Model 350 Processor Board
Introduction
This
high-perfonnance
processor
board
for
the
Model 350
computer
has:
•
25
MHz MC68020
CPU.
• 20 MHz MC68881
Floating
Point
Co-processor.
• 32
Kbyte
direct
mapped
instruction/data
cache.
• Memory
Management
Unit.
• MC6840
Timer.
•
Boot
ROM.
•
Self-Test LEDs.
Figure
3-14 is a block diagraln
of
the
Model 350 processor
board.
Refer
to
this
diagram
for
functional descriptions.
Central Processing Unit
A Motorola MC68020
CPU
functions as
the
CPU
on
this
board.
Its
asynchronous
data
bus
is
32-bits wide for
hoth
addresses
and
data
registers.
The
MC68020
CPU
can
address
32 Mbytes
of
RAM
Operating
at
25
MHz, it is significantly faster
than
other
workstation
CPUs.
The
CPU
has 57
instruction
types
and
handles 7
major
data
types.
It
also has
18
addressing
modes, 6 maskable
and
1 non-maskable
interrupt
levels. Six
stack
frames exist in
the
MC68020.
Coprocessor
A floating-point coprocessor
is
used
to
increase systenl performance when
certain
floating-point
math
routines
are
part
of
the
application. An MC68881 chip performs these functions.
It
implements
the
required
IEEE
standard
capabilities. Performance is significantly
greater
than
the
HP
98635A
Floating-Point
Card.
Floating-point
instructions
become
part
of
the
language
of
the
CPU
in its operation.
Memory Management Unit
The
MMU
translates
CPU
logical addresses into physical addresses
and
handles access rights.
Translation
information
is
contained in
tables
resident in
maiJ;l
memory. MMU
halts
the
pro-
cessor
and
performs bus cycles
to
access these tables.
Translations
are
stored
in a
Translation
Lookaside Buffer (TLB) so
the
tables
do
not
need
to
be
accessed for every
CPU
cycle.
The
CPU
can
run
at
the
same
speed
with
the
MMU on
or
off.
Both
the
logical
and
physical address spaces are divided
up
into 4
Kbyte
pages.
The
MMU
provides
the
capability
to
map
any logical page into any physical page. Write
protection
is
provided by
the
MMU
at
the
page
and
segment level.
The
table
structure
is
the
same
as
the
Model 320
computer's
MMU;
this
similarity allows software compatibility.
74
Functional Description
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