CTAOS v6

SysCore/drivers/cpu/gdt/gdt.asm

@@ -0,0 +1,20 @@
+bits 32
+; !!!  GDT  !!!
+; This will set up our new segment registers. We need to do
+; something special in order to set CS. We do what is called a
+; far jump. A jump that includes a segment as well as an offset.
+; This is declared in C as 'extern void gdt_flush();'
+
+global _i86_GdtFlush     ; Allows the C code to link to this
+extern _gp            ; Says that '_gp' is in another file
+_i86_GdtFlush:
+    lgdt [_gp]        ; Load the GDT with our '_gp' which is a special pointer
+    mov ax, 0x10      ; 0x10 is the offset in the GDT to our data segment
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+    jmp 0x08:flush2   ; 0x08 is the offset to our code segment: Far jump!
+flush2:
+    ret               ; Returns back to the C code!

SysCore/drivers/cpu/gdt/gdt.c

@@ -0,0 +1,76 @@
+/******************************************************************
+ *    gdt.c  -  GLOBAL DESCRIPTOR TABLE                           *
+ *      Contains function prototypes for setting up the GDT       *
+ ******************************************************************/
+#define MAX_DESCRIPTORS 5
+#include "gdt.h"
+
+extern void i86_GdtInstall();
+
+/* Our GDT, with 3 entries, and finally our special GDT pointer */
+struct GdtEntry gdt[MAX_DESCRIPTORS];
+struct GdtPointer gp;
+
+		
+/* Setup a descriptor in the Global Descriptor Table */
+void i86_GdtSetGate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
+{
+    /* Sanity check */
+    if (num >= MAX_DESCRIPTORS) return;
+    
+    /* 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;
+}
+
+/*struct gdt_entry* i86_GdtGetGate(int num)
+{
+      if (num>MAX_DESCRIPTORS) return 0;
+      return &gdt[num];
+}*/
+
+/* Should be called by main. This will setup the special GDT
+*  pointer, set up the first 3 entries in our GDT, and then
+*  finally call gdt_flush() in our assembler file in order
+*  to tell the processor where the new GDT is and update the
+*  new segment registers */
+void i86_GdtInstall()
+{
+    /* Setup the GDT pointer and limit */
+    gp.limit = (sizeof(struct GdtEntry) * 3) - 1;
+    gp.base = (unsigned int)&gdt;
+
+    /* Our NULL descriptor */
+    i86_GdtSetGate(0, 0, 0, 0, 0);
+
+    /* The second entry is our Code Segment. The base address
+    *  is 0, the limit is 4GBytes, it uses 4KByte granularity,
+    *  uses 32-bit opcodes, and is a Code Segment descriptor.
+    *  Please check the table above in the tutorial in order
+    *  to see exactly what each value means */
+    i86_GdtSetGate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+
+    /* The third entry is our Data Segment. It's EXACTLY the
+    *  same as our code segment, but the descriptor type in
+    *  this entry's access byte says it's a Data Segment */
+    i86_GdtSetGate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+
+    /* User mode Code segment*/
+    i86_GdtSetGate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF);
+    
+    /* User mode data segment*/
+    i86_GdtSetGate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF);
+
+    /* Flush out the old GDT and install the new changes! */
+    i86_GdtFlush();
+}
+		

SysCore/drivers/cpu/gdt/gdt.h

@@ -0,0 +1,38 @@
+/******************************************************************
+ *    gdt.h  -  GLOBAL DESCRIPTOR TABLE                           *
+ *      Contains structures and function declarations for GDT     *
+ ******************************************************************/
+
+#ifndef __GDT_H
+#define __GDT_H
+
+/* Defines a GDT entry. We say packed, because it prevents the
+*  compiler from doing things that it thinks is best: Prevent
+*  compiler "optimization" by packing */
+struct GdtEntry
+{
+    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 which includes the limit: The max bytes
+*  taken up by the GDT, minus 1. Again, this NEEDS to be packed */
+struct GdtPointer
+{
+    unsigned short limit;
+    unsigned int base;
+} __attribute__((packed));
+
+
+/* This will be a function in start.asm. We use this to properly
+*  reload the new segment registers */
+extern void i86_GdtInstall();
+extern void i86_GdtFlush();
+extern void i86_GdtSetGate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran);
+//extern struct GdtEntry* i86_GdtGetGate(int num);
+
+#endif