Alto
Hardware Manual
Section
2:
Microprocessor
4
registers. However, the registers themselves do
not
change.
At
the
end
of
the cycle,
if
the microinstruction specifies that one or
more
registers
be
loaded, they
are
loaded instantaneously and simultaneously with the newly-computed values. These
then
serve as
the
initial register values for the next microinstruction. As a result, it
is
possible (and
in
fact very
common)
to
both
read
and
load a register during the same microinstruction.
The
R registers behave similarly
except that it is
not
possible to both read and load
an
R register during the same microinstruction.
R
SELECT
The R select field specifies one
of
the
32
R cells to
be
loaded
or
read
under
control
of
the
bus
source
field,or,
in conjunction with the bus source field, one
of
the
256
locations to
be
read from the
constant·
ROM.
The R field is also used to address registers in S
--
see section
8.
The low order two bits
of
the R address
(but
not the constant
ROM
address) may
be
taken from fields
in
IR
under
control
of
the functions. This allows the emulator to address its central registers easily.
ALU
FUNCTIONS
The
ALUF
field controls the sN74181
ALU.
This device can do a total
of
48
arithmetic
and
logical
operations, most
of
which are relatively useless. The 4-bit field
is
mapped
by
a
PROM
into the 16
most
useful functions.
ALUF
T
FUNCTION
S3
S2
Sl
SO
M
C
OPERA.TION
0
*
BUS
1 1 1 1 1 0
A
1 T 1
0 1 0
1
0
B
2
*
BUS
OR
T 1
1
1 0
1
0 A+B
3
BUS
AND
T 1 0 1 1 1
0
AB
4
BUS
XOR
T 0 1 1 0 1 0
AXORB
5
*
BUS
+ 1 0 0 0 0 0 0
A
PLUS
1
6
*
BUS
-1
1 1 1 1 0 1
A
MINUS
1
7
BUS
+ T
1
0 0
1
0
1 A
PLUS
B
lOB
BUS
- T
0
1
1
0 0 0
A
MINUS
B
llB
BUS
- T - 1
0
1
1
0 0
1 A
MINUS
B
MINUS
1
12B
*
BUS
+ T + 1
1
0
0
1 0
0
A
PLUS
B
PLUS
1
l3B
*
BUS
+
SKIP
0 0 0 0 0
SKIP
A
PLUS
1
14B
*
BUS.
T
(AND)
1
0
1 1 1
0
AB
15B
BUS
AND
NOTT
0 1 1 1 1 0
A &
NOT
B
16B-17B
UNDEFINED
If
T is loaded in an instruction containing an
ALUF
with a * in the T column, it will
be
loaded from
the
ALU
output
rather than from
BUS.
S3-S0
selects the function; M selects logical
or
arithmetic mode by controlling carry propagation; C is
the
carry into the
LSB.
The carry output
is
forced to zero during logical operations
(M
= 0).
input to the
ALU;
T
is
the B input.
BUS
SOURCES
The
bus
data source
(BS)
field
BS
NAME
o
<-RName
1
RName<-
2 (None)
3 Task-specific
specifies one
of
8 data sources for the bus:
SOURCE
Read R
Load R from shifter output (see below)
Enables no source to the
BUS,
leaving
it
all ones
Performs different functions
in
different tasks.
BUS
is
the
A
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