Systems Design & Programming Memory III CMPE 310
1 (Mar. 6, 2002)
UMBC
U M B C
U
N
I
V
E
R
S
I
T
YO
FM
A
R
Y
L
A
N
DB

• Address pin A
0
(or BLE, Bus Low Enable) is used differently.
The 16-bit data bus presents a new problem:
The microprocessor must be able to read and write data to any 16-bit
location in addition to any 8-bit location.
The data bus and memory are divided into banks:
FFFFFF
FFFFFD
000003
000001
8 MB
8 bits
D
15
-D
8
FFFFFE
FFFFFC
000002
000000
8 MB
8 bits
D
7
-D
0
High bank
Low bank
Odd bytes
B
A
L
T
I
M
O
R
EC
O
U
N
T
Y
1

9

6

6
8086 - 80386SX 16-bit Memory Interface
BHE and BLE are used to select one or both:
Bank selection can be accomplished in two ways:
Separate write decoders for each bank (which drive CS).

R
S
I
T
YO
FM
A
R
Y
L
A
N
DB
A
L
T
I
M
O
R
E

CS
CS
CS
CS
CS
CS
M/IO
CS
CS
CS
CS
CS
CS
CS
A
0
A
15
O
0
O
7
...
...
CS
BHE
A
17
BLE
G2A

3
74LS138
A
18
A
19
A
21
A
22
A
23
Data Bus
D
0
to D
7
D
8
to D
15
80386SX
Separate Decoders
(64K X 8)
62512
WE
OE
MWTC
OE
WE

R
S
I
T
YO
FM
A
R
Y
L
A
N
DB
A
L
T
I
M
O
R
E

chip (using a PLD such as the PAL 16L8).
The high clock rates of these processors usually require wait states for
memory access.
We will come back to this later.
Systems Design & Programming Memory III CMPE 310
5 (Mar. 6, 2002)
UMBC
U M B C
U
N
I
V
E
R
S
I
T
YO
FM
A
R
Y
L
A

Therefore, 8 decoders or 8 write strobes are needed as well as 8 memory
banks.
The write strobes are obtained by combining the bank enable signals
(
BEx) with the MWTC signal.
MWTC is generated by combining the M/IO and W/R signals.
BE7
BE6
BE5
BE4
MWTC
BE3
BE2
BE1
BE0
WR7
WR6
WR5
WR4
WR3
WR2
WR1
WR0
W/R
M/
IO
Systems Design & Programming Memory III CMPE 310
6 (Mar. 6, 2002)
UMBC
U M B C

I
M
O
R
EC
O
U
N
T
Y
1

9

6

6
Pentium Memory Interface
I1
I2
I3
I4
I5
I6
I7
I8
I9

O7
O8
A19
A20
A21
A22
A23
A24
A25
A26
A27
A28
A
0
A
15
O
0
O
7
...
...
CE
OE
27512
D
0
-D
7
D

0
A
15
O
0
O
7
...
...
CE
OE
27512
A
0
A
15
O
0
O
7
...
...
CE
OE
27512
A
0
A
15
O

...
CE
OE
27512
A
0
A
15
O
0
O
7
...
...
CE
OE
27512
A
0
A
15
O
0
O
7
...
...
CE
OE
27512


M
A
R
Y
L
A
N
DB
A
L
T
I
M
O
R
EC
O
U
N
T
Y
1


O4
O5
O6
O7
O8
A29
A30
A31
I1
I2
I3
I4
I5
I6
I7
I8
I9
I10
16L8
O1
O2
O3
O4
O5
O6
O7
O8
A19
A20
A21