Philips Semiconductors

Preliminary specification

89C51RB2/89C51RC2/

89C51RD2

80C51 8-bit Flash microcontroller family

16KB/32KB/64KB ISP/IAP Flash with 512B/512B/1KB RAM

1999 Sep 23

30

Expanded Data RAM Addressing

The 89C51RB2/RC2/RD2 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 Register (SFR), and

256 bytes expanded RAM (ERAM) (768 bytes for the RD2).

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 256/768-bytes expanded RAM (ERAM, 00H – 1FFH/2FFH)

are indirectly accessed by move external instruction, MOVX, and

with the EXTRAM bit cleared, see Figure 27.

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.

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). Instructions that

use indirect addressing access the Upper 128 bytes of data RAM.

For example:

MOV @R0,#data

where R0 contains 0A0H, accesses the data byte at address 0A0H,

rather than P2 (whose address is 0A0H).

The ERAM can be accessed by indirect addressing, with EXTRAM

bit cleared and MOVX instructions. This part of memory is physically

located on-chip, logically occupies the first 7936-bytes of external

data memory.

With EXTRAM = 0, the ERAM is indirectly addressed, using the

MOVX instruction in combination with any of the registers R0, R1 of

the selected bank or DPTR. An access to ERAM will not affect ports

P0, P3.6 (WR#) and P3.7 (RD#). P2 SFR is output during external

addressing. For example, with EXTRAM = 0,

MOVX @R0,#data

where R0 contains 0A0H, access the ERAM at address 0A0H rather

than external memory. An access to external data memory locations

higher than the ERAM will be performed with the MOVX DPTR

instructions in the same way as in the standard 80C51, so with P0

and P2 as data/address bus, and P3.6 and P3.7 as write and read

timing signals. Refer to Figure 28.

With EXTRAM = 1, MOVX @Ri and MOVX @DPTR will be similar

to the standard 80C51. MOVX @ Ri will provide an 8-bit address

multiplexed with data on Port 0 and any output port pins can be

used to output higher order address bits. This is to provide the

external paging capability. MOVX @DPTR will generate a 16-bit

address. Port 2 outputs the high-order eight address bits (the

contents of DPH) while Port 0 multiplexes the low-order eight

address bits (DPL) with data. MOVX @Ri and MOVX @DPTR will

generate either read or write signals on P3.6 (WR) and P3.7 (RD).

The stack pointer (SP) may be located anywhere in the 256 bytes

RAM (lower and upper RAM) internal data memory. The stack may

not be located in the ERAM.

AUXR

Reset Value = xxxx xx00B

—

——

——

—

EXTRAM

AO

Not Bit Addressable

Bit:

Symbol

Function

AO

Disable/Enable ALE

AO

Operating Mode

0

1

ALE is active only during a MOVX or MOVC instruction.

EXTRAM

Internal/External RAM access using MOVX @Ri/@DPTR

EXTRAM

Operating Mode

0

Internal ERAM access using MOVX @Ri/@DPTR

1

External data memory access.

—

Not implemented, reserved for future use*.

NOTE:

*User software should not write 1s to reserved bits. These bits may be used in future 8051 family products to invoke new features. In that case, the reset or inactive value of the new

bit will be 0, and its active value will be 1. The value read from a reserved bit is indeterminate.

SU01258

7654

321

0

Address = 8EH

Figure 27. AUXR: Auxiliary Register