Loading GDT in C.

CMakeLists.txt

@@ -47,6 +47,8 @@ if(${CMAKE_SYSTEM_PROCESSOR} MATCHES "i.86")
 	set(SRCS
 		src/kernel/arch/x86/boot.s
 		${SRCS}
+		src/kernel/arch/x86/gdt.s
+		src/kernel/arch/x86/gdt.c
 		src/kernel/arch/x86/tty.c)
 	set(CMAKE_EXE_LINKER_FLAGS
 		"-T ${CMAKE_CURRENT_SOURCE_DIR}/src/kernel/arch/x86/linker.ld -ffreestanding -nostdlib")

src/kernel/arch/x86/boot.s

@@ -45,8 +45,8 @@ stack_bottom:
 .skip 16384  # 16 KiB
 stack_top:
 
-.section .data
+/*.section .data
-.include "gdt.s"
+.include "gdt.s"*/
 
 /*
  * The linker script specifies the `_start' label as the entry point to the kernel
@@ -59,19 +59,10 @@ stack_top:
 _start:
 	# set the position of `%esp' to the top of the stack
 	mov $stack_top, %esp
+	mov %esp, %ebp
 
 	# GDT, paging, and other features
-flush_gdt:
+	call gdt_install
-	cli
-	lgdt (gdtr)
-	movw $0x10, %ax
-	movw %ax, %ds
-	movw %ax, %es
-	movw %ax, %fs
-	movw %ax, %gs
-	movw %ax, %ss
-	ljmp $0x08, $flush_end
-flush_end:
 
 boot_kernel:
 	# enter high-level kernel (C)

src/kernel/arch/x86/gdt.c

@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2018  Ortega Froysa, Nicolás <nortega@themusicinnoise.net>
+ * Author: Ortega Froysa, Nicolás <nortega@themusicinnoise.net>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "gdt.h"
+
+/*
+ * ENTRIES:
+ * 0    - NULL entry
+ * 1/2  - kernel entries
+ * 3/4  - user entries
+ * 5    - TSS entry
+ */
+#define GDT_SIZE 3
+
+struct segdesc gdt[GDT_SIZE];
+struct segreg gdtr;
+
+void gdt_entry_set(struct segdesc *entry, uint32_t base,
+		uint32_t limit, uint8_t access, uint8_t gran) {
+	// base
+	entry->base_low = base & 0xFFFF;
+	entry->base_mid = (base >> 16) & 0xFF;
+	entry->base_high = (base >> 24) & 0xFF;
+
+	// limit
+	entry->limit_low = limit & 0xFFFF;
+	entry->gran = (limit >> 16) & 0x0F;
+
+	// access and granularity flags
+	entry->gran |= gran & 0xF0;
+	entry->access = access;
+}
+
+void gdt_install() {
+	gdtr.base = (uint32_t)gdt;
+	gdtr.limit = sizeof(gdt)-1;
+
+	gdt_entry_set(&gdt[0], 0, 0, 0, 0);
+	gdt_entry_set(&gdt[1], 0, 0xFFFFFFFF, 0x9A, 0xCF);
+	gdt_entry_set(&gdt[2], 0, 0xFFFFFFFF, 0x92, 0xCF);
+
+	gdt_flush(&gdtr);
+}

src/kernel/arch/x86/gdt.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2018  Ortega Froysa, Nicolás <nortega@themusicinnoise.net>
+ * Author: Ortega Froysa, Nicolás <nortega@themusicinnoise.net>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "structs.h"
+
+extern void gdt_flush(struct segreg *gdtr);
+void gdt_entry_set(struct segdesc *entry, uint32_t base,
+		uint32_t limit, uint8_t access, uint8_t gran);
+void gdt_install();

src/kernel/arch/x86/gdt.s

@@ -16,57 +16,22 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-/*
+.section .text
- * Create the GDT
+.global gdt_flush
- */
+.type gdt_flush, @function
-gdt_start:
+gdt_flush:
-gdt_null:
+	pushl %ebp
-.long 0x0
+	movl %esp, %ebp
-.long 0x0
+	movl 8(%ebp), %eax
-
+	cli
-gdt_kcode:
+	lgdt (%eax)
-.word 0xFFFF             # limit
+	movw $0x10, %ax
-.word 0x0                # base
+	movw %ax, %ds
-.byte 0x0                # base
+	movw %ax, %es
-.byte 0b10011010         # 1st flags | type flags
+	movw %ax, %fs
-.byte 0b11001111         # 2nd flags | limit
+	movw %ax, %gs
-.byte 0x0                # base
+	movw %ax, %ss
-
+	ljmp $0x0008, $gdt_flush_end
-gdt_kdata:
+gdt_flush_end:
-.word 0xFFFF             # limit
+	popl %ebp
-.word 0x0                # base
+	ret
-.byte 0x0                # base
-.byte 0b10010010         # 1st flags | type flags
-.byte 0b11001111         # 2nd flags | limit
-.byte 0x0                # base
-
-gdt_ucode:
-.word 0xFFFF             # limit
-.word 0x0                # base
-.byte 0x0                # base
-.byte 0b11111010         # 1st flags | type flags
-.byte 0b11001111         # 2nd flags | limit
-.byte 0x0                # base
-
-gdt_udata:
-.word 0xFFFF             # limit
-.word 0x0                # base
-.byte 0x0                # base
-.byte 0b11110010         # 1st flags | type flags
-.byte 0b11001111         # 2nd flags | limit
-.byte 0x0                # base
-
-# TODO: setup the TSS
-gdt_tss:
-.word 0x0                # size of TSS (set later)
-.word 0x0                # address of TSS (set later)
-.byte 0x0                # address of TSS (set later)
-.byte 0b10010001         # 1st flags | type flags
-.byte 0b01000000         # 2nd flags | size of TSS (set later
-.byte 0x0                # address of TSS (set later)
-
-gdt_end:
-
-gdtr:
-.word (gdt_end - gdt_start - 1)
-.long gdt_start

src/kernel/arch/x86/structs.h

@@ -18,10 +18,12 @@
 
 #pragma once
 
+#include <stdint.h>
+
 struct segdesc {
 	uint16_t limit_low;
 	uint16_t base_low;
-	uint8_t base_middle;
+	uint8_t base_mid;
 	uint8_t access;
 	uint8_t gran;
 	uint8_t base_high;
@@ -72,3 +74,8 @@ struct tss {
 	uint16_t iopb_r;   // reserved
 	uint16_t iopb;
 } __attribute__((packed));
+
+struct segreg {
+	uint16_t limit;
+	uint32_t base;
+} __attribute__((packed));