
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