AT80SND1C-ROTIL datasheet(23/210 Pages) ATMEL

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AT8xC51SND1C

4109E–8051–06/03

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 30.

Figure 15 shows the internal and external data memory spaces organization.

Figure 15. Internal and External Data Memory Organization

Internal Space

Lower 128 Bytes RAM

The lower 128 Bytes of RAM (see Figure 16) 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 28)

select which bank is in use according to Table 25. This allows more efficient use of code

space, since register instructions are shorter than instructions that use direct address-

ing, and can be used for context switching in interrupt service routines.

Table 25. 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.

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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