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Global Descriptor Table (GDT)
randcode-generator edited this page Sep 8, 2014
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5 revisions
In code 1, the code segment is at 0x08 and data segment is at 0x10. Note that in order to jump to print function after GDT is turned on, we need to specify the code segment which is 0x08
jmp 0x08:printIn code 2, the code segment is at 0x10 and data segment is at 0x10. Note that we specify 0x10 instead of 0x08 because the code and data segment has been swapped.
jmp 0x10:printCode 1 : the code segment is at 0x08 and data segment is at 0x10 https://github.com/randcode-generator/RandCodeOS/blob/de7760e31832cb93861421fc5c30505708809938/boot.asm
[ORG 0x7c00]
jmp main
msg db 'Welcome to RandCodeOS', 0
gdt_info:
; null descriptor: 8 bytes of zeros
dd 0
dd 0
; GDT code descriptor -- offset is 0x08
dw 0xFFFF ; segment limit (0:15)
dw 0 ; base address (0:15)
db 0 ; base address (16:23)
db 10011010b ; 1001b-1 for present
; -00 for privilage level
; -1 for descriptor type (code or data)
; 1010b-segment type (execute/read)
db 11001111b ; 1100b-1 for granularity
; -1 for operation size (0-16 bit, 1-32 bit)
; -0 reserved bit
; -0 available for software use
; 1111b segment limit(16:19)
db 0 ; base address (24:31)
; GDT data descriptor -- offset is 0x10
dw 0xFFFF ; segment limit (0:15)
dw 0 ; base address (0:15)
db 0 ; base address (16:23)
db 10010010b ; 1001b-1 for present
; -00 for privilage level
; -1 for descriptor type (code or data)
; 0010b-segment type (read/write)
db 11001111b ; 1100b-1 for granularity
; -1 for operation size (0-16 bit, 1-32 bit)
; -0 reserved bit
; -0 available for software use
; 1111b segment limit(16:19)
db 0 ; base address (24:31)
gdt_info_end:
gdt_entry:
dw gdt_info_end - gdt_info - 1 ; GDT size
dd gdt_info ; base of GDT
main:
cli ; disable interrupts
pusha ; push all registers into stack
lgdt [gdt_entry] ; load GDT
popa ; Restores all registers from stack
; Go into protected mode
; set cr0 register to 1
mov eax, cr0
mov eax, 1
mov cr0, eax
; Jump to print
; Note: 0x08 is our code segment as specified in our GDT
jmp 0x08:print
bits 32
print:
mov eax, 0x10
mov ds, eax ; set ds to 0x10 (as specified in GDT)
mov eax, 0xb8000 ; start of color video memory
mov ebx, 0 ; bx is going to be our string offset
mov edx, 0 ; dx is going to be our memory offset
loop:
; must be aligned as the following
; byte 1: character
; byte 2: the color of character
movzx cx, byte[msg + ebx] ; copy character from msg plus offset into cx
; NOTE: must specify to copy a byte
mov [eax + edx], cx ; copy character (cx) into memory
push cx ; save the character by pushing to stack
mov cx, 5 ; 5 is the color Magenta
mov [eax + edx + 1], cx ; copy into memory
pop cx ; pop value (character) from stack into cx
inc bx ; increment bx (move to next character)
add edx, 2 ; add memory counter by 2
test cx, cx ; test for 0
jnz loop ; if not zero, go to loop
hlt
times 510-($-$$) db 0
db 0x55
db 0xAACode 2 : the data segment is at 0x08 and code segment is at 0x10
[ORG 0x7c00]
jmp main
msg db 'Welcome to RandCodeOS', 0
gdt_info:
; null descriptor: 8 bytes of zeros
dd 0
dd 0
; GDT data descriptor -- offset is 0x08
dw 0xFFFF ; segment limit (0:15)
dw 0 ; base address (0:15)
db 0 ; base address (16:23)
db 10010010b ; 1001b-1 for present
; -00 for privilage level
; -1 for descriptor type (code or data)
; 0010b-segment type (read/write)
db 11001111b ; 1100b-1 for granularity
; -1 for operation size (0-16 bit, 1-32 bit)
; -0 reserved bit
; -0 available for software use
; 1111b segment limit(16:19)
db 0 ; base address (24:31)
; GDT code descriptor -- offset is 0x10
dw 0xFFFF ; segment limit (0:15)
dw 0 ; base address (0:15)
db 0 ; base address (16:23)
db 10011010b ; 1001b-1 for present
; -00 for privilage level
; -1 for descriptor type (code or data)
; 1010b-segment type (execute/read)
db 11001111b ; 1100b-1 for granularity
; -1 for operation size (0-16 bit, 1-32 bit)
; -0 reserved bit
; -0 available for software use
; 1111b segment limit(16:19)
db 0 ; base address (24:31)
gdt_info_end:
gdt_entry:
dw gdt_info_end - gdt_info - 1 ; GDT size
dd gdt_info ; base of GDT
main:
cli ; disable interrupts
pusha ; push all registers into stack
lgdt [gdt_entry] ; load GDT
popa ; Restores all registers from stack
; Go into protected mode
; set cr0 register to 1
mov eax, cr0
mov eax, 1
mov cr0, eax
; Jump to print
; Note: 0x10 is our code segment as specified in our GDT
jmp 0x10:print
bits 32
print:
mov eax, 0x10
mov ds, eax ; set ds to 0x10 (as specified in GDT)
mov eax, 0xb8000 ; start of color video memory
mov ebx, 0 ; bx is going to be our string offset
mov edx, 0 ; dx is going to be our memory offset
loop:
; must be aligned as the following
; byte 1: character
; byte 2: the color of character
movzx cx, byte[msg + ebx] ; copy character from msg plus offset into cx
; NOTE: must specify to copy a byte
mov [eax + edx], cx ; copy character (cx) into memory
push cx ; save the character by pushing to stack
mov cx, 5 ; 5 is the color Magenta
mov [eax + edx + 1], cx ; copy into memory
pop cx ; pop value (character) from stack into cx
inc bx ; increment bx (move to next character)
add edx, 2 ; add memory counter by 2
test cx, cx ; test for 0
jnz loop ; if not zero, go to loop
hlt
times 510-($-$$) db 0
db 0x55
db 0xAA