title: Why isn't loop decrementing cx register?
body: <p>I'm trying to load sectors from the disk into memory with the following assembly code, but as I figured out with some int 0x10 to the terminal, the reason it's not working is it gets stuck in an infinite loop. I thought loop automatically decremented the cx register for me. Below is the code, with the test show_message left in for you to see how I came to the conclusion that loop is not decrementing cx. What I see is msg_2 "Still in loop" over and over and over again without end.</p>
  
  <p>Another great answer would be why the read never succeeds (why the carry flag is never clear) as I'm very confident the values for the registers are good at the time of the call, but other than those registers' contents not being right, if someone can see something I did wrong in setting up for int 0x13, that would be great too.</p>
  
  <p>One last note: I know about ch being the low eight bits of the cylinder, the two high bits of cl being the two high bits of the cylinder, and the low 6 bits of cl being the sector number. My stuff is at cylinder 0, sector 2, so [CYLNUM] is 0x00 and [SECTNUM] is 0x02.</p>
  
  <pre><code>;;;Load Memory;;;
  
  load_mem:
  
   mov cx, 3        ;Try to read the drive 3 times
  
  
  
  read_loop:
  
    xor ah, ah        ;ah=0, reset drive
  
    int 0x13            ;Call drive reset
  
  
  
    mov ax, [SYSADDR]
  
    mov es, ax        ;Destination- es:bx
  
    mov bx, 0
  
  
  
    mov dl, [DRIVENUM]
  
    mov dh, [HEADNUM]
  
    mov al, [NUMKERNELSECTS]
  
    mov ch, [CYLNUM]
  
    mov cl, [SECTNUM]
  
    mov ah, 0x02        ;ah=2, read drive
  
    int 0x13            ;Call read interrupt
  
    jnc exit            ;If carry flag is clear, exit
  ;;test;;
    mov si, msg_2
    call show_message
  ;;test;;
  
    loop read_loop
  
    mov si, read_error    ;Failed 3 times
  
    call show_message
  
  hang:
  
    jmp hang            ;hang for now
  
  exit:
  
    ret
  </code></pre>