.intel_syntax noprefix
.section .boot16, "awx"
.code16
.global _code_16
.global gdt32_desc
.global gdt32_offset_code
.global gdt32_offset_data
.global print_str
.global print_hex
.extern __e820_memory_map_len
.equ TERMINAL_COLOR, 0x0F
.equ VIDEO_PAGE, 0x00
_code_16:
cli
cld
mov si, offset enter_bootstrap
call print_str
mov di, offset __e820_memory_map
call do_e820
jnc e820_success
mov si, offset _e820_fail
call print_str
0: jmp 0b
e820_success:
mov dword ptr [__e820_memory_map_len], ebp
mov dword ptr [__e820_memory_map_len + 4], 0
lgdt [gdt32_desc]
mov eax, cr0
or al, 1
mov cr0, eax
.att_syntax prefix
jmpl $0x08, $0x100000
.intel_syntax noprefix
print_str:
pushad
mov ah, 0x0e # ah=0x0E, int 0x10, print character and move cursor
mov bx, 0 << 8 | 0x0F
1:
lodsb
int 0x10
cmp byte ptr [si], 0
jne 1b
popad
ret
print_hex:
pushad
mov bp, sp
push 0
1:
lodsb
mov ah, al
shr ah, 4
and al, 0x0F
call hex
xchg al, ah
call hex
push ax
loop 1b
mov si, sp
call print_str
mov sp, bp
popad
ret
hex:
add al, 0x30
cmp al, 0x39
jle 0f
add al, 'A' - 0x30 - 0x0A
0:
ret
# uses eax = 0xe820, int 15h to get memory map
# returns number of entries in bp
# places memory map at es:di
do_e820:
xor ebx, ebx # ebx must be 0 to start
xor ebp, ebp # keep an entry count in bp
mov edx, 0x0534D4150 # Place "SMAP" into edx
mov eax, 0xe820
mov dword ptr es:[di + 20], 1 # force a valid ACPI 3.X entry
mov ecx, 24 # ask for 24 bytes
int 0x15
jc short .failed # carry set on first call means "unsupported function"
mov edx, 0x0534D4150 # Some BIOSes apparently trash this register?
cmp eax, edx # on success, eax must have been reset to "SMAP"
jne short .failed
test ebx, ebx # ebx = 0 implies list is only 1 entry long (worthless)
je short .failed
jmp short .jmpin
.e820lp:
mov eax, 0xe820 # eax, ecx get trashed on every int 0x15 call
mov dword ptr es:[di + 20], 1 # force a valid ACPI 3.X entry
mov ecx, 24 # ask for 24 bytes again
int 0x15
jc short .e820f # carry set means "end of list already reached"
mov edx, 0x0534D4150 # repair potentially trashed register
.jmpin:
jcxz .skipent # skip any 0 length entries
cmp cl, 20 # got a 24 byte ACPI 3.X response?
jbe short .notext
test byte ptr es:[di + 20], 1 # if so: is the "ignore this data" bit clear?
je short .skipent
.notext:
mov ecx, es:[di + 8] # get lower uint32_t of memory region length
or ecx, es:[di + 12] # "or" it with upper uint32_t to test for zero
jz .skipent # if length uint64_t is 0, skip entry
inc ebp # got a good entry: ++count, move to next storage spot
add di, 24
.skipent:
test ebx, ebx # if ebx resets to 0, list is complete
jne short .e820lp
.e820f:
clc # there is "jc" on end of list to this point, so the carry must be cleared
ret # returns the number of entries in bp
.failed:
stc # "function unsupported" error exit
ret
enter_bootstrap: .asciz "[+] enter bootstrap stage\r\n"
_e820_fail: .asciz "e820 unsupported."
# https://web.archive.org/web/20190424213806/http://www.osdever.net/tutorials/view/the-world-of-protected-mode
gdt32:
# 0 byte offset
gdt32_null:
.8byte 0 # null segment
# 8 byte offset
gdt32_code:
.2byte 0xffff # segment limit
.2byte 0 # base limit
.byte 0 # base limit continued...
.byte 0b10011010 # access flags
.byte 0b11001111
.byte 0
# 16 byte offset
gdt32_data:
.2byte 0xffff # segment limit
.2byte 0 # base limit
.byte 0 # base limit continued...
.byte 0b10010010 # access flags
.byte 0b11001111
.byte 0
gdt32_end:
gdt32_desc: # gdt descriptor
.2byte gdt32_end - gdt32 - 1 # size of gdt
.4byte gdt32 # address of gdt
.equ gdt32_offset_code, gdt32_code - gdt32
.equ gdt32_offset_data, gdt32_data - gdt32
.att_syntax prefix
