Reduced GDT magic numbers and added documentation

3 files changed, 69 insertions, 44 deletions

diff --git a/src/loader/ent.asm b/src/loader/ent.asm
index c518aad..87b5e53 100644
--- a/src/loader/ent.asm
+++ b/src/loader/ent.asm
@@ -20,10 +20,10 @@ _start:
 
 	lgdt [gdt] ; Load GDT
 
-	jmp 0x18:.reload_cs
+	jmp 0x18:.reload_cs ; Use third GDT segment for code
 
 .reload_cs:
-	mov eax, 0x20
+	mov eax, 0x20 ; Use fourth GDT segment for data/other
 	mov ds, ax
 	mov es, ax
 	mov fs, ax
diff --git a/src/loader/gdt.c b/src/loader/gdt.c
index ea91c33..61f186d 100644
--- a/src/loader/gdt.c
+++ b/src/loader/gdt.c
@@ -2,50 +2,75 @@
 
 #include <gdt.h>
 
+#define GDT_MAX_LIMIT 0xffff // General max access limit without extra nibble
+#define GDT_EXTRA_MAX_LIMIT (0x0f) // Extra limit nibble
+#define GDT_PRESENT (1 << 7) // Makes the descriptor valid
+#define GDT_DESCRIPTOR (1 << 4) // Should be set for code/data segments
+#define GDT_EXECUTABLE (1 << 3) // Makes the segment executable (for code segments)
+#define GDT_READWRITE (1 << 1) // Makes the segment read/writeable
+#define GDT_SIZE (0x40) // Enable 32-Bit
+#define GDT_GRANULARITY (0x80) // Enable 4K granularity for limit
+
 static struct gdt_desc gdt_descs[] = {
+	// NULL descriptor - Offset 0
 	{ 0 },
 
-	{ .limit = 0xffff,
-	  .base_low = 0x0000,
-	  .base_mid = 0x00,
-	  .access = 0x9a,
-	  .granularity = 0x00,
-	  .base_hi = 0x00 },
-
-	{ .limit = 0xffff,
-	  .base_low = 0x0000,
-	  .base_mid = 0x00,
-	  .access = 0x92,
-	  .granularity = 0x00,
-	  .base_hi = 0x00 },
-
-	{ .limit = 0xffff,
-	  .base_low = 0x0000,
-	  .base_mid = 0x00,
-	  .access = 0x9a,
-	  .granularity = 0xcf,
-	  .base_hi = 0x00 },
-
-	{ .limit = 0xffff,
-	  .base_low = 0x0000,
-	  .base_mid = 0x00,
-	  .access = 0x92,
-	  .granularity = 0xcf,
-	  .base_hi = 0x00 },
-
-	{ .limit = 0x0000,
-	  .base_low = 0x0000,
-	  .base_mid = 0x00,
-	  .access = 0x9a,
-	  .granularity = 0x20,
-	  .base_hi = 0x00 },
-
-	{ .limit = 0x0000,
-	  .base_low = 0x0000,
-	  .base_mid = 0x00,
-	  .access = 0x92,
-	  .granularity = 0x00,
-	  .base_hi = 0x00 },
+	/**
+	 * These 16-Bit access segments get used by 16-Bit real mode emulation
+	 *
+	 * Limit + extra limit gets multiplied by 1 (no granularity/size bit)
+	 */
+
+	// Standard code segment // Offset 1
+	// Access from (0) to (0xffff * 1 = 64KiB)
+	{
+		.limit = GDT_MAX_LIMIT,
+		.base_low = 0x0000,
+		.base_mid = 0x00,
+		.access = GDT_DESCRIPTOR | GDT_PRESENT | GDT_READWRITE | GDT_EXECUTABLE,
+		.flags = 0x00,
+		.base_hi = 0x00,
+	},
+
+	// Standard data segment // Offset 2
+	// Access from (0) to (0xffff * 1 = 64KiB)
+	{
+		.limit = GDT_MAX_LIMIT,
+		.base_low = 0x0000,
+		.base_mid = 0x00,
+		.access = GDT_DESCRIPTOR | GDT_PRESENT | GDT_READWRITE,
+		.flags = 0x00,
+		.base_hi = 0x00,
+	},
+
+	/**
+	 * These 32-Bit full access segments get uses by the bootloader to load
+	 * ELF programs no matter whether they want to be loaded to 0-4GiB addr
+	 *
+	 * Limit + extra limit gets multiplied by 4096 (<< 12) with added 1s
+	 */
+
+	// Full access code segment // Offset 3
+	// Access from (0) to ((0xfffff << 12) + 0xfff = 4GiB)
+	{
+		.limit = GDT_MAX_LIMIT,
+		.base_low = 0x0000,
+		.base_mid = 0x00,
+		.access = GDT_DESCRIPTOR | GDT_PRESENT | GDT_READWRITE | GDT_EXECUTABLE,
+		.flags = GDT_GRANULARITY | GDT_SIZE | GDT_EXTRA_MAX_LIMIT,
+		.base_hi = 0x00,
+	},
+
+	// Full access data segment // Offset 4
+	// Access from (0) to ((0xfffff << 12) + 0xfff = 4GiB)
+	{
+		.limit = GDT_MAX_LIMIT,
+		.base_low = 0x0000,
+		.base_mid = 0x00,
+		.access = GDT_DESCRIPTOR | GDT_PRESENT | GDT_READWRITE,
+		.flags = GDT_GRANULARITY | GDT_SIZE | GDT_EXTRA_MAX_LIMIT,
+		.base_hi = 0x00,
+	},
 };
 
 REAL struct gdtr gdt = {
diff --git a/src/loader/inc/gdt.h b/src/loader/inc/gdt.h
index 1ff20b7..556d1e8 100644
--- a/src/loader/inc/gdt.h
+++ b/src/loader/inc/gdt.h
@@ -16,7 +16,7 @@ struct gdt_desc {
 	u16 base_low;
 	u8 base_mid;
 	u8 access;
-	u8 granularity;
+	u8 flags;
 	u8 base_hi;
 } PACKED;