A file learn how to write GDT (Global Descriptor Table) 带中文

gdt.asm

;                                       Segment Descriptor
;
; |------------------------------------------------------|----------------------------------------|
; | 63           56 | 55           52 | 51            48 | 47                40 | 39           32 |
; |  Base (24~31)   |   Flags (3~0)   |   Limit (19~16)  |   Access Byte (7~0)  |   Base (23~16)  |
; |------------------------------------------------------|----------------------------------------|
; | 31                                                16 | 15                                   0 |
; |                  Base (0~15)                         |              Limit (0~15)              |
; |------------------------------------------------------|----------------------------------------|
; 
; Base: A 32-bit value containing the linear address where the segment begins.
;
; Limit: A 20-bit value, tells the maximum addressable unit. In 32-bit mode, max value is 0xfffff (4 GB)
;
; Access:
;     ---------------------------------
;     | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
;     ---------------------------------
;     | P |  DPL  | S | E | DC| RW| A |
;     ---------------------------------
;     P: Present bit
;         Define whether to segment
;         In this <Yes>, so it is 1
;     DPL: Descriptor privilege level field
;         Contains the CPU Privilege level of the segment
;         0 = highest privilege (kernel)
;         3 = lowest privilege (user applications)
;         定义 CPU 特权级别
;         0 = 最高权限 （内核）
;         3 = 最低权限 （用户APP）
;     S: Descriptor type bit
;         0 = the descriptor defines a system segment (eg. a Task State Segment)
;         1 = it defines a code or data segment
;         0 = 系统段（例如：任务状态）
;         1 = 代码段或数据段
;     E: Executable bit
;         0 = data segment, cannot execute
;         1 = code segment, can be executed
;         0 = 数据段，不可执行
;         1 = 代码段，可执行
;     DC: Direction bit/Conforming bit
;         * For Data segment
;             0 = The segment grows up
;             1 = The segment grows down
;             0 = 段从下到上
;             1 = 段从上到下
;         * For Code segment
;             0 = Code in this segment can only be executed from an equal privilege level
;             1 = Code in this segment can be executed from an equal or lower privilege level
;             0 = 此段中的代码<只能>从相等的特权级别执行
;             1 = 此段中的代码可以从相等或更低的特权级别执行
;             简而言之，例如：如果为 1 ，则此段（假设为内核特权）中的代码可以被 用户特权级 的代码执行
;     RW: Readable bit/Writable bit
;         * For Data segment
;             0 = This segment is only readable, not writable
;             1 = This segment is readable and writable
;             0 = 此段只可读，不可写
;             1 = 此段可读也可写
;         * For code segment
;             0 = This segment is neither readable nor writable
;             1 = This segment is readable (all code segment is not writable)
;             0 = 此段不可读也不可写
;             1 = 此段可读，不可写（代码段都不可写）
;     A: Accessed bit
;         Just keep it 0, CPU will set it when the segment is accessed
;         设为 0 即可，CPU在读取这个配置时会把它置 0
; Flags:
; -----------------
; | 3 | 2 | 1 | 0 |
; -----------------
; | G | DB| L |   |
; -----------------
; G: Granularity flag
;     0 = The unit of the `Limit` is 1 bit (byte granularity)
;     1 = The unit of the `Limit` is 4 kB (page granularity)
;     0 = Limit 的单位是 1 bit
;     1 = Limit 的单位是 4 kB
; DB: Size flag
;     0 = This is a 16-bit protected mode segment
;     1 = This is a 32-bit protected mode segment
; L: Long-mode code flag
;     0 = Other mode
;     1 = This is a 64-bit protected mode segment
gdt_nulldesc:
    dd 0, 0
gdt_codedesc:
    dw 0xffff       ; Limit (0~15)
    dw 0x0000       ; Base (0~15)
    db 0x00         ; Base (16~23)
    db 1001_1010B   ; Access Byte (0~7)
    ; 内核特权, 
    db 1100_1111B   ; Flags(0~3) + Limit (16~19)
    ; 单位 1 bit, 32位的段
    db 0x00         ; Base (24~31)
gdt_datadesc:
    dw 0xffff
    dw 0x0000
    db 0x00
    db 1001_0010B
    db 1100_1111B
    db 0x00
gdt_end:

gdt_descriptor:
    gdt_size:
        ; The size of the table in bytes subtracted by 1
        ; 16 bit
        dw gdt_end - gdt_nulldesc - 1
        ; The linear address of the GDT (not the physical address, paging applies)
        ; 32 bit (in 32-bit mode)
        ; 64 bit (in 64-bit mode)
        dd gdt_nulldesc

codeseg euq gdt_codedesc - gdt_nulldesc
dataseg euq gdt_datadesc - gdt_nulldesc