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-rw-r--r--i686/gdt.h55
1 files changed, 55 insertions, 0 deletions
diff --git a/i686/gdt.h b/i686/gdt.h
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+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+enum Ring { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };
+
+struct __attribute__((packed)) Access {
+ bool accessed : 1; // if 0, is set by processor when accessed
+ bool readwrite : 1; // code seg: read toggle; data seg: write toggle
+ bool direction : 1; // code seg: conforming bit; data seg: direction bit
+ bool executable : 1; // executable bit
+ bool segment : 1; // true for code/data; false for gates/tss
+ enum Ring privilege : 2; // descriptor privilege level
+ bool present : 1; // true for every active segment
+};
+_Static_assert(sizeof(struct Access) == 1);
+
+static const struct Access null_access = {};
+static const struct Access ktext_access = {.readwrite = true, .executable = true, .segment = true, .present = true};
+static const struct Access kdata_access = {.readwrite = true, .segment = true, .present = true};
+
+// Segment Descriptor
+// A memory structure (part of a table) that tells the CPU the attributes of a given segment
+// |31| | | | | | |24|23|22|21|20|19| | |16|15| | | | | | | 8| 7| | | | | | | 0|
+// | base_31_24 | G|DB| | A| lim_19_16 | access | base_23_16 |
+// | base_15_0 | limit_15_0 |
+// |31| | | | | | | | | | | | | | |16|15| | | | | | | | | | | | | | | 0|
+// limit size of segment - 1, either in bytes or in 4KiB chunks (check flags)
+// base address of segment
+// access
+// flags defines the segment chunks and 16/32 bit
+struct __attribute__((packed)) SegmentDescriptor_t {
+ uint16_t limit_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 access; // access byte
+ uint8_t limit_19_16 : 4; // high bits of segment limit
+ // flags
+ bool a : 1; // unused, available for software use
+ bool rsv : 1; // reserved
+ bool db : 1; // false => 16-bit seg; true => 32-bit seg
+ bool granularity : 1; // limit scaled by 4k when set
+ uint8_t base_31_24; // high bits of segment address
+};
+_Static_assert(sizeof(struct SegmentDescriptor_t) == 8);
+
+void SegmentDescriptor(struct SegmentDescriptor_t *self, unsigned base, unsigned limit, uint8_t access);
+
+void gdt_install();
+
+enum SegmentIndex {
+ ktextDescriptor = 2 * sizeof(struct SegmentDescriptor_t),
+ kdataDescriptor = 3 * sizeof(struct SegmentDescriptor_t),
+};