COP8SG National Semiconductor, COP8SG Datasheet - Page 15

• Manufacturer Part Number: COP8SG
• Description: 8-Bit CMOS ROM Based and OTP Microcontrollers with 8k to 32k Memory/ Two Comparators and USART
• Manufacturer: National Semiconductor

5.0 Functional Description
memory mapped; therefore, I/O bits and register bits can be
directly and individually set, reset and tested. The accumula-
tor (A) bits can also be directly and individually tested.
Note: RAM contents are undefined upon power-up.
5.4 DATA MEMORY SEGMENT RAM EXTENSION
Data memory address 0FF is used as a memory mapped lo-
cation for the Data Segment Address Register (S).
The data store memory is either addressed directly by a
single byte address within the instruction, or indirectly rela-
tive to the reference of the B, X, or SP pointers (each con-
tains a single-byte address). This single-byte address allows
an addressing range of 256 locations from 00 to FF hex. The
upper bit of this single-byte address divides the data store
memory into two separate sections as outlined previously.
With the exception of the RAM register memory from ad-
dress locations 00F0 to 00FF, all RAM memory is memory
mapped with the upper bit of the single-byte address being
equal to zero. This allows the upper bit of the single-byte ad-
dress to determine whether or not the base address range
(from 0000 to 00FF) is extended. If this upper bit equals one
(representing address range 0080 to 00FF), then address
The instructions that utilize the stack pointer (SP) always ref-
erence the stack as part of the base segment (Segment 0),
regardless of the contents of the S register. The S register is
not changed by these instructions. Consequently, the stack
(used with subroutine linkage and interrupts) is always lo-
cated in the base segment. The stack pointer will be initial-
ized to point at data memory location 006F as a result of re-
set.
The 128 bytes of RAM contained in the base segment are
split between the lower and upper base segments. The first
112 bytes of RAM are resident from address 0000 to 006F in
the lower base segment, while the remaining 16 bytes of
(Continued)
FIGURE 8. RAM Organization
15
extension does not take place. Alternatively, if this upper bit
equals zero, then the data segment extension register S is
used to extend the base address range (from 0000 to 007F)
from XX00 to XX7F, where XX represents the 8 bits from the
S register. Thus the 128-byte data segment extensions are
located from addresses 0100 to 017F for data segment 1,
0200 to 027F for data segment 2, etc., up to FF00 to FF7F
for data segment 255. The base address range from 0000 to
007F represents data segment 0.
Figure 8 illustrates how the S register data memory exten-
sion is used in extending the lower half of the base address
range (00 to 7F hex) into 256 data segments of 128 bytes
each, with a total addressing range of 32 kbytes from XX00
to XX7F. This organization allows a total of 256 data seg-
ments of 128 bytes each with an additional upper base seg-
ment of 128 bytes. Furthermore, all addressing modes are
available for all data segments. The S register must be
changed under program control to move from one data seg-
ment (128 bytes) to another. However, the upper base seg-
ment (containing the 16 memory registers, I/O registers,
control registers, etc.) is always available regardless of the
contents of the S register, since the upper base segment
(address range 0080 to 00FF) is independent of data seg-
ment extension.
RAM represent the 16 data memory registers located at ad-
dresses 00F0 to 00FF of the upper base segment. No RAM
is located at the upper sixteen addresses (0070 to 007F) of
the lower base segment.
Additional RAM beyond these initial 128 bytes, however, will
always be memory mapped in groups of 128 bytes (or less)
at the data segment address extensions (XX00 to XX7F) of
the lower base segment. The additional 384 bytes of RAM in
this device are memory mapped at address locations 0100
to 017F, 0200 to 027F and 0300 to 037F hex.
DS101317-45
www.national.com