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nuclear@7
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1 #include <string.h>
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nuclear@7
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2 #include "segm.h"
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3
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nuclear@8
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4 /* bits for the 3rd 16bt part of the descriptor */
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5 #define BIT_ACCESSED (1 << 8)
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6 #define BIT_WR (1 << 9)
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7 #define BIT_RD (1 << 9)
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8 #define BIT_EXP_DOWN (1 << 10)
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9 #define BIT_CONFORMING (1 << 10)
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10 #define BIT_CODE (1 << 11)
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11 #define BIT_NOSYS (1 << 12)
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12 #define BIT_PRESENT (1 << 15)
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13
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14 /* bits for the last 16bit part of the descriptor */
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15 #define BIT_BIG (1 << 6)
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16 #define BIT_DEFAULT (1 << 6)
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17 #define BIT_GRAN (1 << 7)
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18
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19 enum {TYPE_DATA, TYPE_CODE};
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20
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21 static void segm_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type);
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22
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23 /* these functions are implemented in segm-asm.S */
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24 void setup_selectors(uint16_t code, uint16_t data);
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25 void set_gdt(uint32_t addr, uint16_t limit);
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26
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27
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28 /* our global descriptor table */
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29 static desc_t gdt[4];
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30
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31
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32 void init_segm(void)
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33 {
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34 memset(gdt, 0, sizeof gdt);
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35 segm_desc(gdt + SEGM_KCODE, 0, 0xffffffff, 0, TYPE_CODE);
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36 segm_desc(gdt + SEGM_KDATA, 0, 0xffffffff, 0, TYPE_DATA);
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37
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38 set_gdt((uint32_t)gdt, sizeof gdt - 1);
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39
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40 setup_selectors(selector(SEGM_KCODE, 0), selector(SEGM_KDATA, 0));
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41 }
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42
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43 /* constructs a GDT selector based on index and priviledge level */
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44 uint16_t selector(int idx, int rpl)
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45 {
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46 return (idx << 3) | (rpl & 3);
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47 }
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48
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49 static void segm_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type)
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50 {
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51 desc->d[0] = limit & 0xffff; /* low order 16bits of limit */
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52 desc->d[1] = base & 0xffff; /* low order 16bits of base */
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53
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54 /* third 16bit part contains the last 8 bits of base, the 2 priviledge
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55 * level bits starting on bit 13, present flag on bit 15, and type bits
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56 * starting from bit 8
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57 */
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58 desc->d[2] = ((base >> 16) & 0xff) | ((dpl & 3) << 13) | BIT_PRESENT |
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59 BIT_NOSYS | (type == TYPE_DATA ? BIT_WR : (BIT_RD | BIT_CODE));
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60
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61 /* last 16bit part contains the last nibble of limit, the last byte of
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62 * base, and the granularity and deafult/big flags in bits 23 and 22 resp.
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63 */
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64 desc->d[3] = ((limit >> 16) & 0xf) | ((base >> 16) & 0xff00) | BIT_GRAN | BIT_BIG;
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65 }
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