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x86: Refactor GDT initialization code

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This patch refactors the GDT initialization code in more of a
self-documenting style.
This commit is contained in:
Michael LeMay 2015-07-11 15:53:10 -07:00 committed by Jesus Sanchez-Palencia
parent 9b6b5ce5b0
commit 62fc195d0f
1 changed files with 28 additions and 16 deletions
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44
cpu/x86/init/common/gdt.c
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@ -32,6 +32,20 @@
#define NUM_DESC 3 #define NUM_DESC 3
#define GDT_IDX_NULL 0
#define GDT_IDX_CODE 1
#define GDT_IDX_DATA 2
/* All code in the x86 port of Contiki runs at ring (privilege) level 0 */
#define PRIV_LVL 0
/* Compute GDT selector from descriptor index and requested privilege level */
#define GDT_SEL(IDX, RPL) (((IDX) << 3) | (RPL))
#define GDT_SEL_NULL GDT_SEL(GDT_IDX_NULL, 0)
#define GDT_SEL_CODE GDT_SEL(GDT_IDX_CODE, PRIV_LVL)
#define GDT_SEL_DATA GDT_SEL(GDT_IDX_DATA, PRIV_LVL)
/* Each define here is for a specific flag in the descriptor. Refer to Intel /* Each define here is for a specific flag in the descriptor. Refer to Intel
* Combined Manual (Intel 64 and IA-32 Architectures Software Developer's * Combined Manual (Intel 64 and IA-32 Architectures Software Developer's
* Manual), Vol. 3, Section 3.4.5 for a description of each flag. * Manual), Vol. 3, Section 3.4.5 for a description of each flag.
@ -110,27 +124,25 @@ gdt_init(void)
gdtr.base = (uint32_t) &gdt; gdtr.base = (uint32_t) &gdt;
/* Initialize descriptors */ /* Initialize descriptors */
set_descriptor(0, 0, 0, 0); set_descriptor(GDT_IDX_NULL, 0, 0, 0);
set_descriptor(1, 0, 0x0FFFFF, GDT_CODE_PL0); set_descriptor(GDT_IDX_CODE, 0, 0x0FFFFF, GDT_CODE_PL0);
set_descriptor(2, 0, 0x0FFFFF, GDT_DATA_PL0); set_descriptor(GDT_IDX_DATA, 0, 0x0FFFFF, GDT_DATA_PL0);
/* Load GDTR register and update segment registers. /* Load GDTR register and update segment registers.
* *
* In protected mode, segment registers should be loaded according to * CS register cannot be changed directly. For that reason, we do a far jump.
* the offset in GDT. So DS, SS, ES, FS and GS registers should be
* loadded with 0x10 while CS with 0x08. CS register cannot be changed
* directly. For that reason, we do a far jump.
*/ */
__asm__ ("lgdt %0\n\t" __asm__ ("lgdt %[_gdtr_]\n\t"
"jmp $0x08, $1f\n\t" "jmp %[_cs_], $1f\n\t"
"1:\n\t" "1:\n\t"
"mov $0x10, %%ax\n\t" "mov %[_ds_], %%ds\n\t"
"mov %%ax, %%ds\n\t" "mov %[_ds_], %%ss\n\t"
"mov %%ax, %%ss\n\t" "mov %[_ds_], %%es\n\t"
"mov %%ax, %%es\n\t" "mov %[_ds_], %%fs\n\t"
"mov %%ax, %%fs\n\t" "mov %[_ds_], %%gs\n\t"
"mov %%ax, %%gs\n\t"
: :
: "m" (gdtr) : [_gdtr_] "m" (gdtr),
[_cs_] "i" (GDT_SEL_CODE),
[_ds_] "r" (GDT_SEL_DATA)
); );
} }