SAB80C517A-N18 Siemens Semiconductor Group, SAB80C517A-N18 Datasheet - Page 22

• Manufacturer Part Number: SAB80C517A-N18
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: Siemens Semiconductor Group

Special Function Register XPAGE
Addr. 91
The reset value of XPAGE is 00
XPAGE can be set and read by software.
The register XPAGE provides the upper address byte for accesses to XRAM with MOVX @Ri
instructions. If the address formed from XPAGE and Ri is less than the XRAM address range,
then an external access is performed. For the SAB 80C517A the contents of XPAGE must be
greater or equal than F8H in order to use the XRAM. Of course, the XRAM must be enabled if
it shall be used with MOVX @Ri instructions.
Thus, the register XPAGE is used for addressing of the XRAM; additionally its contents are
used for generating the internal XRAM select. If the contents of XPAGE is less than the XRAM
address range then an external bus access is performed where the upper address byte is
provided by P2 and not by XPAGE!
Therefore, the software has to distinguish two cases, if the MOVX @Ri instructions with paging
shall be used:
a) Access to XRAM:
b) Access to external memory: The upper address byte must be written to P2; XPAGE
Semiconductor Group
H
H
or P2; both writes selects the XRAM address range.
will be loaded with the same address in order to deselect
the XRAM.
The upper address byte must be written to XPAGE
.
21
SAB 80C517A/83C517A-5
XPAGE
1994-05-01