diff options
Diffstat (limited to 'src/kernel/core/gdt.c')
| -rw-r--r-- | src/kernel/core/gdt.c | 90 |
1 files changed, 90 insertions, 0 deletions
diff --git a/src/kernel/core/gdt.c b/src/kernel/core/gdt.c new file mode 100644 index 0000000..eadde5f --- /dev/null +++ b/src/kernel/core/gdt.c @@ -0,0 +1,90 @@ +#include <gdt.h> +#include <string.h> + +#define GDT_ENTRIES 6 // The contents of each entry is defined in gdt_init. + +/* One entry of the table */ +struct gdt_entry { + uint16_t limit_low; + uint16_t base_low; + uint8_t base_middle; + uint8_t access; + uint8_t granularity; + uint8_t base_high; +} __attribute__((packed)); +typedef struct gdt_entry gdt_entry_t; + +/* Structure defining the whole table : address and size (in bytes). */ +struct gdt_ptr { + uint16_t limit; + uint32_t base; +} __attribute__((packed)); +typedef struct gdt_ptr gdt_ptr_t; + +/* The TSS is used for hardware multitasking. + We don't use that, but we still need a TSS so that user mode process exceptions + can be handled correctly by the kernel. */ +struct tss_entry { + uint32_t prev_tss; // The previous TSS - if we used hardware task switching this would form a linked list. + uint32_t esp0; // The stack pointer to load when we change to kernel mode. + uint32_t ss0; // The stack segment to load when we change to kernel mode. + uint32_t esp1; // Unused... + uint32_t ss1; + uint32_t esp2; + uint32_t ss2; + uint32_t cr3; + uint32_t eip; + uint32_t eflags; + uint32_t eax; + uint32_t ecx; + uint32_t edx; + uint32_t ebx; + uint32_t esp; + uint32_t ebp; + uint32_t esi; + uint32_t edi; + uint32_t es; // The value to load into ES when we change to kernel mode. + uint32_t cs; // The value to load into CS when we change to kernel mode. + uint32_t ss; // The value to load into SS when we change to kernel mode. + uint32_t ds; // The value to load into DS when we change to kernel mode. + uint32_t fs; // The value to load into FS when we change to kernel mode. + uint32_t gs; // The value to load into GS when we change to kernel mode. + uint32_t ldt; // Unused... + uint16_t trap; + uint16_t iomap_base; +} __attribute__((packed)); +typedef struct tss_entry tss_entry_t; + +// ========================= // +// Actual definitions + +static gdt_entry_t gdt_entries[GDT_ENTRIES]; +static gdt_ptr_t gdt_ptr; + +/* For internal use only. Writes one entry of the GDT with given parameters. */ +static void gdt_set_gate(int num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran) { + gdt_entries[num].base_low = (base & 0xFFFF); + gdt_entries[num].base_middle = (base >> 16) & 0xFF; + gdt_entries[num].base_high = (base >> 24) & 0xFF; + + gdt_entries[num].limit_low = (limit & 0xFFFF); + gdt_entries[num].granularity = (limit >> 16) & 0x0F; + gdt_entries[num].granularity |= gran & 0xF0; + gdt_entries[num].access = access; +} + +/* Write data to the GDT and enable it. */ +void gdt_init() { + gdt_ptr.limit = (sizeof(gdt_entry_t) * GDT_ENTRIES) - 1; + gdt_ptr.base = (uint32_t)&gdt_entries; + + gdt_set_gate(0, 0, 0, 0, 0); //Null segment + gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); //Kernel code segment 0x08 + gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); //Kernel data segment 0x10 + gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); //User code segment 0x18 + gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); //User data segment 0x20 + + asm volatile ("lgdt %0"::"m"(gdt_ptr):"memory"); +} + +/* vim: set ts=4 sw=4 tw=0 noet :*/ |