11-1.md

Code: AssemblyUtility.asm that can read or write byte from I/O ports

[BITS 64]

SECTION .text

; read a byte from port
; or write a byte to port
global kInPortByte, kOutPortByte

; function that reads one byte from port
; this function follows IA-32e mode function convention
; param:
;   port address (word): memory address where data is stored
kInPortByte:
    push rdx
    mov rax, 0	 ; initialize register to zero
    mov rdx, rdi ; move first parameter (port addr) to rax
    in al, dx    ; read byte

    pop rdx
    ret


; function that writes one byte to port
; this function follows IA-32e mode function convention
; param:
;   port address (word): I/O port address to write data
;   data (byte): data to write
kOutPortByte:
    push rdx
    push rax

    mov rdx, rdi ; move first parameter (port addr) to rdi
    mov rax, rsi ; move second parameter (data) to rax
    out dx, al   ; write byte

    pop rax
    pop rdx
    ret

Explanation

What does this code do?

1. PS/2 Controller (often called a Keyboard controller) is located on the motherboard, and it uses I/O ports to communicate with CPU. The above code is utility functions that keyboard-related code depends on.

2. The functions are simple. kInPortByte reads data from a I/O port address and kOutputByte writes data to a I/O port address

Long mode calling convention

1. Unlike protected mode, long mode utilizes registers first for parameters. If number of registers for parameters is not enough, stack is also utilizes for extra parameters.

2. Depending on parameter type, they are stored in the following order

   • Integer: RDI, RSI, RDX, RCX, R8, R9 (6 registers)
   • Float: XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7 (8 registers)

3. Unlike protected mode, left parameter is inserted into register or stack first. Right-most one is the last.

4. Depending on return type they are stored in different registers in the following order. Only if the return value size is big, two registers are used

   • Integer: RAX, RDX
   • Float: XMM0, XMM1

long mode calling convention