Question

因此，当我看到本教程的源代码时，我试图学习一些汇编代码。现在，我已经对其进行编译并以有效的输出运行它：

Booting from hard disk...
Hello, World
Goodbye
0x12fe

但是问题是，当我查看代码时，有 call 命令用于某些功能，而其他命令未明确调用。我想我的意思是boot_sect_main.asm，该函数print在boot_sect_print.asm文件中被调用，但没有调用 start 或 done 函数，但是这两个函数是仍在运行。有人可以解释为什么会发生这种情况。

boot_sect_main.asm

[org 0x7c00] ; tell the assembler that our offset is bootsector code

; The main routine makes sure the parameters are ready and then calls the function
mov bx, HELLO
call print

call print_nl

mov bx, GOODBYE
call print

call print_nl

mov dx, 0x12fe
call print_hex

; that's it! we can hang now
jmp $

; remember to include subroutines below the hang
%include "boot_sect_print.asm"
%include "boot_sect_print_hex.asm"


; data
HELLO:
    db 'Hello, World', 0

GOODBYE:
    db 'Goodbye', 0

; padding and magic number
times 510-($-$$) db 0
dw 0xaa55

boot_sect_print.asm

print:
    pusha

; keep this in mind:
; while (string[i] != 0) { print string[i]; i++ }

; the comparison for string end (null byte)
start:
    mov al, [bx] ; 'bx' is the base address for the string
    cmp al, 0 
    je done

    ; the part where we print with the BIOS  help
    mov ah, 0x0e
    int 0x10 ; 'al' already contains the char

    ; increment pointer and do next loop
    add bx, 1
    jmp start

done:
    popa
    ret



print_nl:
    pusha

    mov ah, 0x0e
    mov al, 0x0a ; newline char
    int 0x10
    mov al, 0x0d ; carriage return
    int 0x10

popa
ret

boot_sect_print_hex.asm

; receiving the data in 'dx'
; For the examples we'll assume that we're called with dx=0x1234
print_hex:
    pusha

    mov cx, 0 ; our index variable

; Strategy: get the last char of 'dx', then convert to ASCII
; Numeric ASCII values: '0' (ASCII 0x30) to '9' (0x39), so just add 0x30 to byte N.
; For alphabetic characters A-F: 'A' (ASCII 0x41) to 'F' (0x46) we'll add 0x40
; Then, move the ASCII byte to the correct position on the resulting string
hex_loop:
    cmp cx, 4 ; loop 4 times
    je end

    ; 1. convert last char of 'dx' to ascii
    mov ax, dx ; we will use 'ax' as our working register
    and ax, 0x000f  ; 0x1234 -> 0x0004 by masking first three to zeros
    add al, 0x30 ; add 0x30 to N to convert it to ASCII "N"
    cmp al, 0x39 ; if > 9, add extra 8 to represent 'A' to 'F'
    jle step2
    add al, 7 ; 'A' is ASCII 65 instead of 58, so 65-58=7

step2:
    ; 2. get the correct position of the string to place our ASCII char
    ; bx <- base address + string length - index of char
    mov bx, HEX_OUT + 5 ; base + length
    sub bx, cx  ; our index variable
    mov [bx], al ; copy the ASCII char on 'al' to the position pointed by 'bx'
    ror dx, 4 ; 0x1234 -> 0x4123 -> 0x3412 -> 0x2341 -> 0x1234

    ; increment index and loop
    add cx, 1
    jmp hex_loop

end:
    ; prepare the parameter and call the function
    ; remember that print receives parameters in 'bx'
    mov bx, HEX_OUT
    call print

    popa
    ret

HEX_OUT:
    db '0x0000',0 ; reserve memory for our new string

程序集命令正在运行，而我没有明确调用它们

0 个答案: