AT83SND1CXXX-ROTIL datasheet(25/212 Pages) ATMEL

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4109J–8051–10/06

AT8xC51SND1C

Data Memory

The AT8xC51SND1C provides data memory access in 2 different spaces:

The internal space mapped in three separate segments:

–

The lower 128 Bytes RAM segment

–

The upper 128 Bytes RAM segment

–

The expanded 2048 Bytes RAM segment

The external space.

A fourth internal segment is available but dedicated to Special Function Registers, SFRs,

(addresses 80h to FFh) accessible by direct addressing mode. For information on this segment,

refer to the Section “Special Function Registers”, page 32.

Figure 8 shows the internal and external data memory spaces organization.

Figure 8. Internal and External Data Memory Organization

6.1

Internal Space

6.1.1

Lower 128 Bytes RAM

The lower 128 Bytes of RAM (see Figure 9) are accessible from address 00h to 7Fh using direct

or indirect addressing modes. The lowest 32 Bytes are grouped into 4 banks of 8 registers (R0

to R7). 2 bits RS0 and RS1 in PSW register (see Table 18) select which bank is in use according

to Table 15. This allows more efficient use of code space, since register instructions are shorter

than instructions that use direct addressing, and can be used for context switching in interrupt

service routines.

Table 15. Register Bank Selection

2K Bytes

Upper

128 Bytes

Internal RAM

Lower

128 Bytes

Internal RAM

Special

Function

Registers

80h

00h

7FFh

FFh

00h

FFh

64K Bytes

External XRAM

0000h

FFFFh

Direct Addressing

Addressing

0800h

7Fh

Internal ERAM

Direct or Indirect

Indirect Addressing

EXTRAM = 0

EXTRAM = 1

RS1

RS0

Description

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