x86: Tweak IDT and GDT for alignment and readability
--
Changes for v2:
- Renamed to better reflect nature of changes
diff --git a/arch/x86/cpu/start16.S b/arch/x86/cpu/start16.S
index 33e53cd..cc393ff 100644
--- a/arch/x86/cpu/start16.S
+++ b/arch/x86/cpu/start16.S
@@ -86,7 +86,11 @@
.word 0x20 /* limit (32 bytes = 4 GDT entries) */
.long BOOT_SEG + gdt /* base */
- /* The GDT table ...
+/* Some CPUs are picky about GDT alignment... */
+.align 16
+gdt:
+ /*
+ * The GDT table ...
*
* Selector Type
* 0x00 NULL
@@ -94,17 +98,46 @@
* 0x10 32bit code
* 0x18 32bit data/stack
*/
+ /* The NULL Desciptor - Mandatory */
+ .word 0x0000 /* limit_low */
+ .word 0x0000 /* base_low */
+ .byte 0x00 /* base_middle */
+ .byte 0x00 /* access */
+ .byte 0x00 /* flags + limit_high */
+ .byte 0x00 /* base_high */
-gdt:
- .word 0, 0, 0, 0 /* NULL */
- .word 0, 0, 0, 0 /* unused */
+ /* Unused Desciptor - (matches Linux) */
+ .word 0x0000 /* limit_low */
+ .word 0x0000 /* base_low */
+ .byte 0x00 /* base_middle */
+ .byte 0x00 /* access */
+ .byte 0x00 /* flags + limit_high */
+ .byte 0x00 /* base_high */
- .word 0xFFFF /* 4Gb - (0x100000*0x1000 = 4Gb) */
- .word 0 /* base address = 0 */
- .word 0x9B00 /* code read/exec */
- .word 0x00CF /* granularity = 4096, 386 (+5th nibble of limit) */
+ /*
+ * The Code Segment Descriptor:
+ * - Base = 0x00000000
+ * - Size = 4GB
+ * - Access = Present, Ring 0, Exec (Code), Readable
+ * - Flags = 4kB Granularity, 32-bit
+ */
+ .word 0xffff /* limit_low */
+ .word 0x0000 /* base_low */
+ .byte 0x00 /* base_middle */
+ .byte 0x9b /* access */
+ .byte 0xcf /* flags + limit_high */
+ .byte 0x00 /* base_high */
- .word 0xFFFF /* 4Gb - (0x100000*0x1000 = 4Gb) */
- .word 0x0 /* base address = 0 */
- .word 0x9300 /* data read/write */
- .word 0x00CF /* granularity = 4096, 386 (+5th nibble of limit) */
+ /*
+ * The Data Segment Descriptor:
+ * - Base = 0x00000000
+ * - Size = 4GB
+ * - Access = Present, Ring 0, Non-Exec (Data), Writable
+ * - Flags = 4kB Granularity, 32-bit
+ */
+ .word 0xffff /* limit_low */
+ .word 0x0000 /* base_low */
+ .byte 0x00 /* base_middle */
+ .byte 0x93 /* access */
+ .byte 0xcf /* flags + limit_high */
+ .byte 0x00 /* base_high */