AT87F55WD ATMEL [ATMEL Corporation], AT87F55WD Datasheet - Page 7

• Manufacturer Part Number: AT87F55WD
• Manufacturer: ATMEL [ATMEL Corporation]

Memory Organization
MCS-51 devices have a separate address space for Pro-
gram and Data Memory. Up to 64K bytes 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 AT87F55WD, if EA is connected to V
fetches to addresses 0000H through 4FFFH are directed to
internal memory and fetches to addresses 5000H through
FFFFH are directed to external memory.
Data Memory
The AT87F55WD implements 256 bytes of on-chip RAM.
The upper 128 bytes occupy a parallel 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.
CC
, program
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).
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).
Note that stack operations are examples of indirect
addressing, so the upper 128 bytes of data RAM are avail-
able as stack space.
MOV 0A0H, #data
MOV @R0, #data
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