T89C51RD2-RDTIL datasheet(15/104 Pages) ATMEL

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T89C51RD2

4243G–8051–05/03

Expanded RAM (XRAM)

The T89C51RD2 provide additional Bytes of random access memory (RAM) space for

increased data parameter handling and high level language usage.

T89C51RD2 devices have expanded RAM in external data space; Maximum size and

location are described in Table 6.

The T89C51RD2 has internal data memory that is mapped into four separate segments.

The four segments are:

•

1. The Lower 128 bytes of RAM (addresses 00H to 7FH) are directly and indirectly

addressable.

•

2. The Upper 128 bytes of RAM (addresses 80H to FFH) are indirectly addressable

only.

•

3. The Special Function Registers, SFRs, (addresses 80H to FFH) are directly

addressable only.

•

4. The expanded RAM bytes are indirectly accessed by MOVX instructions, and

with the EXTRAM bit cleared in the AUXR register. (See Table 7.)

The Lower 128 bytes can be accessed by either direct or indirect addressing. The Upper

128 bytes can be accessed by indirect addressing only. The Upper 128 bytes occupy

the same address space as the SFR. That means they have the same address, but are

physically separate from SFR space.

Figure 4. Internal and External Data Memory Address

When an instruction accesses an internal location above address 7FH, the CPU knows

whether the access is to the upper 128 bytes of data RAM or to SFR space by the

addressing mode used in the instruction.

•

Instructions that use direct addressing access SFR space. For example: MOV

0A0H, # data ,accesses the SFR at location 0A0H (which is P2).

Table 6. Description of Expanded RAM

Port

XRAM size

Address

Start

End

T89C51RD2

1024

00h

3FFh

XRAM

Upper

128 bytes

Internal

Ram

Lower

128 bytes

Internal

Ram

Special

Function

FF or 3FF

External

Data

Memory

0000

0100 or 0400

FFFF

indirect accesses

direct accesses

direct or indirect

accesses

T89C51RD2-RDTIL 数据表(PDF) 15 Page - ATMEL Corporation

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