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Global Data Memory

3-9

Memory and I/O Spaces

3.4

Global Data Memory

Addresses in the upper 32K words (8000h–FFFFh) of local data memory can

be used for global data memory. The global memory allocation register

(GREG) determines the size of the global data-memory space, which is be-

tween 256 and 32K words. The GREG is connected to the eight LSBs of the

internal data bus and is memory-mapped to data-memory location 0005h.

Table 3–2 shows the allowable GREG values and shows the corresponding

address range set aside for global data memory. Any remaining addresses

within 8000h–FFFFh are available for local data memory.

Note:

Choose only the GREG values listed in Table 3–2. Other values lead to frag-

mented memory maps.

Table 3–2. Global Data Memory Configurations

GREG Value

Local Memory

Global Memory

High Byte Low Byte

Range

Words

Range

Words

XXXX XXXX 0000 0000

0000h–FFFFh

65 536

–

XXXX XXXX 1000 0000

0000h–7FFFh

32 768

8000h–FFFFh

32 768

XXXX XXXX 1100 0000

0000h–BFFFh

49 152

C000h–FFFFh

16 384

XXXX XXXX 1110 0000

0000h–DFFFh

57 344

E000h–FFFFh

8 192

XXXX XXXX 1111 0000

0000h–EFFFh

61 440

F000h–FFFFh

4 096

XXXX XXXX 1111 1000

0000h–F7FFh

63 488

F800h–FFFFh

2 048

XXXX XXXX 1111 1100

0000h–FBFFh

64 512

FC00h–FFFFh

1 024

XXXX XXXX 1111 1110

0000h–FDFFh

65 024

FE00h–FFFFh

512

XXXX XXXX 1111 1111

0000h–FEFFh

65 280

FF00h–FFFFh

256

Note:

X = Don’t care

When a program accesses any data address, the ’C24x drives the DS signal

low. If that address is within the range defined by the GREG as a global ad-

dress, BR signal is also asserted. Because BR differentiates local and global

accesses, the addresses configured by the GREG value are an additional data

space. The external data-address range is extended by the selected amount

of global space (up to 32K words).

Global memory is available only on devices with an external memory interface.

In other devices, this space and the GREG register are reserved.

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