cli
cld
-# Set up data segments and stack.
+# Set up data segments.
subw %ax, %ax
movw %ax, %es
movw %ax, %ds
+# Set up stack segment.
# Stack grows downward starting from us.
-# We don't ever use the stack so this is strictly speaking
-# unnecessary.
+# We don't ever use the stack, but we call into the BIOS,
+# which might.
movw %ax, %ss
movw $0x7c00, %sp
movb $0x88, %ah
int $0x15
jc panic
+ cli # BIOS might have enabled interrupts
addl $1024, %eax # Total kB memory
cmp $0x10000, %eax # Cap at 64 MB
jbe 1f
mov $0x10000, %eax
1: shrl $2, %eax # Total 4 kB pages
- movl %eax, ram_pages
+ movl %eax, ram_pgs
#### Create temporary page directory and page table and set page
#### directory base register.
#### Switch to protected mode.
-# First we turn off interrupts because we don't set up an IDT.
+# Note that interrupts are still off.
- cli
-
-# Then we point the GDTR to our GDT. Protected mode requires a GDT.
+# Point the GDTR to our GDT. Protected mode requires a GDT.
# We need a data32 prefix to ensure that all 32 bits of the GDT
# descriptor are loaded (default is to load only 24 bits).
.long gdt + LOADER_PHYS_BASE # address gdt
#### Fatal error.
-#### Print panicmsg (with help from the BIOS) and spin.
+#### Print panic_message (with help from the BIOS) and spin.
panic: .code16 # We only panic in real mode.
- movw $panicmsg, %si
+ movw $panic_message, %si
movb $0xe, %ah
subb %bh, %bh
1: lodsb
int $0x10
jmp 1b
-panicmsg:
- .ascii "Loader panic!\r\n"
+panic_message:
+ .ascii "Panic!"
.byte 0
-#### Memory size in 4 kB pages.
- .org LOADER_RAM_PAGES - LOADER_BASE
-ram_pages:
+#### Physical memory size in 4 kB pages.
+#### This is initialized by the loader and read by the kernel.
+ .org LOADER_RAM_PGS - LOADER_BASE
+ram_pgs:
.long 0
-#### Command-line arguments inserted by another utility.
-#### The loader doesn't use these, but we note their
-#### location here for easy reference.
- .org LOADER_CMD_LINE - LOADER_BASE
-cmd_line:
+#### Command-line arguments and their count.
+#### This is written by the `pintos' utility and read by the kernel.
+#### The loader itself does not do anything with the command line.
+ .org LOADER_ARG_CNT - LOADER_BASE
+arg_cnt:
+ .long 0
+ .org LOADER_ARGS - LOADER_BASE
+args:
.fill 0x80, 1, 0
-#### Boot-sector signature for BIOS inspection.
- .org LOADER_BIOS_SIG - LOADER_BASE
+#### Boot-sector signature.
+#### The BIOS checks that this is set properly.
+ .org LOADER_SIG - LOADER_BASE
.word 0xaa55