Address Switches (As) - IBM 7090 Instruction-Reference

Switching Units
Switching circuits are comprised of AND and OR logic blocks. They cannot hold
information, and their output is conditioned only as long as the input is conditioned.
The switching units are:
1. Address switches (AS)
2. Adders (AD) (See Section 3. 1. 11 for arithmetic adding applications. )
Binary Counters
A binary counter is a group of binary trigger positions designed to count the number
of input pulses to the low-order position. There are two types of binary counters:
1. Count-Up Counter: Each position, as it goes off, reverses the status
(on-off) of the next higher position (Figure 3.1-3).
2. Count-Down Counter: Each position, as it goes on, reverses the status
of the next higher position (F igure 3. 1-4) .
Each counter position has three inputs (Figure 3.1-5):
1. Set Pulse: Presets counter to some initial status.
2. Step Pulse: Advances (count up) or decreases (count down) a preset
number.
3. Reset Line: Resets counter positions to all zeros.
Speed-Up Counting
In a counter position for a large number, normal circuitry (where one position
feeds the next higher position) can cause a considerable delay. An example of this
delay is shown in Figure 3.1-6. \Vhen each position goes off, it feeds the next
higher position and reverses its status. Because of the high speed of the system,
this delay must be overcome to insure reliable operation. The following system
is used to reduce the delay.
The outputs of several positions are AND'ed, and if all these positions are
impulsed, their AND changes the next higher position, as shown in Figure 3. 1-7.
The stepping of a low-order counter position may step all positions above it.
Therefore, circuits are attached to the counter unit to speed up counter operation.
Figures 3.1-7 A and 3. 1-7B show the logic of the speed-up circuits used with the
shift counter and program counter.
7090 Counters
1. The program counter (PC) (instruction counter) is a count-up type (Systems,
Volume 5, 03.00.05.1-3.05.09.1)0 See Figure 3.1-7A.
2. The shift counter (SC) is a count-down type (Systems, ·Volume 5, 03.04.14.1-
.03.04. 19. 1). See Figure 3. 1-7B.
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