Use Of Rss Bit - NEC uPD784038 Series User Manual

3.7.3 Use of RSS Bit

Basically, the RSS bit should be fixed at 0 at all times.
The following explanation refers to the case where a 78K/III Series program is used, and the program used sets the RSS bit
to 1. This explanation can be skipped if the RSS bit is fixed at 0.
The RSS bit is provided to allow the functions of A (R1), X (R0), B (R3), C (R2), AX (RP0), and BC (RP1) to be used by registers
R4 to R7 (RP2, RP3) as well. Effective use of this bit enables efficient programs to be written in terms of program size and
program execution.
However, careless use can result in unforeseen problems. Therefore, the RSS bit should always be set to 0. The RSS bit
should only be set to 1 when a 78K/III Series program is used.
Use of the RSS bit set to 0 in all programs will improve programming and debugging efficiency.
Even when using a program in which the RSS bit set to 1 is used, it is recommended that the program be amended if possible
so that it does not set the RSS bit to 1.
(1) RSS bit specification
• Registers used by instructions for which the A, X, B, C, and AX registers are directly entered in the operand column of
the operation list (see 27.2.)
• Registers specified as implied by instructions that use the A, AX, B, and C registers by means of implied addressing
• Registers used in addressing by instructions that use the A, B, and C registers in indexed addressing and based indexed
addressing
The registers used in these cases are switched as follows according to 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 U11316EJ4V1UD
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