8-Bit Working Register Addressing - Samsung S3C80A5B User Manual

ADDRESS SPACES

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).
These Address
Bits Indicate 8-bit
Working Register
Addressing
2-16
Selects
RP0 or RP1
1 1
Register Pointer
Provides Five
High-order Bits
8-bit Physical Address
Figure 2-13. 8-Bit Working Register Addressing
Address
0 0
Three low-
order Bits
S3C80A5B
RP0
RP1
8-bit Logical
Address