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Extended 8051 Memory Layout

The extended 8051 8051 architecture includes the following memory classes.


The BIT, DATA, and IDATA memory classes comprise the on-chip memory of the 8051 architecture and are shown in the following figure.

On-chip Data Memory

CODE, ECODE, and Memory Banks

CODE and ECODE MemoryThe CODE and ECODE memory classes are typically stored in ROM that may be either off-chip or on-chip. The CODE memory class may be used for program code and refers to the first 64K (or Bank 0) of this read-only memory area.

The ECODE memory class is used for constant variables declared in code space above the first 64K of ROM.

In several variants the DPTR register is expanded to a 24-bit register with an DPX SFR. For example, the Dallas/Maxim 390 provides new operating modes where this addressing is enabled. You may even use the HCONST and HDATA memory classes with classic 8051 devices by using the memory banking available in LX51.

Bank 0 through Bank 63 allow your program to expand up to 4 Mbytes when using memory banking.


XDATA and HDATA MemoryThe XDATA memory class refers to the first 64K of RAM which may be off-chip or on-chip.

The HDATA memory class is used when RAM memory requirements exceed 64K bytes. This memory class allows you to define up to 16 Mbytes of variable data.

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