AT89C55WD-24AI datasheet(9/37 Pages) ATMEL

9 / 37 page

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.

AT89C55WD-24AI датащи(PDF) 9 Page - ATMEL Corporation