Ada Bare Bones: Difference between revisions

.global startup # Make the startup entry point symbol visible to the linker

# Setup the Multiboot header (see GRUB docs for details)

.set ALIGN, 1<<0 # Align loaded modules on page boundaries

.set MEMINFO, 1<<1 # Provide memory map

.set FLAGS, ALIGN | MEMINFO # This is the Multiboot 'flag' field

.set MAGIC, 0x1BADB002 # 'magic number' lets your bootloader find the header

.set CHECKSUM, -(MAGIC + FLAGS) # Checksum required

header: # Must be in the first 8kb of the kernel file

.align 4, 0x90 # Pad the location counter (in the current subsection) to a 4-byte

# (DWORD) storage boundary. The way alignment is specified can vary

# with host system architecture.

# Reserve initial kernel stack space

.set STACKSIZE, 0x4000 # 0x4000 being 16k.

.lcomm stack, STACKSIZE # Reserve 16k stack on a doubleword (32bit) boundary

.comm mbd, 4 # Declare common symbol mbd, allocate it 4-bytes (DWORD) of

# uninitialized memory.

.comm magic, 4 # Declare common symbol magic, allocate it 4-bytes (DWORD) of

# uninitialized memory.

movl $(stack + STACKSIZE), %esp # Set up the stack

# The following saves the contents of the registers as they will likely be

# overwritten because main is not our actual entry point, Bare_Bones is. We

# will use these 2 symbols inside Bare_Bones.

movl %eax, magic # EAX indicates to the OS that it was loaded by a Multiboot-compliant boot

# loader

movl %ebx, mbd # EBX must contain the physical address of the Multiboot information

# structure

call main # Call main (created by gnatbind)

cli # Disable interrupts. then intentionally hang the system.

# CLI only affects the interrupt flag for the processor on which it is

# executed.

hlt # Because the HLT instruction halts until an interrupt occurs, the

# combination of a CLI followed by a HLT is used to intentionally hang

# the computer if the kernel returns.

# HLT is in a loop just in case a single HLT instruction fails to execute

# for some reason, (such as in the case of an NMI).