Similarly, the source expression <- (RS|[] (A+B))/2 indicates that the output will be the result (i.e., (A+B)), shifted right one bit, with RSI becoming the most significant bit of the output.
After a shift operation one may want to remember the bit that was shifted out. This can be accomplished by using the overflow facility. There is an OVERFLOW\OVF line on the RTM Bus, and each DMgpa has an output connected to this line.(3) After any given operation the value of OVF may be stored in a one bit OVF register in the Kbus module (see below) and be available for use as a Boolean output of Kbus. The values that OVF takes for DMgpa operations where it is of interest are shown in the table in Figure 13.
Fig. 1-3. Table of values of 0VF for associated DMgpa operations.
All 16 bits of the A register, and bits 15 and 0 of the B register (i.e., A<15:0>, B<15,0>) are available as Boolean outputs of a DMgpa.
The DMgpa0 consists of two double height, extended length boards interconnected via a rear connector.
K(bus sense and termination)\Kbus. Each independent Bus in an RTM system requires a centralized control module. It has a register, Bus Sense Register \BSR, which always contains the result of the' last register transfer that took place via the Bus. Each bit of BSR<15:0> is available as a Boolean.
The block diagram of Kbus is shown in Figure 14. Kbus provides for the following functions:
1. It monitors all register transfer operations via the Bus transfer sequencing signals, and supplies the operation completion signal to the Ke modules to indicate that the requested function has been completed. This signal is called Bus DONE\DONE. As indicated, these signals are invisible to the RT level user because they are a prewired part of the Bus.
2. It provides for manual control of an RTM system, if switches are connected to it. A set of suitable switches are provided in the T(lights and switches), described later. The manual START switch causes an output
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3. For 1972 PDP-16's, the OVF Bus signal is not valid when more than one DMgpa is connected to a given Bus.
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