cleaning up a bit

1 files changed, 136 insertions, 0 deletions

diff --git a/kernel/gdt.c b/kernel/gdt.c
new file mode 100644
index 0000000..06a0949
--- /dev/null
+++ b/kernel/gdt.c
@@ -0,0 +1,136 @@
+// http://wiki.osdev.org/GDT_Tutorial
+#include "lib/logger/log.h"
+#include "usermode.h"
+#define FOOLOS_MODULE_NAME "GDT"
+
+#include <stdint.h>
+#define GDT_SIZE 6
+
+extern sys_tss;
+
+typedef struct GDT_struct
+{
+    uint32_t base;
+    uint32_t limit;
+    uint32_t type;
+
+}GDT;
+
+//alternative
+struct gdt_entry_bits
+{
+	unsigned int limit_low:16;
+	unsigned int base_low : 24;
+     //attribute byte split into bitfields
+	unsigned int accessed :1;
+	unsigned int read_write :1; //readable for code, writable for data
+	unsigned int conforming_expand_down :1; //conforming for code, expand down for data
+	unsigned int code :1; //1 for code, 0 for data
+	unsigned int always_1 :1; //should be 1 for everything but TSS and LDT
+	unsigned int DPL :2; //priveledge level
+	unsigned int present :1;
+     //and now into granularity
+	unsigned int limit_high :4;
+	unsigned int available :1;
+	unsigned int always_0 :1; //should always be 0
+	unsigned int big :1; //32bit opcodes for code, uint32_t stack for data
+	unsigned int gran :1; //1 to use 4k page addressing, 0 for byte addressing
+	unsigned int base_high :8;
+} __packed; //or __attribute__((packed))
+
+
+static GDT myGDT[GDT_SIZE];
+static uint8_t gdt_struct[GDT_SIZE*8];
+
+/**
+ * \param target A pointer to the 8-byte GDT entry
+ * \param source An arbitrary structure describing the GDT entry
+ */
+void encodeGdtEntry(uint8_t *target, GDT source)
+{
+    // Check the limit to make sure that it can be encoded
+    if ((source.limit > 65536) && (source.limit & 0xFFF) != 0xFFF)
+    {
+	panic(FOOLOS_MODULE_NAME,"trying to set an invalid GDT source.limit!");
+    }
+    if (source.limit > 65536) {
+        // Adjust granularity if required
+        source.limit = source.limit >> 12;
+        target[6] = 0xC0;
+    } else {
+	target[6] = 0x40;
+    }
+ 
+    // Encode the limit
+    target[0] = source.limit & 0xFF;
+    target[1] = (source.limit >> 8) & 0xFF;
+ 
+    // Encode the base 
+    target[2] = source.base & 0xFF;
+    target[3] = (source.base >> 8) & 0xFF;
+    target[4] = (source.base >> 16) & 0xFF;
+    target[7] = (source.base >> 24) & 0xFF;
+
+    target[5] = source.type;
+    target[6] |= (source.limit >> 16) & 0xF;
+ 
+    // And... Type
+
+    /*
+
+0 1 dw 0xffff	    ;limit
+2 3 dw 0x0	    ;base
+4   db 0x0	    ;base
+5   db 10011010b    ;flags
+6   db 11001111b    ;flags & seg.limit
+7   db 0x0	    ;base
+    */
+
+
+}
+
+void gdt_init()
+{
+    log(FOOLOS_MODULE_NAME,FOOLOS_LOG_INFO,"setting up Global Descriptor Table");
+    //selector 0x0
+    myGDT[0].base=0;
+    myGDT[0].limit=0;
+    myGDT[0].type=0;
+
+    //selector 0x8 code
+    myGDT[1].base=0;
+    myGDT[1].limit=0xffffffff;
+    myGDT[1].type=0x9a;
+
+    //selector 0x10 data
+    myGDT[2].base=0;
+    myGDT[2].limit=0xffffffff;
+    myGDT[2].type=0x92;
+
+    //selector 0x18 code user
+    myGDT[3].base=0;
+    myGDT[3].limit=0xffffffff;
+    myGDT[3].type=0xFa;
+
+    //selector 0x20 data user
+    myGDT[4].base=0;
+    myGDT[4].limit=0xffffffff;
+    myGDT[4].type=0xF2;
+
+    //TSS 0x28
+    myGDT[5].base=&sys_tss; //tss start?
+    myGDT[5].limit=sizeof(tss_struct); //tss end?
+    myGDT[5].type=0x89;
+
+
+    // transcript to format the processor wants
+    for(int i=0;i<GDT_SIZE;i++)
+        encodeGdtEntry(&gdt_struct[8*i],myGDT[i]);
+
+
+    // updat
+    install_tss();
+    setup_gdt(&gdt_struct[0],8*GDT_SIZE);
+
+
+}