Editing IBM 650 (section)

==Sample program==

This one-card program, taken from the ''650 Programming Bulletin 5, IBM, 1956, 22-6314-0'', will set most of the drum storage to minus zeros. The program includes examples of instructions being executed from the console switches and from an accumulator.

To begin, a load card is keypunched with 80 consecutive digits (the second column below) so that, when read, drum locations 0001 through 0008 contents will be as shown.<ref>A 12 punch can be used to identify cards as ''load cards''.  Load cards are directly read into words 1-8 of the specified storage band</ref>

     0001 0000010000
     0002 0000000000-
     0003 1000018003
     0004 6100080007
     0005 2400008003
     0006 0100008000
     0007 6900060005
     0008 2019990003

The console digit switches (address 8000) are manually set to a Read instruction with data address 0004.

     loc-       op|data|next
     ation        |addr|instruction
                  |    |addr

     8000  RD   70 0004 xxxx  Read load card into first band read area

Each drum band has a read area; these read areas are in locations 0001-0010, 0051-0060, 0101-0110 and so on.  Any address in a band can be used to identify that band for a read instruction; the address 0004 identifies the first band.  Execution begins then, from the console with the reading of the eight words on the load card into locations 0001-0008 of the first memory band. In the case of reading a load card, the "next instruction address" is taken from the data address field, not the next instruction address field (shown above as xxxx).  Thus execution continues at 0004

     0004  RSU  61 0008 0007  Reset entire accumulator, subtract into upper (8003) the value 2019990003
     0007  LD   69 0006 0005  Load distributor with 0100008000
     0005  STD  24 0000 8003  Store distributor in location 0000, next instruction is in 8003 (the upper accumulator)
                              Note: the moving of data or instructions from one drum location to another
                                requires two instructions: LD, STD.

Now a two-instruction loop executes:

     8003  STL  20 1999 0003  Store lower accumulator (that accumulator was reset to 0- by the RSU instruction above)
                              The "1999" data address is decremented, below, on each iteration.
                              This instruction was placed in the upper accumulator by the RSU instruction above.
                              Note: this instruction, now in the upper accumulator, will be decremented and then
                                executed again while still in the accumulator.

     0003  AU   10 0001 8003  Decrement data address of the instruction in the accumulator by 1
                              (by adding 10000 to a negative number)

The STL's data address will, eventually, be decremented to 0003, and the AU ... instruction at 0003 will be overwritten with zeros.  When that occurs (the STL's next instruction address remains 0003) execution continues as follows:

     0003  NOOP 00 0000 0000  No-operation instruction, next instruction address is 0000
     0000  HALT 01 0000 8000  Halt, next instruction address is the console
                              (this Halt instruction was stored in 0000 by the STD instruction above)