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40 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Operator Manual for the RCA CDS II CDP18S005
Memory Addressing
and Expansion
To aid the user in interfacing the COSMAC
Development System to added memory hardware,
Memory Organization
"
)
The total directly addressable memory space (65,
536_bytesl may be considered as being divided into 16
hanks of 4096 bytes each. The 16-bit memory ad-
dress, A(l5:01, can be divided into two fields:
A(l5:121 being a bank number and A(ll:OI selecting
the byte within a bank, (see Fig. 1
n
A bank may be
further subdivided into four blocks of 1024 bytes
each (in which case A(ll:lOl is a block number within
a bank and A(9:01 is the address of a byte within the
selected blockl or into eight blocks of 512 bytes each
(A(l1:91 defining the block number and A(8:01
identifying the byte within the selected block).
MEMORY ADDRESS BITS
~~
,9
8
7
6
5
4
3
2
I
v
°
,
BANK
BLOCK
NUMBER
NUMBER
A [ 15
:
12)
A [ II
:
10]
'-----y--'
HALF-
BLOCK
NUMBER
A [11:9J
4 PAGES WITHIN BLOCK
A [9
,
0]
~-----'
vr------J
'
256 BYTES
=
I PAGE
A [7:0)
~------~vr------~
2 PAGES WITHIN I HALF-BLOCK
A [8
:
0]
9
2CS
-
28
2
11
Fig.
17 -
Memory address bit
assignments.
Each memory plug-in module comprises a bank or
a block of memory. When a module is inserted in a
given plug-in slot, external wiring of appropriate pins
on the connector in that position defines its bank and
block numbers (the range of addresses to which it
respondsl. Thus, users may define arbitrary address
ranges for the memory modules they use.
The COSMAC Development System CDPI8S005
is supplied with a 4-kilobyte RAM module wired to
occupy the lowest memory address range (starting at
address 00001. The CDS is also equipped with a
ROM containing the Utility Program UT20. UT20's
address range is 8000 to 83FF. Another ROM
containing the disk loader program occupies ad-
dresses 8400 to 87FF. UT20 also uses a dedicated
RAM of 32 bytes starting at address 8COO. Refer to
Table III for a list of module position assignments.
this section discusses memory module addressing and
the use of custom memory modules.
RCA Modules
Each RCA memory module includes a sufficient
number of enable inputs which can be used to assure
that it will respond only over its assigned address
range. Every module has a bank select input at pin X.
When this signal is high, the module is ACTIV ATED
or ENABLED. When it is low, the module is
DISABLED
or
DESELECTED.
Whenever
a
module
is added or moved in the address space. this
overriding SELECT input must be connected to the
proper enabling source.
In the Development System. the supplied ROM
memory is enabled bv BS8-P, so that the UT20
progra~ begins at add~ess ~WOO. The RAM
space,
on
the other hand,
is
design ed to begin at location zero.
See Fig. W for a CDS memory map. The memory
bank !Select module decode!' ; the upper four address
bits to provide sixteen RAM bank-select signals. The
supplied 4-kilobyte RAM (in slot 81 is enabled at pin
X by th
e
lowest of these outputs. called BSO-P.
DECIMAL ADDRESS
HEX A-DDRESS
0-4095
4- KILOBYTE RAM
OOOO-
OFFF
4096 -32767
UNUSED
1000-7FFF
32768-33792
UT20
8000-83FF
33793- 3 4817
DISK LOADER
8400
-
871'F
34818-35839
UNUSED
8800-8BFF
35840-35871
REGISTER STORAGE
8COO-8CIF
35872-65535
UNUSED
8C2G-FFFF
92CS-
2
96
2
0
Fig. 18 - CDS CDP18S005 memory map.
(
I
I
j
I
t
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