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// -- BOOT TEXT SECTION ---------------------------------------------------------
.code16
.intel_syntax noprefix
.section .boot, "ax", @progbits
// Disable interrupts.
cli
// Clear segment selectors.
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov fs, ax
mov gs, ax
// Set cs to 0x0000, as some BIOSes load the MBR to either 07c0:0000 or 0000:7c000.
jmp 0x0000:entry_rm16
entry_rm16:
// Get current video mode [1].
// Return:
// ah number of columns
// al display mode (see table in [2])
//
// [1] http://www.ctyme.com/intr/rb-0108.htm
// [2] http://www.ctyme.com/intr/rb-0069.htm
mov ah, 0xf
int 0x10
// Execpt that the bios initializes text mode 0x3.
// * 80x25 text mode (cols x rows)
// * 2 byte per character
// [15] blink [14:12] bg color [11:8] fg color [7:0] char
// * 0xB80000 screen address
cmp al, 0x3
// Else we indicate an error with a blue screen.
je 2f
// Set background color [1].
//
// [1] http://www.ctyme.com/intr/rb-0101.htm
mov ah, 0xb
mov bx, 1
int 0x10
1:
hlt
jmp 1b
2:
// Enable A20 address line.
in al, 0x92
or al, 2
out 0x92, al
// Load GDT descriptor.
lgdt [gdt_desc]
// Enable protected mode (set CR0.PE bit).
mov eax, cr0
or eax, (1 << 0)
mov cr0, eax
// Far jump which loads segment selector (0x0008) into cs.
// 0x0008 -> RPL=0, TI=0(GDT), I=1
jmp 0x0008:entry_pm32
.code32
entry_pm32:
// Select data segment selector (0x0010) for ds.
mov ax, gdt_data - gdt
mov ds, ax
// Initialize stack pointer.
// Real Mode memory (https://wiki.osdev.org/Memory_Map_(x86)
// 0x00000500 - 0x00007BFF | 29.75 KiB | conventional memory
mov esp, 0x7c00
// Enter zmbr.zig:kmain.
// Should not return, but for safety we emit a call rather than a jmp.
call kmain
1:
hlt
jmp 1b
// -- RODATA SECTION ------------------------------------------------------------
.section .rodata, "a", @progbits
.balign 8
gdt:
.8byte 0x0000000000000000 // 0x00 | null descriptor
.8byte 0x00cf9a000000ffff // 0x08 | 32 bit, code (rx), present, dpl=0, g=4K, base=0, limit=fffff
gdt_data:
.8byte 0x00cf92000000ffff // 0x10 | 32 bit, data (rw), present, dpl=0, g=4K, base=0, limit=fffff
gdt_desc:
.2byte (. - gdt - 1) // size
.4byte gdt // address
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