AT80C51RB2-3CSIM datasheet(14/83 Pages) ATMEL

14 / 83 page

AT80C51RD2/AT83C51Rx2

4113B–8051–03/05

Expanded RAM

(XRAM)

The T8xc51Rx2 devices provide additional Bytes of Random Access Memory (RAM)

space for increased data parameter handling and high level language usage.

The devices have expanded RAM in external data space; maximum size and location

are described in Table 7.

Table 7. Expanded RAM

The T8xc51Rx2 has internal data memory that is mapped into four separate segments.

The four segments are:

The Lower 128 bytes of RAM (addresses 00h to 7Fh) are directly and indirectly

addressable.

The Upper 128 bytes of RAM (addresses 80h to FFh) are indirectly addressable

only.

The Special Function Registers (SFRs) (addresses 80h to FFh) are directly

addressable only.

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 5. Internal and External Data Memory Address

XRAM size

Address

Start

End

T83C51RB2/RC2

T80C51RD2

1024

00h

3FFh

XRAM

Upper

128 Bytes

Internal

RAM

Lower

128 Bytes

Internal

RAM

Special

Function

80h

0FFh or 3FFh

0FFh

External

Data

Memory

0000

00FFh up to 03FFh

0FFFFh

indirect accesses

Direct Accesses

Direct or Indirect

Accesses

7Fh

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