124 lines
4 KiB
C
Executable file
124 lines
4 KiB
C
Executable file
/*
|
|
File: gdt.C
|
|
|
|
Date : 09/03/02
|
|
|
|
Description: Management of the Global Descriptor Table (GDT)
|
|
|
|
The GDT describes memory access priviledges for memory segments.
|
|
|
|
While the table is initialized by GRUB already, it may be a good idea to
|
|
do this again in the kernel code.
|
|
|
|
For details see Section 5 of Brandon Friesen's Tutorial
|
|
on OS Kernel Development.
|
|
URL: http://www.osdever.net/bkerndev/Docs/title.htm
|
|
|
|
* bkerndev - Bran's Kernel Development Tutorial
|
|
* By: Brandon F. (friesenb@gmail.com)
|
|
* Desc: Interrupt Descriptor Table management
|
|
*
|
|
* Notes: No warranty expressed or implied. Use at own risk.
|
|
*/
|
|
|
|
/*--------------------------------------------------------------------------*/
|
|
/* DEFINES */
|
|
/*--------------------------------------------------------------------------*/
|
|
|
|
/* -- (none) -- */
|
|
|
|
/*--------------------------------------------------------------------------*/
|
|
/* INCLUDES */
|
|
/*--------------------------------------------------------------------------*/
|
|
|
|
//#include "assert.H"
|
|
#include "utils.H"
|
|
#include "gdt.H"
|
|
|
|
/*--------------------------------------------------------------------------*/
|
|
/* DATA STRUCTURES */
|
|
/*--------------------------------------------------------------------------*/
|
|
|
|
/* Defines a GDT entry. We use the 'packed' directive to prevent the
|
|
compiler from optimizing by aligning structure elements at word
|
|
boundaries. */
|
|
struct gdt_entry {
|
|
unsigned short limit_low;
|
|
unsigned short base_low;
|
|
unsigned char base_middle;
|
|
unsigned char access;
|
|
unsigned char granularity;
|
|
unsigned char base_high;
|
|
} __attribute__((packed));
|
|
|
|
/* Special pointer that includes the limit: The max bytes
|
|
taken up by the GDT, minus 1. Again, this NEEDS to be packed. */
|
|
struct gdt_ptr {
|
|
unsigned short limit;
|
|
unsigned int base;
|
|
} __attribute__((packed));
|
|
|
|
/*--------------------------------------------------------------------------*/
|
|
/* VARIABLES */
|
|
/*--------------------------------------------------------------------------*/
|
|
|
|
static struct gdt_entry gdt[GDT::SIZE];
|
|
struct gdt_ptr gp;
|
|
|
|
/*--------------------------------------------------------------------------*/
|
|
/* EXTERNS */
|
|
/*--------------------------------------------------------------------------*/
|
|
|
|
/* This function is defined in 'gdt_low.asm', which in turn is included in
|
|
'start.asm'. */
|
|
extern "C" void gdt_flush();
|
|
|
|
/*--------------------------------------------------------------------------*/
|
|
/* EXPORTED FUNCTIONS */
|
|
/*--------------------------------------------------------------------------*/
|
|
|
|
/* Use this function to set up an entry in the GDT. */
|
|
void GDT::set_gate(int num,
|
|
unsigned long base, unsigned long limit,
|
|
unsigned char access, unsigned char gran) {
|
|
|
|
/* Setup the descriptor base address */
|
|
gdt[num].base_low = (base & 0xFFFF);
|
|
gdt[num].base_middle = (base >> 16) & 0xFF;
|
|
gdt[num].base_high = (base >> 24) & 0xFF;
|
|
|
|
/* Setup the descriptor limits */
|
|
gdt[num].limit_low = (limit & 0xFFFF);
|
|
gdt[num].granularity = ((limit >> 16) & 0x0F);
|
|
|
|
/* Finally, set up the granularity and access flags */
|
|
gdt[num].granularity |= (gran & 0xF0);
|
|
gdt[num].access = access;
|
|
}
|
|
|
|
|
|
/* Installs the GDT */
|
|
void GDT::init() {
|
|
|
|
/* Sets up the special GDT pointer. */
|
|
gp.limit = (sizeof (struct gdt_entry) * SIZE) - 1;
|
|
gp.base = (unsigned int)&gdt;
|
|
|
|
/* Our NULL descriptor */
|
|
set_gate(0, 0, 0, 0, 0);
|
|
|
|
/* The second entry is our Code Segment. The base address
|
|
is 0, the limit is 4GByte, it uses 4kB granularity,
|
|
uses 32-bit opcodes, and is a Code Segment descriptor.
|
|
Please check the GDT section in Bran's Kernel Development
|
|
tutorial to see exactly what each value means. */
|
|
set_gate(1, 0, 0xFFFFFFFF, 0x9a, 0xCF);
|
|
|
|
/* The third entry is our Data Segment. It's EXACTLY the
|
|
same as the code segment, but the descriptor type in
|
|
this entry's access byte says it's a Data Segment. */
|
|
set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
|
|
|
|
/* Flush out the old GDT, and install the new changes. */
|
|
gdt_flush();
|
|
}
|