88 lines
2.3 KiB
PHP
88 lines
2.3 KiB
PHP
%define DIV_ROUNDUP(a, b) (((a) + ((b) - 1)) / (b))
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%define ALIGN_UP(x, a) (DIV_ROUNDUP((x), (a)) * (a))
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; The GDT is copied to <start of EBDA> - gdt.size, which will also serve
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; as the upper limit for balloc()
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load_gdt:
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pusha
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push es
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mov ax, word [0x40e] ; 0x40e contains the value of a segment pointing to the EBDA
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sub ax, ALIGN_UP(gdt.size, 16) / 16
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mov es, ax
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mov word [gdt.ptr], ax
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shl dword [gdt.ptr], 4
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xor di, di
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mov si, gdt.start
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mov cx, gdt.size
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rep movsb
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lgdt [gdt]
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pop es
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popa
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ret
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gdt:
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dw .size - 1 ; GDT size
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.ptr:
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dd 0 ; GDT start address (calculated at runtime)
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.start:
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; Null descriptor (required)
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dw 0x0000 ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 00000000b ; Access
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db 00000000b ; Granularity
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db 0x00 ; Base (high 8 bits)
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; 16-bit code
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dw 0xffff ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 10011010b ; Access
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db 00000000b ; Granularity
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db 0x00 ; Base (high 8 bits)
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; 16-bit data
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dw 0xffff ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 10010010b ; Access
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db 00000000b ; Granularity
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db 0x00 ; Base (high 8 bits)
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; 32-bit code
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dw 0xffff ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 10011010b ; Access
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db 11001111b ; Granularity
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db 0x00 ; Base (high 8 bits)
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; 32-bit data
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dw 0xffff ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 10010010b ; Access
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db 11001111b ; Granularity
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db 0x00 ; Base (high 8 bits)
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; 64-bit code
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dw 0x0000 ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 10011010b ; Access
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db 00100000b ; Granularity
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db 0x00 ; Base (high 8 bits)
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; 64-bit data
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dw 0x0000 ; Limit
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dw 0x0000 ; Base (low 16 bits)
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db 0x00 ; Base (mid 8 bits)
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db 10010010b ; Access
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db 00000000b ; Granularity
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db 0x00 ; Base (high 8 bits)
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.end:
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.size: equ .end - .start
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