UPD784938 Datasheet, PDF (86/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 3 CPU ARCHITECTURE
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 28.2)
• 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
Registers used other than those mentioned above are always the same irrespective of the value of the RSS bit. With the
NEC assembler (RA78K4), the register operation code generated when the A, X, B, C, AX, and BC registers are described by
those names is determined by the assembler RSS pseudo-instruction.
When the RSS bit is set or reset, an RSS pseudo-instruction must be written immediately before (or immediately after) the
relevant instruction (see example below).
<Program example>
• When RSS is set to 0
RSS 0
; RSS pseudo-instruction
CLR1 PSWL. 5
MOV B, A ; This description is equivalent to “MOV R3, R1”.
• When RSS is set to 1
RSS 1
; RSS pseudo-instruction
SET1 PSWL. 5
MOV B, A ; This description is equivalent to “MOV R7, R5”.
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Preliminary User’s Manual U13987EJ1V0UM00