| ??? 11/13/08 09:29 Read: times |
#160009 - Agreed, but... Responding to: ???'s previous message |
Richard Erlacher said:
Yes, the off-chip RAM high-address-byte appears on P2, on most chips (not necessarily on SiLabs) but it's really addressed by DPTR and not by P2. Not with "classic" (see below) PDATA. A "classic" PDATA access doesn't use the DPTR, and doesn't modify P2. When using "classic" PDATA with off-chip memory, only the low address byte is modified during the memory access by a MOVX @Rn instruction; it is entirely up to the programmer to ensure that their high address byte (if the hardware has one!) is set appropriately. By "classic" PDATA I mean memory accessed via the "classic" 8051 external memory interface, with P0 and P2 - Of course, how this happens in a device with on-chip PDATA/XDATA (like SiLabs?), or some other interface, is entirely device-dependent... It's getting messy, as 805x architecture expansion continues, and we need to be very precise with nomenclature in order to avoid mixing up details such as where the memory lives, physically, with how it's addressed. Thoroughly agreed! SiLabs provides 8 KB of what's addressable as external (sic) memory. I think calling it "external" is unhelpful here - because it is internal to the physical device! Especially as some SiLabs chips do also support off-chip memory! I think the best way to describe it is as XDATA or, more precisely, as "MOVX-addressed" ie, we take "XDATA" to be synonymous with "MOVX-addressed" which may or may not be physically off the chip. I think PDATA is enough of a special case that it need only be mentioned when specifically used? The trick is to keep things straight in our collective understanding, so that when we refer to a given thing, it's clear to everybody what is meant. Agreed. Unfortunately, a lot of marketing materials aren't concerned about such precision. This is also a great example of where out-of-date materials can cause confusion: in the original 8051, XDATA was literally external; so "off-chip" and "external" were synonymous back then - unfortunately, that assumption still persists in some places today when it is blatantly no longer true! |
