AT89C51RC2-3CSIM datasheet(23/127 Pages) ATMEL

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AT89C51RB2/RC2

4180C–8051–12/03

Expanded RAM

(XRAM)

The AT89C51RB2/RC2 provides additional bytes of random access memory (RAM)

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

AT89C51RB2/RC2 devices have expanded RAM in external data space; maximum size

and location are described in Table 18.

Table 18. Expanded RAM

The AT89C51RB2/RC2 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 18).

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 8. 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).

Part Number

XRAM Size

Address

Start

End

AT89C51RB2/RC2

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

AT89C51RC2-3CSIM Datasheet(PDF) 23 Page - ATMEL Corporation

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