Alto Hardware Manual Section
2:
Microprocessor
6
5
6
7
ALUCY
MD<-
<-CONSTANT
NEXT<-
NEXT
OR
ALUco.
ALUCO
is
the carry produced
by
the
ALU
during
the most recent microinstruction that loaded
L.
It
is not the carry
produced during execution
of
the microinstruction
that
contains the
ALUCY
function.
Deliver
BUS
data
to
memory (see section 2.3)
Same
as
Fl=7
*Note that the value
of
the
SHIFTER
Ourpur
is
determined by the value
of
L as the
microinstruction
begins
execution
and
the shifter function
(L
LSH
1,
L
RSH
1,
or
L
LCY
8)
specified during the current microinstruction
(if
no
shifter function is specified, the shifter
output
is
equal to
L).
2.2
Constant Memory
The constant memory
is
a 256 x
16
PROM
that holds arbitrary constants.
The
constant memory is gated
to
the bus
by
Fl=7,
F2=7, or BS>4. The constant memory
is
addressed by the
(8
bit) concatenation
of
RSELECT
and
BS.
The intent in enabling constants with
Bs>4
is
to provide a masking facility, particularly
for the
<-MOUSE
and
<-DISP
bus sources. This works because the processor bus
ANDS
if
more than
one
source
is
gated to it. Up to
32
such mask constants can
be
provided for each
of
the four bus sources
>4.
Alto
I:
Note that it
is
not possible
to
use a constant
other
than
-1
with the
+-
MD
bus
source, because
memory parity is calculated
on
the bus, and a parity
error
will result
if
bits are masked
off
in a word
fetched from memory.
2.3
Main Memory
Main memory references are handled differently on Alto I and Alto II.
It
is, however, possible to write
most microcode so that it will operate correctly
on
both machines.
BASICS
Memory
is
addressed by a 16-bit number that refers to a 16-bit word
in
the memory. Addresses 0
through
176777B
are true memory storage locations; addresses 177000B through 177777B are used to
control
1/0
devices that are attached to the Alto memory bus. Some operations
on
memory are
performed on "double-words." The double-word beginning at location
adr
(adr is even) is a 32-bit
quantity equivalent to the 16-bit contents
of
location adr, together with the 16-bit contents
of
location
adr+
1.
(Double-word references operate correctly only on true memory locations,
not
on
I/O device
locations.)
MEMORY
REFERENCES
Alto I and Alto
II:
A memory reference
is
initiated by executing
Fl
=
1,
MAR
<-.
The
results
of
a
read
operation are delivered somewhat later onto the bus with
BS
=
5,
"MD.
A store into the addressed
memory location is achieved with F2=6,
MD<-.
The microprogram partially controls memory timing,
and
must observe certain rules to insure correct operation.
a)
A minimum
of
one microinstruction must intervene between the initiation
of
a memory
reference and an
MD<-
or
<-MD.
b) On both Alto I
and
Alto II, memory cycles last a total
of
5 micro-cycles, although double-
word operations may extend the memory cycle
to
take a total
of
6 micro-cycles. Although
the exact details
of
memory timing differ
on
Alto I
and
Alto II, both machines share
the
property that the processor will suspend execution
of
microinstructions
if
the memory
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