Siemens EF 88H Series Manual do Utilizador descarregar pdf (Página 18)

Semiconductor Group 3 - 10

Memory Organization

Accesses to XRAM using the Registers R0/R1

The 8051 architecture provides also instructions for access to external data memory range which

use only an 8-bit address (indirect addressing with registers R0 or R1). The instructions are:

MOVX A, @ Ri (Read)

MOVX @Ri, A (Write)

In application systems, either a real 8-bit bus (with 8-bit address) is used or Port 2 serves as page

general purpose I/0 or as "page address" is made by the external system design. From the device’s

point of view it cannot be decided whether the Port 2 data is used externally as address or as I/0

data!

Hence, a special page register is implemented into the SAB 80C517A to provide the possibility of

accessing the XRAM also with the MOVX @Ri instructions, i.e. XPAGE serves the same function

for the XRAM as Port 2 for external data memory.

Special Function Register XPAGE

The reset value of XPAGE is 00

XPAGE can be set and read by software.

Figures 3-2 to 3-4 show the dependencies of XPAGE- and Port 2 - addressing in order to explain

the differencies in accessing XRAM, ext. RAM or what is to do when Port 2 is used as an I/O-port.

76543210

MSB LSB

Bit No.

XPAGEAddr. 91

1 2 ... 13 14 15 16 17 18 19 20 21 22 23 ... 136 137

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Siemens EF 88H Series Manual do Utilizador Página 18