Intel x86 asm打印字符串函数

我试图编写一个打印字符串函数;

 45 ; the address is stored in si                                                                                       
 46 print_string:                                                                   
 47     pusha                                                                       
 48     ; load character from si                                                    
 49     mov al, [si]                                                                
 50     cmp al, 0x00                                                                
 51     jz print_string_end                                                         
 52     call print_char ; print the char using the print_char function              
 53     inc si ; increment the string printing index si                             
 54 print_string_end:                                                               
 55     popa                                                                        
 56     ret                                                                         
 57                                                                                 
 58 ; print function: print a single character                                      
 59 ; the character is stored in al                                                 
 60 print_char:                                                                     
 61     pusha                                                                       
 62     mov ah, 0x0e                                                                
 63     int 0x16                                                                    
 64     popa            ; don't know what registers int 0x16 modifies               
 65     ret                                                                         
 66                                                                                 
 67 mystring:                                                                       
 68 db "loading operating system",0x00  

这将组装并运行。然而,在我改为使用之前 si 对于要打印的当前字符的地址,我正在使用寄存器 dx 。由于某种原因,从 硅 到 dx 导致以下nasm错误:

bootsector.asm:49: error: invalid effective address

为什么不 mov al, [dx] 工作,但是 mov al, [si] 做