Alto Hardwa.re Manual Section 3: Emulator
12
3.0 EMULATOR
The lowest-priority
Alto
task
is
called the Emulator
task.
This task
is
always requesting wakeup,
but
can
be interrupted by a
wakeup
request from
any
other
task.
In effect, the emulator task is the "background
job." The standard
Alto
microcode
ROM
includes standard emulator task microcode for fetching from
Alto
memory, decoding,· and interpreting instructions from the Standard Instruction Set. In the rest
of
this chapter
we
shall frequently
use
the term "emulator"
to
mean "standard emulator task microcode."
This standard microcode can be extended or replaced, usually
by
executing special emulator task
microcode in the microinstruction
RAM.
This section describes microcode versions installed after June
1976.
To determine the vintage
of
a
machine's microcode,
see
descriptions
of
SIO
and
VERS
(section
3.2).
3.1
Standard Instruction
Set
REGISTERS
The emulator state
is
carried
from
instruction
to
instruction in several registers:
PC:
The "program counter,"
which
contains the 16-bit address
of
the next instruction
to
be
fetched and executed.
It
is
actually
implemented
as
R -register
6.
ACO,
ACl,
AC2,
Ac3:
The accumulators,
each
of which contains
16
bits. Instructions are available
for
transferring contents
of
accumulators
to
and from memory registers and for performing
arithmetic and logical operations
among
accumulators. The notation AC(n)
is
often used to
refer
to
the contents
of
accumulator n
(n=
0,1,2,3).
These accumulators are implemented
as
R -registers
3-0
respectively.
C:
The "carry" bit which
is
modified
by
most
arithmetic operations.
It
is
implemented
as
special
hardware
(see
section
3.5).
MEMORY:
The Alto has "64K" 16-bit memory
words,
addressed by values ranging from 0 to
176777B.
Addresses
177000B
to
177777B
are reserved for various
I/O
device uses (see
Appendix
B).
Memory on
Alto
IIs can
be
extended
to
256K
in
64K
banks (see Section
2.3).
Additional R - and S-registers
may
be used temporarily during emulation
of
a single instruction.
INSTRUCTION
FORMAT
The standard instruction set
is
best described
by
breaking it into four groups according to the
way
the
instructions are formatted (see Figure
3).
Several
of
the instructions compute an "effective address" based on the values
of
the I (indirect), X
(index) and
DISP
(displacement)
fields
of
the
M-group,
J-group and some s-group instructions. The
effective address calculation
is
best described
by
a brief "program." First
we
define the function
SignExtend(x)
to
represent the sign-extension of the 8-bit number
x:
SignExtend(x) =
if
x >
200B
then
x+177400B
else
x.
Then EffAddrO, the function
to
compute the effective address
is:
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