SAB80C515A-5 Datasheet, PDF (20/108 Pages) Siemens Semiconductor Group – 8-Bit CMOS Single-Chip Microcontroller Family

Memory Organization
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 80C515A 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:
The upper address byte must be written to XPAGE or P2;
both writes selects the XRAM address range.
b) Access to external memory: The upper address byte must be written to P2; XPAGE
will be loaded with the same address in order to deselect the XRAM.
The behaviour of Port0, Port2 and the RD/WR signals depends on the state of pin EA and on the
control bits XMAP0 and XMAP1 in register SYSCON.
Semiconductor Group
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