142 Part 2 The instruction-set processor: main-line computers
Section 1 Processors with one address per instruction
Fig. 7. Arithmetic element.
Arithmetic element
The arithmetic element, see Fig. 7, consists of three registers, a counter, and a control.
The first register is an accumulator (AC) which actually consists of a partial-sum or adding register and a carry register. The accumulator holds the product during multiplication.
The second or A-register holds the multiplicand during multiplication. All numbers entering the arithmetic element do so through AR.
The third or B-register holds the multiplier during multiplication. The accumulator and B-register shift right or left. A high-speed carry is provided for addition. Subtraction is by 9's complement and end-around-carry. Multiplication is by successive additions, division by successive subtractions, and shift orders provide for shifting right or left by an arbitrary number of steps, with or without roundoff.
The arithmetic element is straightforward except for a few special orders and the high speed at which it operates. Addition takes 3 microseconds complete with carry; multiplication, 16 microseconds average including sign correction.
In Fig. 8 are shown several digits of the arithmetic element. The large panels are accumulator digits. Above the accumulator is the B-register, below it the A-register.
Test control
Test control, shown in Fig. 9, is used at present both for operating and for trouble shooting the computer. The control includes:
1 Power supply control and meters.
2 Neon indicators for all flip-flops in the machine.
3 Switches for setting up special conditions.
4 Manual intervention switches.
5 Oscilloscopes for viewing wave forms. A probe and amplifier system allows viewing any wave form in the computer on one scope at test control.
6 Test equipment to provide synchronizing, stop, or delay pulses at any step of any order of a program, allowing viewing wave forms on the fly anywhere in the machine.
An important part of the test facilities is the test storage, a group of 32 toggle-switch registers plus five flip-flop registers that can be inserted in place of any five of the toggle-switch registers. This storage has proved invaluable not only for testing control and
Fig. 8. View of arithmetic element.