Fig. 18.2.2 shows a simple program in the style of the CSC-1 computer that does the CPU's end of the transfer. This program is actually too simple, since it has no provision for stopping and the STD instruction is extremely unrealistic since it stores the bytes into the same variable, overwriting the old values. A real program would probably put the data bytes into an array.

          LDI  001b         ;form the code for the START command
          OUT  2            ;send START command to the tape reader
          LDI  01b          ;put CPU's status into its status port
          OUT  0            ;which the TR reads as "Go read for me now!"

WHILE0:   NOP               ;get data bytes until done (see below...)

                            ;a spin loop to wait until the TR has delivered next byte
WHILE1:   IN   1            ;read the tape reader's status
          SUB  DATAREADY    ;compare to 10b, which is data is ready to pickup
          JZ   ENDWHILE1    ;if equal, then done
          JMP  WHILE1       ;else go back to top of loop
ENDWHILE1:NOP

          IN   3            ;read data byte on port 3 from the tape reader
          STD  X            ;store into main memory somewhere (X)
          LDI  10b          ;get ready to write 1 into data accepted
          OUT  0            ;write to status port so tape reader sees it
                            ;the tape reader is now spinning, waiting for CPU
          LDI  100          ;pause the loop to allow the tape reader
                            ;to catch up

WHILE2:   JZ   ENDWHILE2    ;busy loop to waste 100 time units here because TR is slower than CPU
          SUB  ONE          ;It counts down from 100 to 0 by subtracing 1
          JMP  WHILE2       ;...sometime inside this loop, the tape reader sets the status
                            ;   register back to "00" so the CPU will see it
ENDWHILE2:NOP

          JMP  WHILE0       ;do it all over again to read next byte
ENDWHILE0:HLT

DATAREADY:NUM  10b          ;"10" is the code for the Tape Reader saying data is ready to get
ONE:      NUM  1