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S3F80JB ADDRESS SPACES
2-18
8-BIT WORKING REGISTER ADDRESSING
You can also use 8-bit working register addressing to access registers in a selected working register area. To
initiate 8-bit working register addressing, the upper four bits of the instruction address must contain the value
1100B. This 4-bit value (1100B) indicates that the remaining four bits have the same effect as 4-bit working
register addressing.
As shown in Figure 2-13, the lower nibble of the 8-bit address is concatenated in much the same way as for 4-bit
addressing: Bit 3 selects either RP0 or RP1, which then supplies the five high-order bits of the final address. The
three low-order bits of the complete address are provided by the original instruction.
Figure 2-14 shows an example of 8-bit working register addressing. The four high-order bits of the instruction
address (1100B) specify 8-bit working register addressing. Bit 4 ("1") selects RP1 and the five high-order bits in
RP1 (10101B) become the five high-order bits of the register address. The three low-order bits of the register
address (011) are provided by the three low-order bits of the 8-bit instruction address. The five-address bits from
RP1 and the three address bits from the instruction are concatenated to form the complete register address,
0ABH (10101011B).
8-bit physical address
Register pointer
provides five
high-order bits
Address
Selects
RP0 or RP1
RP1
RP0
8-bit logical
address
1 1 0 0
These address
bits indicate
8-bit working
register
addressing
Three low-
order bits
Figure 2-14. 8-Bit Working Register Addressing