在16位x86匯編中注冊中斷

Question

我在16位x86匯編中編寫自己的操作系統，我正在嘗試注冊自己的中斷，如MS-DOS中的INT 21H。 我在網上找不到任何東西。 我正在使用NASM作為匯編程序。

Answer 1

您可以關閉中斷，然后直接修改中斷向量表（IVT）。 實模式IVT可以在0x0000：0x0000到0x0000：0x03FF找到。 表中的每個條目都是4個字節。 前2個字節是中斷服務程序（ISR）的偏移量，后2個字節是段。

作為示例，鍵盤中斷（IRQ1）是中斷向量9.它將位於偏移9 * 4 = 36 = 0x0024，因此IVT條目將位於0x0000：0x0024。 下面是一個掛鈎鍵盤中斷的簡單引導程序示例：

KBD_BUFSIZE equ 32                 ; Keyboard Buffer length. **Must** be a power of 2
                                   ;     Maximum buffer size is 2^15 (32768)
KBD_IVT_OFFSET equ 9*4             ; Base address of keyboard interrupt (IRQ) in IVT

bits 16
org 0x7c00

start:
    xor ax, ax                     ; AX=0
    mov ds, ax                     ; DS=0 since we use an ORG of 0x7c00.
                                   ;     0x0000<<4+0x7C00=0x07C00
    mov ss, ax
    mov sp, 0x7c00                 ; SS:SP stack pointer set below bootloader

    ; ****** Hooks the keyboard interrupt here ******
    cli                            ; Don't want to be interrupted when updating IVT
    mov word [KBD_IVT_OFFSET], kbd_isr
                                   ; DS set to 0x0000 above. These MOV are relative to DS 
                                   ; 0x0000:0x0024 = IRQ1 offset in IVT
    mov [KBD_IVT_OFFSET+2], ax     ; 0x0000:0x0026 = IRQ1 segment in IVT
    sti                            ; Enable interrupts

    mov ax, 0xb800
    mov es, ax                     ; Set ES to text mode segment (page 0)
    xor di, di                     ; DI screen offset = 0 (upper left)
    mov ah, 0x1F                   ; AH = White on Blue screen attribute
    mov bx, keyboard_map           ; BX = address of translate table used by XLAT
    cld                            ; String instructions set to forward direction

.main_loop:
    hlt                            ; Halt processor until next interrupt
    mov si, [kbd_read_pos]
    cmp si, [kbd_write_pos]
    je .main_loop                  ; If (read_pos == write_pos) then buffer empty and
                                   ;     we're finished

    lea cx, [si+1]                 ; Index of next read (tmp = read_pos + 1)
    and si, KBD_BUFSIZE-1          ; Normalize read_pos to be within 0 to KBD_BUFSIZE
    mov al, [kbd_buffer+si]        ; Get next scancode
    mov [kbd_read_pos], cx         ; read_pos++ (read_pos = tmp)
    test al, 0x80                  ; Is scancode a key up event?
    jne .main_loop                 ;     If so we are finished

    xlat                           ; Translate scancode to ASCII character
    test al, al
    je .main_loop                  ; If character to print is NUL we are finished
    stosw                          ; Display character on console in white on blue

    jmp .main_loop

; Keyboard ISR (IRQ1)
kbd_isr:
    push ax                        ; Save all registers we modify
    push si
    push cx

    in al, 0x60                    ; Get keystroke

    mov cx, [cs:kbd_write_pos]
    mov si, cx
    sub cx, [cs:kbd_read_pos]
    cmp cx, KBD_BUFSIZE            ; If (write_pos-read_pos)==KBD_BUFSIZE then buffer full
    je .end                        ;    If buffer full throw char away, we're finished

    lea cx, [si+1]                 ; Index of next write (tmp = write_pos + 1)
    and si, KBD_BUFSIZE-1          ; Normalize write_pos to be within 0 to KBD_BUFSIZE
    mov [cs:kbd_buffer+si], al     ; Save character to buffer
    mov [cs:kbd_write_pos], cx     ; write_pos++ (write_pos = tmp)

.end:
    mov al, 0x20
    out 0x20, al                   ; Send EOI to Master PIC

    pop cx                         ; Restore all modified registers
    pop si
    pop ax
    iret

align 2
kbd_read_pos:  dw 0
kbd_write_pos: dw 0
kbd_buffer:    times KBD_BUFSIZE db 0

; Scancode to ASCII character translation table
keyboard_map:
    db  0,  27, '1', '2', '3', '4', '5', '6', '7', '8'    ; 9
    db '9', '0', '-', '=', 0x08                           ; Backspace
    db 0x09                                               ; Tab
    db 'q', 'w', 'e', 'r'                                 ; 19
    db 't', 'y', 'u', 'i', 'o', 'p', '[', ']', 0x0a       ; Enter key
    db 0                                                  ; 29   - Control
    db 'a', 's', 'd', 'f', 'g', 'h', 'j', 'k', 'l', ';'   ; 39
    db "'", '`', 0                                        ; Left shift
    db "\", 'z', 'x', 'c', 'v', 'b', 'n'                  ; 49
    db 'm', ',', '.', '/', 0                              ; Right shift
    db '*'
    db 0                                                  ; Alt
    db ' '                                                ; Space bar
    db 0                                                  ; Caps lock
    db 0                                                  ; 59 - F1 key ... >
    db 0,   0,   0,   0,   0,   0,   0,   0
    db 0                                                  ; < ... F10
    db 0                                                  ; 69 - Num lock
    db 0                                                  ; Scroll Lock
    db 0                                                  ; Home key
    db 0                                                  ; Up Arrow
    db 0                                                  ; Page Up
    db '-'
    db 0                                                  ; Left Arrow
    db 0
    db 0                                                  ; Right Arrow
    db '+'
    db 0                                                  ; 79 - End key
    db 0                                                  ; Down Arrow
    db 0                                                  ; Page Down
    db 0                                                  ; Insert Key
    db 0                                                  ; Delete Key
    db 0,   0,   0
    db 0                                                  ; F11 Key
    db 0                                                  ; F12 Key
    times 128 - ($-keyboard_map) db 0                     ; All other keys are undefined

times 510 - ($-$$) db 0                                   ; Boot signature
dw 0xAA55

從BOCHS運行時，它看起來像：

有關此特定鍵盤ISR如何工作的更多信息，您可以看到我以前的Stackoverflow答案

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如果您打算創建自己的Int 21h處理程序，則需要像上面的示例一樣更新IVT，但IVT中的偏移量將為0x21 * 4 = 0x0000：0x0084。