The IRP bit is not used on the 16C(F)84 chips. It would be used if
the RAM map exceeded 256 locations, hence the need for 9 bits.

RP1 and RP0 are used for DIRECT addressing, not INDIRECT addressing.
These bits are needed because the instructions which specify a RAM address
are only 7 bits long.  Thus to make an 8 bit address to access the
256 bytes of RAM you have to use RP0. RP1 is not used on the
16(F)C84 chips.

The FSR is an 8 bit register.

The address MUX can only use 7 bits of the RAM address data from the
FSR. Bit 7 of the FSR is used to set the Ram Page. Bit 7 = 0, RAM Page 0, Bit 7
= 1, RAM Page 1. Thus the FSR by itself can indirectly address all of the 256
RAM locations. The IRP bit is not used on the 16(F)C84, because there
are only 8 bits needed to access the RAM.

$81 in the FSR will always indirectly address the OPTION register.
$01 in the FSR will always indirectly address the TMR0(RTCC) register.
RP0 has no effect on INDIRECT addressing.

If this still confuses you, check out the web site below.


Regards

Tony


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