Apple sends:
   One byte: "C" ($C3)
   Byte stream: directory name (null terminated)

 Host sends: return code:
   $00 = ok
   $06 = Unable to change directory

 Apple sends: 
   One byte: "G" ($C7)
   Byte stream: file name (null terminated)

 Host sends: return code:
   $00 = Ok
   $02 = File not found (Apple aborts)

 Apple sends:
   $06 = ACK

 (parts = filesize /(40 * 512), remainder = filesize % (40 * 512))
 (loop: parts * 40 blocks + remainder * blocks)

   Host sends:
     2 bytes: current block number (LSB, MSB)
     1 byte: half-block number (2 = bytes 0-255, 1 = bytes 256-511)
     Half-block, RLE encoded
     two bytes:
       CRC (lo)
       CRC (hi)

   Apple sends:
     One byte acknowledgement:
       $06 = ACK (Apple understood the packet, CRC is correct; send next one)
       $15 = NAK (Resend this same packet again: block number tells what Apple
                 is still waiting to get a good packet for)
     Two bytes: current block number (LSB, MSB)
     One byte: half-block number (2 = bytes 0-255, 1 = bytes 256-511)

   Note: if the host receives a NAK, and the Apple is expecting a packet 
         that is one greater than what's just been sent, then an ACK got 
         lost.  Recovery is for the host to go ahead and advance to the next
         block and send that.

 (loop end)

 Apple sends: One byte: number of errors encountered

Apple sends:
  One byte: "Z" ($DA)
  Byte stream: file name (null terminated)

Host sends:
  Two bytes: file size in blocks (LSB, MSB)
  One byte: return code:
    $00 = File exists (and file size is valid)
    $02 = File does not exist
    $04 = File exists, not recognized as a valid disk image

Apple sends:
  One byte: "P" ($D0)
  Byte stream: file name (null terminated)
  Two bytes:
    Number of blocks to follow (lsb)
    Number of blocks to follow (msb)

Host sends: return code:
  $00 = Ok
  $02 = Unable to write (Apple aborts)

Apple sends:
  $06 = ACK

(parts = filesize /(40 * 512), remainder = filesize % (40 * 512))
(loop: parts * 40 blocks + remainder * blocks)

  Apple sends:
    Two bytes: current block number (LSB, MSB)
    One byte: half-block number (2 = bytes 0-255, 1 = bytes 256-511)
    Half-block, RLE encoded
    two bytes:
      CRC (LSB)
      CRC (MSB)

  Host sends one byte acknowledgement:
    $06 = ACK (Host understood the packet, CRC is correct; send next one)
              (Also: this is a duplicate packet, already received; acknowledge
              so the Apple sends the next one)
    $15 = NAK (Resend the same packet again)

(end loop)

Apple sends: One byte: number of errors encountered

Apple sends: One byte: "R" ($D2)
Apple sends: Byte stream: file name (null terminated)

Host sends one byte return code:
  $00 = ok
  $1a = file can't be opened (Old ADT message number)
  $1e = not a 140k image (Old ADT message number)

(Assuming zero return code...)
(loop: 5 parts * 7 tracks * 16 sectors)

  Apple sends one byte acknowledgement:
    $06 = ACK (always ACK for the first sector)
    $15 = NAK

  Host sends:
    Next sector, RLE encoded
    two bytes:
      CRC (lo)
      CRC (hi)

(loop end)

Apple sends:
  One byte: number of errors encountered

Apple sends: One byte: "S" ($D3)

Host sends one byte return code:
  $00 = Ok
  $1c = File exists
  $1a = Can't open
  $1e = Disk full

(Assuming zero return code...)
(loop: 5 parts * 7 tracks * 16 sectors)

  Host sends:
    $00 = ready to commence

  Apple sends:
    Next sector, RLE encoded
    two bytes:
      CRC (lo)
      CRC (hi)

  Host sends one byte acknowledgement:
    $06 = ACK
    $15 = NAK

(loop end)

Apple sends:
  One byte: number of errors encountered

Apple sends: One byte: "D" ($C4)

Host sends: 
  Screen-formatted data of directory contents; null-terminated

  Final byte:
    $00 = last screen
    $01 = another screen follows

If final byte is $01:
  Apple sends one byte:
      "D" ($C4) = retrieve subsequent screen
      $00 = abort