x86: add small mbr exampleHEADmaster

1 files changed, 117 insertions, 0 deletions

diff --git a/src/arch/x86/mbr/mbr.S b/src/arch/x86/mbr/mbr.S
new file mode 100644
index 0000000..b74bae5
--- /dev/null
+++ b/src/arch/x86/mbr/mbr.S
@@ -0,0 +1,117 @@
+.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:
+    // Set video mode 3, see [1].
+    //   * 80x25 text mode
+    //   * 640x200 pixel resolution (8x8 pixel per char)
+    //   * 16 colors (4bit)
+    //   * 4 pages
+    //   * 0xB800 screen address
+    //
+    // [1] http://www.ctyme.com/intr/rb-0069.htm
+    mov ax, 0x3
+    int 0x10
+
+    // Move cursor to second row.
+    // http://www.ctyme.com/intr/rb-0087.htm
+    mov ah, 0x02
+    mov bh, 0  // page
+    mov dh, 1  // row
+    mov dl, 0  // col
+    int 0x10
+
+    // Clear direction flag for lodsb below.
+    cld
+
+    // Load pointer to msg_rm string (null terminated).
+    lea si, [msg_rm]
+
+    // Teletype output char at current cursor position.
+    // http://www.ctyme.com/intr/rb-0106.htm
+    mov ah, 0x0e
+1:
+    lodsb         // al <- ds:si ; si+=1 ; (al char to write)
+    test al,al    // test for null terminator
+    jz 2f
+    int 0x10
+    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
+
+    // Write through VGA interface (video memory).
+    // Each character is represented by 2 bytes.
+    //   4 bit bg | 4 bit fg | 8 bit ascii char
+    //
+    // Start writing at third line.
+    mov edi, 0xb8000 + (80 * 2 * 2) //
+
+    lea esi, [msg_pm]
+1:
+    lodsb           // al <- ds:esi ; esi+=1
+    test al, al     // test for null terminator
+    jz 2f
+    or eax, 0x1f00  // blue bg, white fg
+    stosw           // ds:[edi] <- ax; edi+=2
+    jmp 1b
+2:
+    hlt
+    jmp 2b
+
+// For simplicity keep data used by boot sector in the same section.
+.balign 8
+msg_rm:
+    .asciz "Hello from Real Mode!"
+msg_pm:
+    .asciz "Hello from Protected Mode!"
+
+.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
+
+// Write MBR boot magic value.
+.fill 510 - (. - .boot), 1, 0x00
+.2byte 0xaa55