[coreboot] what does 0x80 I/O address mean when initializing CPU?

[Star Liu]

Greetings!

I'm reading the machine code of my bios(got by flashrom), and below is
the assembly code(non formal) trace i have read

FFFFFFF0        jmp F000:FFAA
FFFFFFAA        jmp relative C3
FFFF0070        jmp relative 021D
FFFF0290        CLI
                        CLD
                        mov esp,eax
                        mov ax,cs
                        mov ss,ax
                        mov ebp,edx
                        jmp relative FE43
FFFF00E2        jmp relative 01BA
FFFF029F        mov al,D0
                        out 80,AL

I understand the code trace quite well until the "out 80,AL", why we
need to output "D0" to I/O address 80? what does the I/O address 80
mean?