Using The Rss Bit - NEC mPD784225 Series User Manual

3.7.3 Using the RSS bit

Basically, always use with the RSS bit fixed at 0.
The following descriptions discuss using a 78K/III Series program and a program that sets the RSS bit to 1. Reading
is not necessary if the RSS bit is fixed at 0.
The RSS bit enables the functions in A (R1), X (R0), B (R3), C (R2), AX (RP0), and BC (RP1) to also be used
in registers R4 to R7 (RP2, RP3). When this bit is effectively used, efficient programs in terms of program size and
program execution can be written.
Sometimes, however, unexpected problems arise if used carelessly. Consequently, always set the RSS bit to 0.
Use with the RSS bit set to 1 only when 78K/III Series programs will be used.
By setting the RSS bit to 0 in all programs, writing and debugging programs become more efficient.
Even if a program where the RSS bit is set to 1 is used, when possible, it is recommended to use the program
after modifying the program so that the RSS bit is not set to 1.
(1) Using the RSS bit
• Registers used in instructions where the A, X, B, C, and AX registers are directly described in the operand
column of the operation list (see 28.2)
• Registers that are implicitly specified in instructions that use the A, AX, B, and C registers by implied addressing
• Registers that are used in addressing in instructions that use the A, B, and C registers in indexed addressing
and based indexed addressing
The registers used in these cases are switched in the following ways by the RSS bit.
• When RSS = 0
A→R1, X→R0, B→R3, C→R2, AX→RP0, BC→RP1
• When RSS = 1
A→R5, X→R4, B→R7, C→R6, AX→RP2, BC→RP3
CHAPTER 3 CPU ARCHITECTURE
User's Manual U12697EJ3V0UM
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