unreal gdt now has 2 selectors 1 code, 1 data
ss, and e/f/g segments are loaded to the same flat 4gb segment moved gdt notes to documentation
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61
docs/gdt.txt
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61
docs/gdt.txt
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@@ -0,0 +1,61 @@
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GDT documentation below:
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Pr: Present bit. This must be 1 for all valid selectors.
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Privl: Privilege, 2 bits. Contains the ring level,
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0 = highest (kernel), 3 = lowest (user applications).
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S: Descriptor type. This bit should be set for code or data segments
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and should be cleared for system segments (eg. a Task State Segment)
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Ex: Executable bit. If 1 code in this segment can be executed
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ie. a code selector. If 0 it is a data selector.
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DC: Direction bit/Conforming bit.
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Direction bit for data selectors: Tells the direction.
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0 the segment grows up. 1 the segment grows down, ie. the offset has to be greater than the limit.
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Conforming bit for code selectors:
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If 1 code in this segment can be executed from an equal or lower privilege level.
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For example, code in ring 3 can far-jump to conforming code in a ring 2 segment.
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The privl-bits represent the highest privilege level that is allowed to execute the segment.
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For example, code in ring 0 cannot far-jump to a conforming code segment with privl==0x2
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while code in ring 2 and 3 can. Note that the privilege level remains the same
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ie. a far-jump form ring 3 to a privl==2-segment remains in ring 3 after the jump.
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If 0 code in this segment can only be executed from the ring set in privl.
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RW: Readable bit/Writable bit.
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Readable bit for code selectors: Whether read access for this segment is allowed. Write access is never allowed for code segments.
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Writable bit for data selectors: Whether write access for this segment is allowed. Read access is always allowed for data segments.
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Ac: Accessed bit. Just set to 0. The CPU sets this to 1 when the segment is accessed.
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Gr: Granularity bit. If 0 the limit is in 1 B blocks (byte granularity), if 1 the limit is in 4 KiB blocks (page granularity).
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Sz: Size bit. If 0 the selector defines 16 bit protected mode. If 1 it defines 32 bit protected mode.
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You can have both 16 bit and 32 bit selectors at once.
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AvL: Availible to software bit, the CPU does not use this field and software can read/write to it
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D/B bit: The default operand-size bit is found in code-segment and data-segment descriptors but not in system-segment descriptors. Setting
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this bit to 1 indicates a 32-bit default operand size, and clearing it indicates a 16-bit default size.
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E bit: Expand down bit: Setting this bit to 1 identifies the data segment as expand-down.
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In expand-down segments, the segment limit defines the lower segment boundary while the base is the upper boundary
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A GDT entry is 8 bytes and is constructed as follows:
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First DWORD
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0-15 Limit 0:15 First 16 bits in the segment limiter
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16-31 Base 0:15 First 16 bits in the base address
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2nd DWORD
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0:7 Base 16:23 Bits 16-23 in the base address
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8:12 S/Type Segment type and attributes, S = bit 12, Type = 8:11, Type is either [1, DC, RW, Ac] <code> or [0, E, RW, Ac] <data>
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13:14 Privl 0 = Highest privilege (OS), 3 = Lowest privilege (User applications)
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15 Pr Set to 1 if segment is present
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16:19 Limit 16:19 Bits 16-19 in the segment limiter
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20:22 Attributes Different attributes, depending on the segment type
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23 Gr Used together with the limiter, to determine the size of the segment
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24:31 Base 24:31 The last 24-31 bits in the base address
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@@ -287,22 +287,50 @@ EnterUnrealMode:
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__CDECL16_ENTRY
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.func:
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cli ; no interrupts
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push ds ; save real mode data selector
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push ds ; save real mode data/stack selectors
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push es
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push fs
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push gs
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push ss
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push cs ; save real mode code selector
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lgdt [unreal_gdt_info]
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mov eax, cr0 ; switch to pmode
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or al,1 ; set pmode bit
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mov cr0, eax
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jmp $+2
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jmp $+2 ; clear instruction cache
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;jmp far 0x0008:EnterUnrealMode.load_cs
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db 0xEA ; jmp far imm16:imm16
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dw EnterUnrealMode.load_cs ; error_far_ptr
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dw 0x0008 ; error_far_seg
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.load_cs:
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mov bx, 0x10 ; select descriptor 2
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mov ds, bx ; 10h = 0001_0000b
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mov bx, 0x08 ; select descriptor 1
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mov ds, bx ; 8h = 1000b
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mov ss, bx
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mov es, bx
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mov fs, bx
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mov gs, bx ; other data/stack to desc. 2
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and al,0xFE ; back to realmode
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mov cr0, eax ; by toggling bit again
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jmp $+2
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pop ds ; get back old segment
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pop ax ; save cs to ax to setup far jump
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mov word [ds:__UNREAL_SEGMENT], ax
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;jmp far 0x0008:EnterUnrealMode.unload_cs
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db 0xEA ; jmp far imm16:imm16
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dw EnterUnrealMode.unload_cs ; error_far_ptr
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__UNREAL_SEGMENT:
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dw 0x0000 ; error_far_seg
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EnterUnrealMode.unload_cs:
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pop ss
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pop gs
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pop fs
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pop es
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pop ds ; get back old segments
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sti
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.endp:
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__CDECL16_EXIT
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@@ -339,71 +367,7 @@ stage2_resb_1:
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partition_offset_ptr:
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dw 0x0000
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; GDT documentation below:
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;
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; Pr: Present bit. This must be 1 for all valid selectors.
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;
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; Privl: Privilege, 2 bits. Contains the ring level,
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; 0 = highest (kernel), 3 = lowest (user applications).
