Initial implementation of GDT

kernel/Makefile

@@ -32,20 +32,20 @@ $(KERNEL_ARCH_OBJS) \
 kernel/kernel.o \
 
 OBJS=\
-$(ARCHDIR)/crti.o \
+$(ARCHDIR)/boot/crti.o \
 $(ARCHDIR)/crtbegin.o \
 $(KERNEL_OBJS) \
 $(ARCHDIR)/crtend.o \
-$(ARCHDIR)/crtn.o \
+$(ARCHDIR)/boot/crtn.o \
 
 LINK_LIST=\
 $(LDFLAGS) \
-$(ARCHDIR)/crti.o \
+$(ARCHDIR)/boot/crti.o \
 $(ARCHDIR)/crtbegin.o \
 $(KERNEL_OBJS) \
 $(LIBS) \
 $(ARCHDIR)/crtend.o \
-$(ARCHDIR)/crtn.o \
+$(ARCHDIR)/boot/crtn.o \
 
 .PHONY: all clean install install-headers install-kernel
 .SUFFIXES: .o .c .S

kernel/arch/i386/boot/boot.S

@@ -25,10 +25,11 @@ stack_top:
 .type _start, @function
 _start:
     movl $stack_top, %esp
-    call _init          # call global constructors
+    cli
+    call gdt_init          # call global constructors
     call kernel_main    # transfer control to main kernel
     # Hang if kernel_main unexpectedly returns
-    cli
 1:  hlt
     jmp 1b
+
 .size _start, . - _start

kernel/arch/i386/boot/gdt.c

@@ -0,0 +1,48 @@
+/**
+ *  Referenced from https://github.com/tendstofortytwo/clay
+ *  kernel/kernel.c
+ */
+
+#include <kernel/gdt.h>
+#include <stdio.h>
+#include <stdint.h>
+
+uint32_t TSS[32];
+gdt_entry GDT[GDT_ENTRIES];
+gdt_ptr gp;
+
+void gdt_fill(gdt_entry* target, gdt* source) {
+    if(source->limit > 0xFFFFF) {
+        printf("Invalid GDT limit %d\n", source->limit);
+        for(;;);
+    }
+
+    target->limit_lo = source->limit & 0xFFFF;
+    target->limit_hi_flags = (source->limit >> 16) & 0x0F;
+    target->base_lo = source->base & 0xFFFF;
+    target->base_mid = (source->base >> 16) & 0xFF;
+    target->base_hi = (source->base >> 24) & 0xFF;
+    target->access = source->access;
+    target->limit_hi_flags |= (source->flags << 4);
+}
+
+void gdt_init(void) {
+    uint32_t tssRef = (uint32_t) TSS;
+    gdt rows[GDT_ENTRIES] = {
+        {0, 0x00000, 0x00, 0x0}, // 0x0000 Null Descriptor
+        {0, 0xFFFFF, 0x9A, 0xC}, // 0x0008 Kernel Mode Code Segment
+        {0, 0xFFFFF, 0x92, 0xC}, // 0x0010 Kernel Mode Data Segment
+        {0, 0xFFFFF, 0xFA, 0xC}, // 0x0018 User Mode Code Segment
+        {0, 0xFFFFF, 0xF2, 0xC}, // 0x0020 User Mode Data Segment
+        {tssRef, sizeof(TSS), 0x89, 0x0}, // 0x0028 Task State Segment
+    };
+
+    for(uint32_t i = 0; i < GDT_ENTRIES; ++i) {
+        gdt_fill(GDT+i, rows+i);
+    }
+
+    gp.base = (void*) GDT;
+    gp.limit = (GDT_ENTRIES * sizeof(gdt_entry)) - 1;
+
+    gdt_load();
+}

kernel/arch/i386/boot/gdt_load.S

@@ -0,0 +1,20 @@
+# Referenced from https://github.com/tendstofortytwo/clay
+# kernel/boot/gdt.s
+
+.global gdt_load
+.type gdt_load, @function
+gdt_load:
+    lgdt (gp)
+    mov %cr0, %eax
+    or $0x1, %eax
+    mov %eax, %cr0
+    jmp $0x08, $reload
+
+reload:
+    mov $0x10, %ax
+    mov %ax, %ds
+    mov %ax, %es
+    mov %ax, %fs
+    mov %ax, %gs
+    mov %ax, %ss
+    ret

kernel/arch/i386/make.config

@@ -4,5 +4,7 @@ KERNEL_ARCH_LDFLAGS=
 KERNEL_ARCH_LIBS=
 
 KERNEL_ARCH_OBJS=\
-$(ARCHDIR)/boot.o \
-$(ARCHDIR)/tty.o \
+$(ARCHDIR)/boot/boot.o \
+$(ARCHDIR)/vga/tty.o \
+$(ARCHDIR)/boot/gdt.o \
+$(ARCHDIR)/boot/gdt_load.o

kernel/include/kernel/gdt.h

@@ -0,0 +1,42 @@
+/**
+ * Referenced from https://tendstofortytwo/clay
+ * kernel/kernel.c
+ */
+
+#ifndef _KERNEL_GDT_H
+#define _KERNEL_GDT_H
+
+#include <stdint.h>
+
+#define GDT_ENTRIES 0x6
+
+typedef struct {
+    uint16_t limit;
+    uint32_t base;
+}__attribute__((packed)) gdt_ptr;
+
+typedef struct {
+    uint16_t limit_lo;
+    uint16_t base_lo;
+    uint8_t base_mid;
+    uint8_t access;
+    uint8_t limit_hi_flags;
+    uint8_t base_hi;
+}__attribute__((packed)) gdt_entry;
+
+typedef struct {
+    uint32_t base;
+    uint32_t limit;
+    uint8_t access;
+    uint8_t flags;
+}__attribute__((packed)) gdt;
+
+void gdt_fill(gdt_entry*, gdt*);
+void gdt_init(void);
+extern void gdt_load();
+
+extern uint32_t TSS[32];
+extern gdt_entry GDT[GDT_ENTRIES];
+extern gdt_ptr gp;
+
+#endif

kernel/kernel/kernel.c

@@ -1,6 +1,7 @@
 #include <stdio.h>
 
 #include <kernel/tty.h>
+#include <kernel/gdt.h>
 
 void kernel_main(void) {
     terminal_initialize();