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S3F80JB ADDRESS SPACES
2-16
PROGRAMMING TIP — Addressing the Common Working Register Area
As the following examples show, you should access working registers in the common area, locations C0H–CFH,
using working register addressing mode only.
Example 1:
LD 0C2H,40H ; Invalid addressing mode!
Use working register addressing instead:
SRP #0C0H
LD R2,40H ; R2 (C2H) ← the value in location 40H
Example 2:
ADD 0C3H,#45H ; Invalid addressing mode!
Use working register addressing instead:
SRP #0C0H
ADD R3,#45H ; R3 (C3H) ← R3 + 45H
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 very 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; "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 2-11, the result of this operation is that the five high-order bits from the register pointer are
concatenated with the three low-order bits from the instruction address to form the complete address. As long as
the address stored in the register pointer remains unchanged, the three bits from the address will always point to
an address in the same 8-byte register slice.
Figure 2-12 shows a typical example of 4-bit working register addressing. The high-order bit of the instruction
'INC R6' is "0", which selects RP0. The five high-order bits stored in RP0 (01110B) are concatenated with the
three low-order bits of the instruction's 4-bit address (110B) to produce the register address 76H (01110110B).