Technical data

Central Processor Unit, Continued
through a scatter/gather map that allows physically discontiguous
pages of data to appear to be contiguous to the bus adapter option.
Sound
Generator
Sound output uses the DTMF tone generation capability of
the 79C30 chip. Two tone generators may be individually
programmed for frequency and amplitude; their outputs appear
summed using either the loudspeaker integral to the system
unit, or to headphones, or an external loudspeaker if plugged
in to the jack at the front of the machine. The resolution of the
frequency generators is eight bits, giving a frequency range of 8
Hz to approximately 2 kHz.
Serial Line
Controller
The VAXstation 4000 Model 90 system board serial line controller
handles four asynchronous serial lines. The controller consists of
the DC7085 QUART and a 64 entry FIFO RAM shared by all four
receive lines.
Time-Of-Year
Clock
The time-of-year (TOY) clock consists of an MC146818BM CMOS
watch chip that keeps the date and time of day and contains 50
bytes of general purpose RAM storage. This chip includes a time
base oscillator and a lithium battery on-chip. The battery powers
the chip logic and oscillator while the system power is off.
Station
Address ROM
A 32-byte ROM on the system board contains a unique network
address for each system. This ROM is installed in a socket so it
can be moved in the event that a system’s CPU board is replaced.
Memory
Control
Subsystem
The memory control subsystem provides support for the KA49
memory subsystem. A key feature of the KA49 memory
subsystem is the use of ownership bits to maintain a sense
of ownership over each hexaword (32 bytes) of main memory.
This ownership mechanism serves the dual function of
maintaining coherency between main memory and the NVAX
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