Functional Description; Program Levels - IBM 3745 Maintenance Information Reference

Functional Description
Program Levels
The central control unit (CCU) is an interrupt-driven processor with a stored
program (called the 'control program' in this manual) that controls the data
transfers on the channel and transmission interfaces.
The CCU:
• Executes the machine instruction set (CCU cycle = 75 ns for TCM or 56 ns
for PUC). to perform arithmetic or logical operations, exchange data
between main storage and the working registers, and also between the
local store and the work registers.
Data transit between the CCU and main storage is achieved via a high-
speed buffer (cache) under the control of the SCTl logic.
Data can also transit directly between main storage and high-speed
adapters via the 'Direct Memory Access' (DMA) logic.
• Communicates with adapters through the IOC logic in PIO or AIO mode:
PIO mode: The exchange operation is initiated by IOH/IOHI instructions in
the CCU.
AIO mode: The operation uses cycle steals for data exchange between
adapters and main storage without control program intervention.
Two buses: IOC1 and IOC2 give access to the adapters' environment
through the IOC switch.
• Communicates with the MOSS through the MOSS IOC. The operations per-
formed can be direct or indirect (MIOH/MIOHI). MOSS uses the CCU level
scan sensitive design (lSSD) to read or write any CCU discrete latch.
The controller hardware has five operational program levels:
.• Program level 1
This is the highest priority program level. Interrupt requests assigned to
level 1 include all critical check conditions such as CCU checks, program
checks, addressing exceptions, and adapter checks. Initial Program load
(IPl) and address compare interrupts are also handled at this level.
• Program level 2
Normal operational interrupt requests from the communication adapters are
assigned to this program level and certain program controlled interrupts
(PCls) are also assigned to this level.
• Program level 3
Normal operational interrupt requests from the channel adapters, interval
timers and program-controlled interrupt 3 '(PCI) requests and panel inter-
rupts are assigned to this level.
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IBM 3745 Maintenance Information Reference (MIR)