Alto Hardware Manual Section
2:
Microprocessor
3
2.0 MICROPROCESSOR
This section describes the Alto microprocessor structure.
If
your programming needs on the Alto do not
extend to writing
new
microcode, this section
is
best left untackled.
If
you do need to decipher what
follows,
it may be helpful
to
have a listing
of
the "standard" Alto microcode at your side.
The microprocessor
is
shown schematically
in
Figures 'I and
2.
A principal design goal in this system
was
to
achieve the simplest structure adequate for the required
tasks.
As
a result, the central portion
of
the processor contains very little application-specific logic, and no specialized data paths. The entire
system
is
synchronous, with a
clock
interval of approximately
170
nsec.
All
microinstructions require one -
cycle for their execution.
A second design
goal
was
to
minimize the amount of hardware in the
liD
controllers. This
is
achieved
by doing most
of
the processing associated with 110 transfers with microprograms. To allow devices to
proceed in parallel with each other and
with
CPU
activity, a control structure
was
devised which allows
the microprocessor
to
be shared among up
to
16
fixed
priority
tasks.
Switching among tasks requires
very
little overhead, and occurs typically every
few
microseconds.
2.1
Arithmetic Section
The arithmetic section
of
the processor
consists
of
two
32-word by 16-bit register files
Rand
S,
and five
registers,
T,
L,
M,
MAR,
and IR, The registers are connected
to
the memory and to an
ALU
with a 16-bit
parallel bus. For historical
reasons,
the
sand
M registers are viewed
as
part
of
the microinstruction
RAM
and are described in section
8.
The
ALU
is
a
sl\'74181
type, restricted
so
that it can
do
only
16
arithmetic and logical functions. The
ALU
output
feeds
the
L,
M,
and
MAR
registers. T
may
also
be loaded from the
ALU
output under certain
conditions. L
is
connected
to
a shifter capable
of
left and right shifts by one place, and cycles of
8.
It
has a mode in which it does the peculiar 17-bit shifts
of
the standard instruction set, and a mode which
allows double-length shifts
to
be done.
The
IR
register
is
used
by
the emulator
to
hold
the
current emulated instruction
--
see
section
3.5.
Attached to the bus
is
a 256-word read
only
memory
(ROM)
which holds arbitrary 16-bit constants.
The fields
of
the 32-bit microinstruction are:
FIELD
NAME
MEANING
0-4
RSELECT
R Register Select
5-8
ALUF
ALU
Function
9-11
BS
Bus
Data Source
12-15
Fl Function 1
16-19
F2 Function 2
20
T Load T
21
L
Load L
& M
22-31
NEXT
Next microinstruction
address (subject to modifiers)
When microprogramming the
Alto,
it
is
important
to
understand where the machine's state resides
and
how it changes.
At
the beginning
of
a microinstruction
cycle,
the various registers (principally T,
L,
M,
and
IR,
but also various bits
of
state such
as
ALUCO)
contain values that remain unchanged throughout
execution
of
the microinstruction. During this
time,
the various non-state-retaining data paths and
elements, such
as
the bus,
ALU,
and shifter, compute results based entirely on the initial values
of
these
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