S3F84B8_UM_REV 1.00 2 ADDRESS SPACES
2-15
2.4.2 4-BIT WORKING REGISTER ADDRESSING
Each register pointer defines a movable 8 byte slice of working register space. The address information stored in
a register pointer serves as an addressing “window” that makes it possible for instructions to access working
registers efficiently using short 4-bit addresses. When an instruction addresses a location in the selected working
register area, the address bits are concatenated in the following way to form a complete 8-bit address.
The high-order bit of the 4-bit address selects one of the register pointers (“0” selects RP0 and “1” selects
RP1).
The five high-order bits in the register pointer select an 8 byte slice of the register space.
The three low-order bits of the 4-bit address select one of the eight registers in the slice.
As shown in Figure 2-12
, the result of this operation is that the five high-order bits from register pointer are
concatenated with the three low-order bits from instruction address to form the complete address. If the address
stored in register pointer remains unchanged, the three bits from the address will always point to an address in the
same 8 byte register slice.
Figure 2-13
shows a typical example of 4-bit working register addressing. The high-order bit of instruction
“INC R6” is “0”, which selects the RP0. The five high-order bits stored in RP0 (01110B) are concatenated with
three low-order bits of instruction’s 4-bit address (110B) to produce the register address 76H (01110110B).
Together they create an
8-bit register address
Register pointer
provides five
high-order bits
Address OPCODE
Selects
RP0 or RP1
RP1
RP0
4-bit address
provides three
low-order bits
Figure 2-12 4-Bit Working Register Addressing
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