From f15f5f4fc4233aae79f532ef82a93fc0eae71985 Mon Sep 17 00:00:00 2001
From: Gitea <gitea@fake.local>
Date: Tue, 1 Dec 2020 20:59:57 -0600
Subject: [PATCH] Some vague attempts at GDT stuff

---
 arch/i386/gdt.S | 34 ++++++++++++++++++++++++++++++++++
 arch/i386/gdt.c | 28 ++++++++++++++++++++++++++++
 arch/i386/gdt.h | 14 ++++++++++++++
 3 files changed, 76 insertions(+)
 create mode 100644 arch/i386/gdt.S
 create mode 100644 arch/i386/gdt.c
 create mode 100644 arch/i386/gdt.h

diff --git a/arch/i386/gdt.S b/arch/i386/gdt.S
new file mode 100644
index 0000000..1a9740a
--- /dev/null
+++ b/arch/i386/gdt.S
@@ -0,0 +1,34 @@
+	.globl set_gdt
+	.type set_gdt,%function
+
+gdtr:
+	.word 0
+	.int 0
+
+set_gdt:
+	.fnstart
+	mov 4(%esp), %eax
+	mov %eax, 2($gdtr)
+	mov 8(%esp), %ax
+	mov %ax, $gdtr
+	lgdt $gdtr
+	ret
+	.fnend
+
+	.globl reload_segments
+	.type reload_segments,%function
+	
+reload_segments:
+	.fnstart
+	jmp 0x08:reload_CS
+	.fnend
+	
+reload_CS:
+	mov 0x10, %ax
+	mov %ax, %ds
+	mov %ax, %es
+	mov %ax, %fs
+	mov %ax, %gs
+	mov %ax, %ss
+	ret
+	
diff --git a/arch/i386/gdt.c b/arch/i386/gdt.c
new file mode 100644
index 0000000..b3f9168
--- /dev/null
+++ b/arch/i386/gdt.c
@@ -0,0 +1,28 @@
+void encode_gdt_entry(unsigned short int * target, struct GDT source) {
+  if((source.limit > 65536) && ((source.limit & 0xFFF) != 0xFFF)) {
+    /* This needs to error out. */
+    return;
+  }
+  if(source.limit > 65536) {
+    /* Granularity adjustments */
+    source.limit = source.limit >> 12;
+    target[6] = 0xC0;
+  }
+  else {
+    target[6] = 0x40;
+  }
+
+  /* Encode limit */
+  target[0] = source.limit & 0xFF;
+  target[1] = (source.limit >> 8) & 0xFF;
+  target[6] |= (source.limit >> 16) & 0xF;
+
+  /* Encode base */
+  target[2] = source.base & 0xFF;
+  target[3] = (source.base >> 8) & 0xFF;
+  target[4] = (source.base >> 16) & 0xFF;
+  target[7] = (source.base >> 24) & 0xFF;
+
+  /* Encode type */
+  target[5] = source.type;
+}
diff --git a/arch/i386/gdt.h b/arch/i386/gdt.h
new file mode 100644
index 0000000..b8581df
--- /dev/null
+++ b/arch/i386/gdt.h
@@ -0,0 +1,14 @@
+#ifndef _ARCH_I386_GDT_H
+#define _ARCH_I386_GDT_H
+
+struct GDT {
+  unsigned int limit;
+  unsigned long int base;
+  unsigned short int type;
+};
+
+void encode_gdt_entry(unsigned short int *, struct GDT);
+extern void set_gdt(void);
+extern void reload_segments(void);
+
+#endif