Question

I’ve already got the program that takes input from the keyboard and print it on the screen but I am having problem understanding few lines of the code,

here is the code,

     MOV DS, AX
      MOV ES, AX        ;Why Move AX content to ES ???

      MOV DX, OFFSET PNAME      ; PRINT NAME: 
      MOV AH, 09H               
      INT 21H                   


      MOV BYTE PTR SNAME, 21    

      MOV DX, OFFSET SNAME      
      MOV AH, 0AH
      INT 21H

      MOV SI, 0002              

      LEA DX, SNAME[SI]         ; PRINT NAME ENTERED
      MOV AH, 09H
      INT 21H

Why we first move 21 of size byte into SNAME ????
How Result of Keyboard input interrupt service automatically stored in SNAME instead of AL ???

Answer 1

Check your favorite MSDOS function reference.

You can find there this:

Format of DOS input buffer:

Offset  Size    Description     (Table 01344)
00h    BYTE    maximum characters buffer can hold
01h    BYTE    (call) number of chars from last input which may be recalled
(ret) number of characters actually read, excluding CR
02h  N BYTEs   actual characters read, including the final carriage return

So, 21 sets the maximum number of character that you can read from the keyboard into the buffer.

As for “How Result of Keyboard input interrupt service automatically stored in SNAME instead of AL ???“, the question is unclear.