0.00-050613_boot.s

!    boot.s
!
! It then loads the system at 0x10000, using BIOS interrupts. Thereafter
! it disables all interrupts, changes to protected mode, and calls the 

BOOTSEG = 0x07c0
SYSSEG  = 0x1000            ! system loaded at 0x10000 (65536).
SYSLEN  = 17                ! sectors occupied.

entry start
start:
    jmpi    go,#BOOTSEG
go:    mov    ax,cs
    mov    ds,ax
    mov    ss,ax
    mov    sp,#0x400        ! arbitrary value >>512

! ok, we've written the message, now
load_system:
    mov    dx,#0x0000
    mov    cx,#0x0002
    mov    ax,#SYSSEG
    mov    es,ax
    xor    bx,bx
    mov    ax,#0x200+SYSLEN
    int     0x13
    jnc    ok_load
die:    jmp    die

! now we want to move to protected mode ...
ok_load:
    cli            ! no interrupts allowed !
    mov    ax, #SYSSEG
    mov    ds, ax
    xor    ax, ax
    mov    es, ax
    mov    cx, #0x2000
    sub    si,si
    sub    di,di
    rep
    movw
    mov    ax, #BOOTSEG
    mov    ds, ax
    lidt    idt_48        ! load idt with 0,0
    lgdt    gdt_48        ! load gdt with whatever appropriate

! absolute address 0x00000, in 32-bit protected mode.
    mov    ax,#0x0001    ! protected mode (PE) bit
    lmsw    ax        ! This is it!
    jmpi    0,8        ! jmp offset 0 of segment 8 (cs)

gdt:    .word    0,0,0,0        ! dummy

    .word    0x07FF        ! 8Mb - limit=2047 (2048*4096=8Mb)
    .word    0x0000        ! base address=0x00000
    .word    0x9A00        ! code read/exec
    .word    0x00C0        ! granularity=4096, 386

    .word    0x07FF        ! 8Mb - limit=2047 (2048*4096=8Mb)
    .word    0x0000        ! base address=0x00000
    .word    0x9200        ! data read/write
    .word    0x00C0        ! granularity=4096, 386

idt_48: .word    0        ! idt limit=0
    .word    0,0        ! idt base=0L
gdt_48: .word    0x7ff        ! gdt limit=2048, 256 GDT entries
    .word    0x7c00+gdt,0    ! gdt base = 07xxx
.org 510
    .word   0xAA55

 

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