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System architecture 15
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asynchronous (handshake) bus operation
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16 file registers used to hold address and data for all operations
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a sense (interrupt) input line to indicate that a particular device (tape,
TTY, PE) requires action by the CPU
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a trap facility which, when activated by an external signal,
causes the
CPU to immediately begin executing instructions starting at a particular
address; this facility is used to enter a recovery routine when a fault is
detected
Mass Storage Unit
A mass storage unit equipped with two floppy diskettes and/or a Winchester
hard disk is used for high speed loading of the resident operating programs
and office data into system memory. The loading process is controlled by
instructions held in the Read-Only-Memory (ROM) fiiware.
When
loading is complete, the diskettes remain in the mass storage unit to provide
a non-volatile store for automatic loading purposes in the event of software
being erased from memory during a power failure. Non-resident software is
loaded from the disk automatically or by manual request when required.
The Mass Storage Interface (MSI) card is designed to interface with
external devices that are compatible with the Small Computer System
.._
Interface (SCSI). This storage capability is provided by the following
hardware configurations:
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NT8D68AA Floppy Disk Unit: two 3.5” 1.2 Mbyte floppy disk drives
(providing 2.4 Mbytes of formatted capacity) as a standard system
offering,
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NTSD69 AA Multi Disk Unit: a Winchester Hard Disk with 10 Mbytes
of formatted capacity. When this option is equipped, the floppy disks
are used for backup and system loading.
System overview 553-3001-100
C.