8163A/B, 8164A/B & 8166A/B Mainframes Programming Guide 23
Introduction to Programming 1
The Status Model
Status Registers
Each node of the status circuitry has three registers:
• A condition register (CONDition), which contains the current status.
This register is updated continuously. It is not changed by having its
contents read.
• The event register (EVENt), which contains details of any positive
transitions in the corresponding condition register, that is, when a bit
changes from 0 -> 1. The contents of this register are cleared when it is
read. The contents of any higher-level registers are affected with regard
to the appropriate bit.
• The enable register (ENABle), which enables changes in the event
register to affect the next stage of registers.
The structures of the Operational and Questionable Status Systems are
similar. Figure 4 on page -26describes how the Questionable Status Bit
(QSB) and the Operational Status Bit (OSB) of the Status Byte Register are
determined.
The event register is the only kind of register that can affect the next stage of
registers.
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