Transfer of data between the PC and the microcontroller:
    Buffer1 shiftReg1 ---------  shiftReg2 Buffer2
Write to board:
    parallel write                     parallel read
    to Buffer1    bits shifted out     from Buffer2
                  one at a time           

===================== 

From the Perspective of the PC:

assembler:  translates source --> S19 records, userProg.s19
            Let's suppose our program is hexmon.s19.
       
XDL downloader:   
       sends a 256 byte program, P1,  to the mini-board
       then sends hexmon.s19 (in ascii)

XDL serial communication:  I/O to the mini-board, e.g., W123455 (in ascii)

===================== 

From the Perspective of the Mini-board:

  Mini-board is in download mode, and reset is pressed:

  PC is loaded with the contents of a memory location in 
     ROM, $BFFE 

     $BFFE contains the address of a program in ROM that 
       1. reads exactly 256 bytes from the serial port, 
          and stores them in RAM, starting at $0000;
          the 256 bytes are the program named P1 above. 
          
          
       2. when that is done, loads the PC with $0000

      What does P1 do?
      It reads S-records, char by char (by reading from
      Buffer2 above), translating each character
      from ASCII into Hex.  In our example, it is
      reading hexmon.s19, which is being transferred
      over the seriel port. 

      P1 burns the values into
      EEPROM at the appropriate places 

Recall S-records: S1 count address data checksum
Example:

S10500000F08E3   
    0F --> 00; 08 --> 01.   **But see caution below!**

S111F8009600D6013DCE000202DF027EF80B18   
    ldaa 0, ldab 1, mul, ldx #2, idiv, ...

S105FFFEF80005
    org reset; fdb start

S9030000FC

Following is pseudo code for P1:
   
Read type; 
while type == S1 {
   read count;
   read address;
   curaddress = address;
   for (i = 0; i < count-1; i++) {
       read the next 2 characters from Buffer2;
       translate them from ASCII into a 1-byte HEX value;
       burn the HEX value into address curaddress;
       curaddress++;
      }
   read and check checksum;
   read type;
}

===================== 
Since P1 is stored in RAM, don't use fdb, fcb instructions in RAM!
They would overwrite P1, corrupting the loading process.

===================== 
Now, put the board in run mode, and press reset.
PC will be loaded with the contents of $FFFE, 
which contains the starting address of the downloaded
user program (here, Hexmon)

Suppose that the PC is now in serial communication mode, 
and it sends the command W123455 over the seriel port.  
Hexmon reads the characters one-by-one from Buffer2, translates 
the numbers into Hex, and performs the write operation (to RAM).