Alto Hardware Manual Section 5: Miscellaneous Peripherals
41
chipNumber+-
15
+ column + 14*row
Alto
II
The
Alto II memory system is organized around 32-bit doublewords. Stored along with each
double
word is 6 bits
of
hamming code and a parity
bit
for a total
of
39
bits:
bits
0-15
bits 16-31
bits 32-37
bit
38
even data word
odd
data word
Hamming code
parity bit
Things are further complicated by the fact that two types
of
memory chips are used: 16K chips
in
machines with the extended memory option (see section 2.3),
and
4K
chips
for
all others.
The
bits in a I-chip deep slice
of
memory are called a
group.
A group contains
4K
or
16K
double
words, depending on chip type. The bits
of
a group
on
a single
board
are called a subgroup.
Thus
a
subgroup contains
10
of
the 40 bits in a group. There are 8 subgroups
on
a memory board. Subgroups
are numbered from the high 3 bits
of
the address: for
4K
chips this means
MAR[0-2];
for 16K chips (Le.
an
Alto with extended memory) this means
BANK.MAR[O]:
Subgroup
chip positions
7
6
5
4
3
2
1
o
81-90
71-80
61-70
51-60
41-50
31-40
21-30
11-20
Nearest the edge connector
The
location
of
the bits in group 0
is:
CARD
1
CARD
2
32
24
16
08 00
I 33 25
17
09
01
36
28
20 12 04 I 37 29
21
13
05
chip posltlon 11
CARD
3
34
26
18 10
02
38 30
22
14 06
l'
CARD
4
35
27
19 11
03
XX
31 23 15
07
l'
Chips 15,
25,
35,
45,
55,
65, 75,
and
85
on board 4 aren't used.
If
you are
out
of
replacement
memory·
chips, you can use one
of
these,
but
then the board with the missing chips will only work in Slot 4.
The
algorithm for converting
address
and bit into cardSlot and chipNumber is (the variable 'xm' is
true
if
the Alto has extended memory):
if
odd(address) then bit+-bit+16
a:
if
switch = 1 then
address[O]
+-
address[Ol xor 1
cardSlot+- (bit
mod
4) + 1
chipNumber+-
bitl8
+
16
-
(if
odd(bitl4)
then
5 else 0) +
10
* (if xm
then
address[O]
else address[O-
2])
+
(if xm then bank*20 else
0)
A second entry to this algorithm
is
with an
address
(usually read from
MEAR),
and
a syndrome (usually
read from
MESR,
but remember that
it
must
be
complemented: syndrome+-(rv(MESR»)[8-13]
XOR
77B».
To Next Page
Loading...
Need help?
General
