With these 20-pin parts, it would be inconvenient to have physically external XRAM. The two are addressed differently, so there is an obvious distinction.

I made an error in reference to the datasheet in an earlier post, in that I referred to page 3 of the datasheet. That was the wrong datasheet, being for the LPC934 and 935.

The LPC924/25 datasheet says little, actually, about RAM aside from
" ...
8.11 Data RAM arrangement
The 256 bytes of on-chip RAM are organized as shown in Table 5.

Table 5: On-chip data memory usages
Type Data RAM Size (bytes)
DATA Memory that can be addressed directly and indirectly 128
IDATA Memory that can be addressed indirectly 256
..."

All of which is quite confusing, in that they make the distinction between the two memory types even less clear. IDATA normally applies to the memory that can be accessed via MOV instructions, and MOV @ instructions. Memory outside that 256-byte range is generally not accessible except by MOVC or MOVX, which makes the distinction obvious. This particular datasheet doesn't make further reference to RAM. That leaves me wondering whether they have 256 bytes of normal 805x-style "internal" memory, plus 128 bytes of on-chip XRAM, or 128 bytes of normal 805x-style "internal" memory together with 256 bytes of on-chip XRAM.

What Jesus has pointed out he's discovered, is that they may be labelling devices, otherwise identical in construction, in accordance with test results, as Erik has suggested, thereby misleading users to attempt to utilize features not warranted by their labelling. Of course it could just be a poor set of documentation. Does anyone know for sure?

RE