Alto Hardware Manual
Ether Boot:
3.4 Hardware
Section 3: Emulator
30
The single disk sector at the given address is read: the 256 data words are read
into locations 1
to
400B
inclusive; the label
is
read into locations
402B
to
411B
inclusive. When the transfer
is
complete, pc+-1, and the emulator is started.
The disk status
is
stored in location
2,
so the bootstrapping code
must
skip this
location.
If
the
<BS)
key
is
depressed, the microcode anticipates breathing life into the
Alto via the Ethernet. The Ethernet hardware
is
set up to read any packet with
destination Alto number
377B'
into locations 1 to
400B
inclusive.
If
a packet
arrives with good status and with memory location 2
(Le.,
the second word
of
the
packet) equal to
602B
(a "Breath-of-Life" packet),
pc+-3,
and the emulator is
started.
More information regarding boot loaders and boot file formats
is
found with
Buildboot documentation in the Alto Subsystems Manual.
There
is
a small amount
of
special hardware which
is
used exclusively by the emulator. This hardware is
controlled by the task specific
F2's,
and by the
~
DISP
bus source.
The
IR
register
is
used to hold the current instruction.
It
is
loaded with
IR~
(F2=14B).
IR~
also merges
bus bits
0,5,6
and 7 into
NEXT[6-9],
which does a first level instruction dispatch.
The high order bits
of
IR
cannot be read directly, but the displacement field
of
IR
(8
low order bits), may
be read with the
..
DISP
bus source.
If
the x field
of
the instruction is zero
(Le.,
it
specifies page 0
addressing) then the
DISP
field
of
the instruction
is
put
on
Bus[8-15]
and
BUS[O-7]
is
zeroed.
If
the x
field
of
the instruction
is
nonzero
(Le.
it specifies pC-relative or base-register addressing) then the
DISP
field is sign-extended and
put
on the bus.
Bus[8-15]~
IR[8-15]
Bus[0-7]
+-
if
IR[
6-7]
= 0 then 0 elseif
IR[8]
= 0 then 0
else-1
There are
two
additional
F2'S
which assist in instruction decoding,
IDISP
and
+-
ACSOURCE.
The
Imsp
function (F2=
15B)
does a
16
way
dispatch under control
of
a
PROM
and a multiplexer. The values are
tabulated below:
Conditions
if
IR[O]
= 1
elseif IR[l-
2]
= 0
elseif IR[l-
2]
= 1
elseif IR[l-
2]
= 2
elseif
IR[4-7]
= 0
elseif
IR[4-7]
= 1
elseif
IR[4-7]
= 6
elseif
IR[4-7]
=
16B
else
ORed
onto
NEXT
then
3-IR[8-9]
then
IR[3-4]
then 4
then 5
then 1
then
0
then
16B
then 6
IR[4-7]
Comment
complement
of
SH
field
of
IR
JMP,
JSR,
ISZ,
DSZ
IDA
STA
CONVERT
~ACSOURCE
(F2=16B) has two roles. First, it replaces the two-low order bits
of
the R select field with
the complement
of
the
SrcAC
field
of
IR,
(IR[1-2]
XOR
3),
allowing the emulator to address its
accumulators (which are assigned to
RO-R3).
Second, a dispatch is performed:
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