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UPD784938 Datasheet(PDF) 86 Page - NEC |
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UPD784938 Datasheet(HTML) 86 Page - NEC |
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86 / 733 page  86 CHAPTER 3 CPU ARCHITECTURE Preliminary User’s Manual U13987EJ1V0UM00 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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