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AT85C51SND3Bx

7632A–MP3–03/06

Table 63. Register Bank Selection

The next 16 Bytes above the register banks form a block of bit-addressable memory

space. The C51 instruction set includes a wide selection of single-bit instructions, and

the 128 bits in this area can be directly addressed by these instructions. The bit

addresses in this area are 00h to 7Fh.

Figure 26. Lower 128 Bytes Internal RAM Organization

Upper 128 Bytes The upper 128 Bytes of RAM are accessible from address 80h to FFh using only indirect

addressing mode. Using direct addressing mode within this address range selects the

Special Function Registers,

SFRs. For information on this segment, refer to the

Section “Special Function Registers”, page 36.

XDATA Segment The on-chip expanded RAM (XRAM) are accessible using indirect addressing mode

through MOVX instructions.

Memory Configuration As shown in Figure 25, the 64KB addressing space of the C51 is artificially increased by

usage of logical address over a physical one. For example, the boot memory which con-

tains the bootstrap software is implemented at physical address 10000h but is starting at

logical address code 0000h which means that the bootstrap is first executed when a

system reset occurs.

To achieve such logical mapping over the physical memory, some registers have been

implemented to give the base address of the memory segments and their size:

•MEMCBAX (see Table 65) for the code segment base address.

•MEMDBAX (see Table 66) for the data segment base address.

• MEMXBAX (see Table 67) for the xdata segment base address.

• MEMCSX (see Table 68) for the code segment size.

• MEMXSX (see Table 69) for the code segment size.

The data segment is not programmable in size as it is a fixed 256-byte segment.

RS1 RS0 Description

0 0 Register bank 0 from 00h to 07h

0 1 Register bank 1 from 08h to 0Fh

1 0 Register bank 2 from 10h to 17h

1 1 Register bank 3 from 18h to 1Fh

Bit-Addressable Space

4 Banks of

8 Registers

R0-R7

30h

7Fh

(Bit Addresses 0-7Fh)

20h

2Fh

18h

1Fh

10h

17h

08h

0Fh

00h

07h

1 2 ... 45 46 47 48 49 50 51 52 53 54 55 ... 262 263

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