census logo

[bootcensus] / src / segm.c

diff --git a/src/segm.c b/src/segm.c
index d2f940a..c6baffd 100644 (file)
--- a/src/segm.c
+++ b/src/segm.c
@@ -1,3 +1,20 @@
+/*
+pcboot - bootable PC demo/game kernel
+Copyright (C) 2018  John Tsiombikas <nuclear@member.fsf.org>
+
+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 <https://www.gnu.org/licenses/>.
+*/
 #include <string.h>
 #include "segm.h"
 #include "desc.h"
@@ -26,6 +43,7 @@ enum {TYPE_DATA, TYPE_CODE};
 #define TSS_TYPE_BITS  (9 << 8)
 
 static void segm_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type);
+static void segm_desc16(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type);
 static void task_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl);
 
 /* these functions are implemented in segm-asm.S */
@@ -35,7 +53,7 @@ void set_task_reg(uint16_t tss_selector);
 
 
 /* our global descriptor table */
-static desc_t gdt[6] __attribute__((aligned(8)));
+static desc_t gdt[NUM_SEGMENTS] __attribute__((aligned(8)));
 
 
 void init_segm(void)
@@ -45,6 +63,7 @@ void init_segm(void)
        segm_desc(gdt + SEGM_KDATA, 0, 0xffffffff, 0, TYPE_DATA);
        segm_desc(gdt + SEGM_UCODE, 0, 0xffffffff, 3, TYPE_CODE);
        segm_desc(gdt + SEGM_UDATA, 0, 0xffffffff, 3, TYPE_DATA);
+       segm_desc16(gdt + SEGM_CODE16, 0, 0xffff, 0, TYPE_CODE);
 
        set_gdt((uint32_t)gdt, sizeof gdt - 1);
 
@@ -81,6 +100,22 @@ static void segm_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int
        desc->d[3] = ((limit >> 16) & 0xf) | ((base >> 16) & 0xff00) | BIT_GRAN | BIT_BIG;
 }
 
+static void segm_desc16(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type)
+{
+       desc->d[0] = limit & 0xffff; /* low order 16bits of limit */
+       desc->d[1] = base & 0xffff;  /* low order 16bits of base */
+
+       /* third 16bit part contains the last 8 bits of base, the 2 priviledge
+        * level bits starting on bit 13, present flag on bit 15, and type bits
+        * starting from bit 8
+        */
+       desc->d[2] = ((base >> 16) & 0xff) | ((dpl & 3) << 13) | BIT_PRESENT |
+               BIT_NOSYS | (type == TYPE_DATA ? BIT_WR : (BIT_RD | BIT_CODE));
+
+       /* 16bit descriptors have the upper word 0 */
+       desc->d[3] = 0;
+}
+
 static void task_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl)
 {
        desc->d[0] = limit & 0xffff;