What's the full part No - there was one with a H for high speed (default 6clock).

Do you use real external memory or the internal-external memory on chip setting EXTRAM in the AUX register?

Auxiliary register (address 8EH) bit description
Bit Symbol Description
7 to 2 - Reserved for future use. Should be set to ‘0’ by user programs.
1 EXTRAM Internal/External RAM access using MOVX @Ri/@DPTR. When ‘1’,
accesses internal XRAM with address specified in MOVX instruction.
If address supplied with this instruction exceeds on-chip available
XRAM, off-chip RAM is accessed. When ‘0’, every MOVX instructions
targets external data memory by default.

When EXTRAM = 0, MOVX @Ri and MOVX @DPTR will be similar to the standard 8051.
Using MOVX @Ri provides an 8-bit address with multiplexed data on Port 0. Other output
port pins can be used to output higher order address bits. This provides external paging
capabilities. Using MOVX @DPTR generates a 16-bit address. This allows external
addressing up the 64 kB. Port 2 provides the high-order eight address bits (DPH), and
Port 0 multiplexes the low order eight address bits (DPL) with data. Both MOVX @Ri and
MOVX @DPTR generates the necessary read and write signals (P3[6] - WR and P3[7] -
RD) for external memory use. Table 7 shows external data memory RD, WR operation
with EXTRAM bit.
The stack pointer (SP) can be located anywhere within the 256 B of internal RAM (lower
128 B and upper 128 B). The stack pointer may not be located in any part of the expanded
RAM.


Joe