COP8FG datasheet(13/59 Pages) NSC

13 / 59 page

5.0 Functional Description (Continued)

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

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.

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

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.

DS101116-45

FIGURE 8. RAM Organization

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