Rainbow Electronics AT89C55WD - Manual (Page 9)

9
AT89C55WD
1921B–MICRO–09/02
Memory
Organization
MCS-51 devices have a separate address space for Program and Data Memory. Up to
64 Kbytes each of external Program and Data Memory can be addressed.
Program Memory If the EA pin is connected to GND, all program fetches are directed to external memory.
On the AT89C55WD, if EA is connected to V
CC
, program fetches to addresses 0000H through
4FFFH are directed to internal memory and fetches to addresses 5000H through FFFFH are
to external memory.
Data Memory The AT89C55WD implements 256 bytes of on-chip RAM. The upper 128 bytes occupy a par-
allel address space to the Special Function Registers. That means the upper 128 bytes have
the same addresses as the SFR space but are physically separate from SFR space.
When an instruction accesses an internal location above address 7FH, the address mode
used in the instruction specifies whether the CPU accesses the upper 128 bytes of RAM or the
SFR space. Instructions that use direct addressing access SFR space.
For example, the following direct addressing instruction accesses the SFR at location 0A0H
(which is P2).
MOV 0A0H, #data
Instructions that use indirect addressing access the upper 128 bytes of RAM. For example, the
following indirect addressing instruction, where R0 contains 0A0H, accesses the data byte at
address 0A0H, rather than P2 (whose address is 0A0H).
MOV @R0, #data
Note that stack operations are examples of indirect addressing, so the upper 128 bytes of data
RAM are available as stack space.