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;
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; S: Descriptor type. This bit should be set for code or data segments
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; and should be cleared for system segments (eg. a Task State Segment)
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;
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; Ex: Executable bit. If 1 code in this segment can be executed
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; ie. a code selector. If 0 it is a data selector.
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;
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; DC: Direction bit/Conforming bit.
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; Direction bit for data selectors: Tells the direction.
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; 0 the segment grows up. 1 the segment grows down, ie. the offset has to be greater than the limit.
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;
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; Conforming bit for code selectors:
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; If 1 code in this segment can be executed from an equal or lower privilege level.
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; For example, code in ring 3 can far-jump to conforming code in a ring 2 segment.
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; The privl-bits represent the highest privilege level that is allowed to execute the segment.
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; For example, code in ring 0 cannot far-jump to a conforming code segment with privl==0x2
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; while code in ring 2 and 3 can. Note that the privilege level remains the same
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; ie. a far-jump form ring 3 to a privl==2-segment remains in ring 3 after the jump.
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;
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; If 0 code in this segment can only be executed from the ring set in privl.
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;
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; RW: Readable bit/Writable bit.
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; Readable bit for code selectors: Whether read access for this segment is allowed. Write access is never allowed for code segments.
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; Writable bit for data selectors: Whether write access for this segment is allowed. Read access is always allowed for data segments.
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;
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; Ac: Accessed bit. Just set to 0. The CPU sets this to 1 when the segment is accessed.
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;
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; Gr: Granularity bit. If 0 the limit is in 1 B blocks (byte granularity), if 1 the limit is in 4 KiB blocks (page granularity).
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;
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; Sz: Size bit. If 0 the selector defines 16 bit protected mode. If 1 it defines 32 bit protected mode.
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; You can have both 16 bit and 32 bit selectors at once.
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;
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; AvL: Availible to software bit, the CPU does not use this field and software can read/write to it
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;
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; D/B bit: The default operand-size bit is found in code-segment and data-segment descriptors but not in system-segment descriptors. Setting
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; this bit to 1 indicates a 32-bit default operand size, and clearing it indicates a 16-bit default size.
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;
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; E bit: Expand down bit: Setting this bit to 1 identifies the data segment as expand-down.
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; In expand-down segments, the segment limit defines the lower segment boundary while the base is the upper boundary
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;
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; A GDT entry is 8 bytes and is constructed as follows:
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; First DWORD
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; 0-15 Limit 0:15 First 16 bits in the segment limiter
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; 16-31 Base 0:15 First 16 bits in the base address
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;
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; 2nd DWORD
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;
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; 0:7 Base 16:23 Bits 16-23 in the base address
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; 8:12 S/Type Segment type and attributes, S = bit 12, Type = 8:11, Type is either [1, DC, RW, Ac] <code> or [0, E, RW, Ac] <data>
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; 13:14 Privl 0 = Highest privilege (OS), 3 = Lowest privilege (User applications)
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; 15 Pr Set to 1 if segment is present
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; 16:19 Limit 16:19 Bits 16-19 in the segment limiter
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; 20:22 Attributes Different attributes, depending on the segment type
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; 23 Gr Used together with the limiter, to determine the size of the segment
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; 24:31 Base 24:31 The last 24-31 bits in the base address
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;
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;
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;
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; see docs/gdt.txt for a quick refresher on GDT
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ALIGN 4, db 0
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unreal_gdt_info:
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unreal_gdt_size: dw (unreal_gdt_end - unreal_gdt_start) - 1
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@@ -413,11 +377,17 @@ unreal_gdt_start:
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; entry 0
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dq 0 ; first entry is null
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; entry 1 (4 GiB flat data map)
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; first dword
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; entry 1 (4 GiB flat code map)
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dw 0xFFFF ; Segment Limit 15:0 (Same large limit as data segment)
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dw 0x0000 ; Base Address 15:0
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db 0x00 ; Base Address 23:16
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db 1001_1010b ; Access Byte: 1001_1010b for executable code
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db 1000_1111b ; Flags and Segment Limit 19:16 (Same as data segment)
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db 0x00 ; Base Address 31:24
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; entry 2 (4 GiB flat data map)
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dw 0xFFFF ; 0:15 limit
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dw 0x0000 ; 0:15 base
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; second dword
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db 0x00 ; 16:23 base
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db 1001_0010b ; bit 0:4 = S/Type, [1, DC, RW, Ac] <code> or [0, E, RW, Ac] <data>
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; bit 5:6 = Privl
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@@ -453,18 +423,32 @@ gdt32_info:
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gdt32_start:
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dq 0
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.gdt32_code:
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dw 0xFFFF ; code segment (RX)
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dw 0x0000
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db 0x00
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db 1001_1000b ; Access: readable, executable
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db 1100_1111b ; 4KB granularity, 32-bit
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db 0x00
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.gdt32_data: ; data segment (RW)
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dw 0xFFFF
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dw 0x0000
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db 0x00
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db 1001_1000b
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db 1100_1111b
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db 1001_0010b ; Access: readable, writable
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db 1100_1111b ; 4KB granularity, 32-bit
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db 0x00
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.gdt32_data:
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.gdt32_stack: ; Stack segment (RW)
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dw 0xFFFF
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dw 0x0000
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db 0x00
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db 1001_0010b
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db 1100_1111b
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db 1001_0010b ; Access: readable, writable
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db 1100_1111b ; 4KB granularity, 32-bit
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db 0x00
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.gdt32_ro_data: ; Read-only data segment (RO)
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dw 0xFFFF
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dw 0x0000
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db 0x00
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db 1001_0000b ; Access: readable, not writable
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db 1100_1111b ; 4KB granularity, 32-bit
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db 0x00
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gdt32_end:
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