From fb88c62f678dc588f7a474d21e92d782471e056d Mon Sep 17 00:00:00 2001
From: Alexander Myltsev <a.myltsev@gmail.com>
Date: Tue, 22 Nov 2022 21:24:49 +0300
Subject: [PATCH] Install new GDT in C code.

---
 Makefile  |  2 +-
 cpu/gdt.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 cpu/gdt.h | 19 +++++++++++++++++++
 kernel.c  |  2 ++
 mbr.S     |  3 +--
 5 files changed, 72 insertions(+), 3 deletions(-)
 create mode 100644 cpu/gdt.c
 create mode 100644 cpu/gdt.h

diff --git a/Makefile b/Makefile
index 760de1a..733507b 100644
--- a/Makefile
+++ b/Makefile
@@ -39,7 +39,7 @@ image.bin: mbr.bin fs.img
 	cat $^ >$@
 
 kernel.bin: kernel.o console.o drivers/vga.o drivers/keyboard.o \
-	string.o drivers/ata.o cpu/vectors.o cpu/idt.o drivers/uart.o
+	string.o drivers/ata.o cpu/vectors.o cpu/idt.o cpu/gdt.o drivers/uart.o
 	$(LD) $(LDFLAGS) -o $@ -Ttext 0x1000 $^
 
 %.o: %.c
diff --git a/cpu/gdt.c b/cpu/gdt.c
new file mode 100644
index 0000000..4b1b6d2
--- /dev/null
+++ b/cpu/gdt.c
@@ -0,0 +1,49 @@
+#include "gdt.h"
+
+#include <stdint.h>
+
+struct seg_desc_t {
+    uint16_t lim_15_0;      // Low bits of segment limit
+    uint16_t base_15_0;     // Low bits of segment base address
+    uint8_t base_23_16;     // Middle bits of segment base address
+    uint8_t type : 4;       // Segment type (see STS_ constants)
+    uint8_t s : 1;          // 0 = system, 1 = application
+    uint8_t dpl : 2;        // Descriptor Privilege Level
+    uint8_t p : 1;          // Present
+    uint8_t lim_19_16 : 4;  // High bits of segment limit
+    uint8_t avl : 1;        // Unused (available for software use)
+    uint8_t rsv1 : 1;       // Reserved
+    uint8_t db : 1;         // 0 = 16-bit segment, 1 = 32-bit segment
+    uint8_t g : 1;          // Granularity: limit scaled by 4K when set
+    uint8_t base_31_24;     // High bits of segment base address
+} __attribute__((packed));
+
+typedef unsigned uint;
+
+#define SEG(type, base, lim, dpl) (struct seg_desc_t)    \
+{ ((lim) >> 12) & 0xffff, (uint)(base) & 0xffff,      \
+  ((uint)(base) >> 16) & 0xff, type, 1, dpl, 1,       \
+  (uint)(lim) >> 28, 0, 0, 1, 1, (uint)(base) >> 24 }
+
+struct seg_desc_t seg_desc[6];
+
+void init_seg_desc() {
+    seg_desc[SEG_KCODE] = SEG(STA_X|STA_R, 0,         0xffffffff, 0);
+    seg_desc[SEG_KDATA] = SEG(STA_W,       0,         0xffffffff, 0);
+    seg_desc[SEG_UCODE] = SEG(STA_X|STA_R, USER_BASE, 0xffffffff - USER_BASE, DPL_USER);
+    seg_desc[SEG_UDATA] = SEG(STA_W,       USER_BASE, 0xffffffff - USER_BASE, DPL_USER);
+}
+
+struct gdt_desc_t {
+    uint16_t size;
+    void* ptr;
+} __attribute__((packed));
+
+void load_gdt() {
+    init_seg_desc();
+
+    struct gdt_desc_t gdt_desc;
+    gdt_desc.size = sizeof(seg_desc) - 1;
+    gdt_desc.ptr = seg_desc;
+    asm("lgdt %0": : "m"(gdt_desc));
+}
diff --git a/cpu/gdt.h b/cpu/gdt.h
new file mode 100644
index 0000000..578fcc8
--- /dev/null
+++ b/cpu/gdt.h
@@ -0,0 +1,19 @@
+#pragma once
+
+#define STA_X     0x8       // Executable segment
+#define STA_W     0x2       // Writeable (non-executable segments)
+#define STA_R     0x2       // Readable (executable segments)
+
+#define DPL_USER 3
+
+#define SEG_KCODE 1
+#define SEG_KDATA 2
+#define SEG_UCODE 3
+#define SEG_UDATA 4
+#define SEG_TSS   5
+
+#define USER_BASE 0x400000 // 4 MB
+
+#ifndef __ASSEMBLER__
+void load_gdt();
+#endif
diff --git a/kernel.c b/kernel.c
index 2cb823c..430c8d5 100644
--- a/kernel.c
+++ b/kernel.c
@@ -2,6 +2,7 @@ asm(".asciz \"kernel start\"");
 
 #include "console.h"
 #include "cpu/isr.h"
+#include "cpu/gdt.h"
 #include "drivers/keyboard.h"
 #include "drivers/vga.h"
 #include "drivers/ata.h"
@@ -9,6 +10,7 @@ asm(".asciz \"kernel start\"");
 #include "drivers/uart.h"
 
 void _start() {
+    load_gdt();
     init_keyboard();
     uartinit();
     load_idt();
diff --git a/mbr.S b/mbr.S
index dac62bb..a214220 100644
--- a/mbr.S
+++ b/mbr.S
@@ -80,8 +80,7 @@ bios_disk_read:
 1:
     mov $2, %ah             // read mode
     int $0x13
-    test %ah, %ah
-    jnz fail
+    jc fail
     add $SECTOR_SIZE, %bx
     inc %cl
     dec %si