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Bendix G15-D

After Computing Devices became affiliated with the Bendix Corporation, the company became the Canadian representative for Bendix equipment. The 102-D in their Ottawa offices was replaced with a Bendix G-15D. Another G-15D was soon installed in the Department of Electrical Engineering at the University of Manitoba. I can't recall how many G-15Ds were sold in Canada, but I can remember visiting the University of Guelph where Gordon Ashton was using one for statistical calculations. At one time the company installed a G15-D in a van which was used for giving demonstrations in various parts of Western Canada.

There were apparently several models of the Bendix G-15 computer, designated by a suffix "A", "C" and "D", and they were roughly comparable. The basic system was contained in a cabinet 32 inches by 27 inches by 61 inches and weighing 650 pounds. There was a control panel on the front of the cabinet and also a paper tape reader which accommodated a removable tape magazine. A Flexowriter was used for manual input and for low-speed paper tape input and output. Neither a separate power supply nor air conditioning was required. The cost of the basic system was about $45,000. Optional equipment included a card reader and punch, magnetic tape, graph plotter, and a digital differential analyzer. The accompanying photograph shows the G15-D at Computing Devices. The person seated at the card punch is Peggy Steen whom we shall mention later in the discusion of programming. The clipbord shown on the table in the lower left probably contains the booking schedule for use of the G-15D.

The magnetic drum memory consisted of 2160 29-bit words. Internal operation was binary with single-, double- and arbitrary-precision arithmetic. Addition times were about 15 milliseconds including access, and multiplication and division times were approximately 32 milliseconds. A two-address instruction was used where the first address specified the operation and the second the address of the next command. There were fifty basic commands which were considered, according to one of the G15 manuals, "sufficient to program any problem", and which could be modified to give about 1300 different commands which would permit an increase in efficiency and speed of computation. Machine-language programming required considerable skill and patience as occasionally a single coding error could take a day or more to locate and correct.

In addition to machine-language programming there was an interpretive system called "Intercom" and a compiler called "Pogo". In 1960 an "Algo" compiler based on the Algol language was introduced; it will be discussed in a later section. Intercom 1000 Single-Precision had a capacity of 1200 five-digit words while Intercom 1000 Double-Precision allowed up to 1000 twelve-digit words. There was a total of about 50 one-address commands with the format "Op Addr" for floating-point arithmetic, transfer of control, input-output, and the use of index registers as well as number of arithmetic and input-output subroutines. The Intercom 1000 system provided a great simplication over the very complicated machine-language programming for the G15. I can still remember Peggy Steen, working from a carefully prepared flow diagram that was provided her, coding in her very neat handwriting page after page of almost error-free Intercom commands.

The present simulation of Intercom for the sample problem required implementing the following nine commands, where "A" refers as usual to the Accumulator:
     41 Subtract: A <- (A) - (Addr)
     42 Clear and Add: A <- (Addr)
     43 Add: A <- (A) + (Addr)
     49 Store: Addr <- (A)
     20 Trans. if (A) >= 0: If (A) >= 0, take next instruction from Addr
     23 Trans. if (A) = 0: If (A) = 0, take next instruction from Addr
     29 Trans.: Take next instruction from Addr
     33 Print: Type fixed point number and tab
     55 Read: Read from paper tape into the channel specified by the first two digits
           of Addr and word positions 00, 01, ... . For example, "55 0803" will read
           three numbers into 0800, 0801 and 0802.
The J script file is given in G15.ijs, and the program for the sample problem is given below.

        0700   42 0722	A <- 0
	0701   49 0725	N <- 0
	0702   49 0726	Sum <- 0
	0703   49 0727	Max <- 0
	0704   55 0801	Read P
	0705   42 0800	A <- P
	0706   23 0718	Is P = 0?
 	0707   43 0726	Sum <- Sum + P
	0708   49 0726
	0709   42 0725	N <- N +1	
	0710   43 0723
	0711   49 0725
	0712   42 0727	A <- Max
	0713   41 0800	Max - P
	0714   20 0704	Is Max - P >= 0?
	0715   42 0800	Max <- P
	0716   49 0727
	0717   29 0704	Trans. to 0704
	0718   33 0725	Print N
	0719   33 0726	Print Sum
	0720   33 0727	Print Max
	0721   67 0000	Halt
	0722         0
	0723         1
	0724		Working storage
Bendix G15D Intercom

Information about the G-15 may be found at the websites of Paul Pierce of Portland, Oregon and the Australia Computer Museum.

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