Question

为什么这个程序不打印X？有人可以一步一步地解释发生什么事情，使其打印出一个铲子作为输出。

mov ah, 0x0e ; Set higher bit of ax to 0e (probably the instruction opcode)
mov al, the_secret ; Set lower bit of ax to bytes of H
int 0x10

the_secret:
    db 'X'

jmp $ ;Jump to the current address forever

; Padding

times 510 - ($-$$) db 0

dw 0xaa55

Answer 1

...使其打印锹作为输出。

...现在我真的无法为您提供帮助，因为我不知道♠️字符的确切十六进制-> ASCII值...尽管，我可以帮助您理解简单的步骤来打印标准（按键）ASCII字符。

这是完整的 NASM 示例，说明如何将字符打印到监视器显示屏上；我将其分解为特定于视频显示的块，以帮助您更好地理解该过程（请注意，我仅使用了 REAL MODE em>示例中的汇编指令（16位）：

[bits 16] ; Declare CPU Instructions as [REAL_MODE] [org 0x0000] ; Organize Memory to Start @ [0x0000] ; We'll Manually setup Memory Organization Later... jmp _glEntry ; Relocate to Function: ;======================================================== ; This Function Prints an Entire Character Stream until ; NULL Terminated Hex Character (0x00) is Detected... ;======================================================== output16_char_array: ; Legacy BIOS Output CHAR_STREAM Function ;push ax ; Store [AX] Register to [STACK_SPACE] xor ax, ax ; Zero [AX] Register mov ah, 0x0E ; Assign [0x0E] to [AX] : (Legacy BIOS Function Code for BIOS Display) ; Continue Code Execution: .output16_char_array_loop: lodsb ; Collect Single Byte from [DS:SI] and increment SI by 1 : ([AL] = [DS:SI]; inc [SI]) or al, al ; Check Whether [AL] is [0x00] jz .output16_char_array_done ; [AL] == [0x00] Proceed to Function: int 0x10 ; [AL] != [0x00] Continue to Display Hex->ASCII [CHAR] jmp .output16_char_array_loop ; Relocate [CS] to create an Infinite Loop... .output16_char_array_done: ;pop ax ; Restore [AX] Register from [STACK_SPACE] ret ; Return to Calling [CS] Memory Position... ;======================================================== ; For Simplicity I've created a Single Character Output ; Method, but in the form of a MACRO ; ; There is really no difference in the method if Output ; other than that this method only prints a single ; Character and doesn't require a NULL Terminated Hex ; Character (0x00) for EndOf(..) CHAR_STREAM... ;======================================================== %macro output16_char_byte 1 ; Define Macro with a single input method... ;push ax ; Store [AX] Register to [STACK_SPACE] xor ax, ax ; Zero [AX] Register mov ah, 0x0E ; Assign [0x0E] to [AH] : (Legacy BIOS Function Code for BIOS Display) mov al, %1 ; Assign [MACRO_INPUT] to [AL] : (Refer to ['%macro output16_char_byte <..>']) int 0x10 ; Legacy BIOS Interrupt for Teletype Output xor ax, ax ; Zero [AX] Register ;pop ax ; Restore [AX] Register from [STACK_SPACE] %endmacro ; End of ['output16_char_byte'] Macro WELCOME_MSG db "Hello, World!", 0x0D, 0x0A, 0x00 ;======================================================== ; This Function is the Entry Point for the Global Master ; Boot Record ;======================================================== _glEntry: cli ; Disable CPU Interrupts xor ax, ax ; Zero [AX] Register mov ax, 0x07E0 ; Assign [0x07E0] to [AX] : (0x07E0) mov ss, ax ; Assign [AX] to [SS] : ([AX]*[0x0010]) : (0x7E00) mov sp, 0x1000 ; Assign [0x1000] to [SP] : (0x1000) ([0x07E0]:[0x1000]) sti ; Enable CPU Interrupts ; Continue Code Execution: xor ax, ax ; Zero [AX] Register mov ax, 0x07C0 ; Assign [0x07C0] to [AX] : (0x07C0) mov ds, ax ; Assign [AX] to [DS] : ([AX]*[0x0010]) : (0x7C00) ; Continue Code Execution: mov si, WELCOME_MSG ; Assign [WELCOME_MSG] to [SI] : (0x[....]) call output16_char_array ; Call our Output [CHAR_STREAM] Function mov si, WELCOME_MSG ; Assign [WELCOME_MSG] to [SI] : (0x[....]) output16_char_byte BYTE [ds:si+0x0008] ; Assign (BYTE) [DS:SI+0x0008] to Macro Input Method... ; we should have the (9th - 1) letter from [WELCOME_MSG] 'o' ; Continue Code Execution on Return: xor ax, ax ; Zero [AX] Register int 0x16 ; Call BIOS Await Keystroke Interrupt xor ax, ax ; Zero [AX] Register int 0x19 ; Call BIOS Reboot Interrupt ; Continue Code Execution if interrupts fail... cli ; Disable CPU Interrupts hlt ; Halt CPU Execution: times 510 - ( $ - $$ ) db 0x00 ; Pad [BOOTSECTOR] with [0x00] up to 510 Bytes... dw 0xAA55 ; Assign [MAGIC_MBR] Value...

好的，这就是将字符打印到监视器/显示器上的整个过程...

最常见/最有效的将字符打印/显示到显示器的方法：

;======================================================== ; This Function Prints an Entire Character Stream until ; NULL Terminated Hex Character (0x00) is Detected... ;======================================================== output16_char_array: ; Legacy BIOS Output CHAR_STREAM Function ;push ax ; Store [AX] Register to [STACK_SPACE] xor ax, ax ; Zero [AX] Register mov ah, 0x0E ; Assign [0x0E] to [AX] : (Legacy BIOS Function Code for BIOS Display) ; Continue Code Execution: .output16_char_array_loop: lodsb ; Collect Single Byte from [DS:SI] and increment SI by 1 : ([AL] = [DS:SI]; inc [SI]) or al, al ; Check Whether [AL] is [0x00] jz .output16_char_array_done ; [AL] == [0x00] Proceed to Function: int 0x10 ; [AL] != [0x00] Continue to Display Hex->ASCII [CHAR] jmp .output16_char_array_loop ; Relocate [CS] to create an Infinite Loop... .output16_char_array_done: ;pop ax ; Restore [AX] Register from [STACK_SPACE] ret ; Return to Calling [CS] Memory Position...

我想指出，我已评论push ax和pop ax，因为在此简单示例中不需要这些说明。尽管，这些指令只是（在主题之外）预先执行了一个存储器堆栈存储（push）和/或恢复（pop）到AX寄存器。

mov ah, 0x0E非常简单...它将值0x0E分配给AH，这就是int 0x10的{{3}}。

lodsb这是执行以下操作的更有效方式：

mov al, BYTE [SI] ; Assign Single [BYTE] from [SI] to [AL] inc si ; Increment [SI] by 1 equivalent to [SI]++

or al, al检查[AL]是否等于0x00。这是字符流的NULL终止字符。

jz .output16_char_array_done如果[AL]等于0x00，则在.output16_char_array_done执行代码...如果[AL] 不等于0x00继续执行代码...

jmp .output16_char_array_loop一种简单的方法，可以在不使用loop指令的情况下循环或迭代某个功能/句柄（loop更多的硬件资源需要执行，因此除非在您的特定情况下更有效，否则通常避免使用此指令。

ret一条简单的指令，返回到先前的（调用）内存地址。

int 0x10这是Teletype Output Function

为什么不打印X？部件

1 个答案: