AT89C51RC-24AI datasheet(9/37 Pages) ATMEL

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AT89C51RC

1920B–MICRO–11/02

Memory Organization MCS-51 devices have a separate address space for Program and Data Memory. Up to

64K bytes each of external Program and Data Memory can be addressed.

Program Memory

If the EA pin is connected to GND, all program fetches are directed to external memory.

On the AT89C51RC, if EA is connected to V

CC, program fetches to addresses 0000H

through 7FFFH are directed to internal memory and fetches to addresses 8000H

through FFFFH are to external memory.

Data Memory

The AT89C51RC has internal data memory that is mapped into four separate segments:

the lower 128 bytes of RAM, upper 128 bytes of RAM, 128 bytes special function regis-

ter (SFR) and 256 bytes expanded RAM (ERAM).

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 256-byte expanded RAM (ERAM, 00H-FFH) is indirectly accessed by MOVX

instructions, and with the EXTRAM bit cleared.

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. This means they have the same address, but are

physically separate from the SFR space.

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

Table 3b. AUXR1: Auxiliary Register 1

AUXR1

Address = A2H

Reset Value = XXXXXXX0B

Not Bit Addressable

––

–

DPS

Bit

–

Reserved for future expansion

DPS

Data Pointer Register Select

DPS

Selects DPTR Registers DP0L, DP0H

Selects DPTR Registers DP1L, DP1H

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