1. Program Memory: in the classic 8051 this is a 64KB spacethat is called CODE.This region is typically a ROM space that is used for program code and constants.With the BL51 you may expand the physical program code memory to 32 code bankswith 64KB each. Constants are fetched with the MOVC instruction. In extended8051 variants and the 251 you may have program memory of up to 16MB that iscalledECODEand HCONST.
2. Internal Data Memory:in the classic 8051 this is the on-chip RAMspace with a maximum of 256 Bytes that contains register banks, BIT space,direct addressableDATA space, and indirect addressableIDATAspace. This region should be used for frequently used variables. In the 80C51MXand the 251 this space is expanded to up to 64KB with anEDATA space.
3. External Data Memory:in classic 8051 devices this area, calledXDATA,is off-chip RAM with a space of up to 64KB. However several new 8051 devices haveadditional on-chip RAM that is mapped into the XDATA space. Usually you need toenable this additional on-chip RAM via dedicated SFR registers. In extendedvariants and the 251 you may have external data memory of up to 16MB that iscalledHDATA.