diff options
Diffstat (limited to 'arch/x86/boot/bootloader/head.s')
| -rw-r--r-- | arch/x86/boot/bootloader/head.s | 35 |
1 files changed, 35 insertions, 0 deletions
diff --git a/arch/x86/boot/bootloader/head.s b/arch/x86/boot/bootloader/head.s new file mode 100644 index 0000000..d24e36c --- /dev/null +++ b/arch/x86/boot/bootloader/head.s @@ -0,0 +1,35 @@ +/* + head.s is loaded at 0x1000 (by second stage), its main goal + is run 32-bit startup code. + + After that manipulations it jumps to kernel written by C +*/ + +.code32 # Tell GAS to generate 32 bit code +.extern kernel_main + +.set CODESEG, 0x08 +.set DATASEG, 0x10 + +.global _start +_start: + mov $DATASEG, %ax # Point segment registers to the + mov %ax, %ds # data selector we defined in our GDT + mov %ax, %es + mov %ax, %ss + mov %ax, %fs + mov %ax, %gs + + mov $0x90000, %ebp # Update stack position so it is right + mov %ebp, %esp # at the top of the free space. + +check_a20: + mov $0x112345, %edi # Odd megabyte address + mov $0x012345, %esi # Even megabyte address + mov %esi, (%esi) # If A20 line is disabled two pointers + mov %edi, (%edi) # would point to the address 0x012345 + cmpsd # Compare values at addresses + je . # If equivalent, loop forever + + call kernel_main + jmp . # infinite loop |