BIOSによりA20以上のアドレスバスを有効にする

code:query_a20_gate_support.asm

query_a20_support:

mov ax, 2403h

int 15h

jb not_supported

cmp ah, 0

jnz not_supported

; al bit 0 keyboard controller supported

; al bit 1 fast a20 gate supported

query_a20_status:

mov ax, 2402h

int 15h

jb failed

cmp ah, 0

jnz failed

cmp al, 1

jz activated

enable_a20_by_bios:

mov ax, 2401h

int 15h

jb failed

cmp ah, 0

jnz failed