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UPD780138 Datasheet(PDF) 84 Page - NEC |
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UPD780138 Datasheet(HTML) 84 Page - NEC |
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84 / 558 page  CHAPTER 3 CPU ARCHITECTURE User’s Manual U16228EJ2V0UD 84 3.4.4 Short direct addressing [Function] The memory to be manipulated in the fixed space is directly addressed with 8-bit data in an instruction word. This addressing is applied to the 256-byte space FE20H to FF1FH. Internal RAM and special function registers (SFRs) are mapped at FE20H to FEFFH and FF00H to FF1FH, respectively. The SFR area (FF00H to FF1FH) where short direct addressing is applied is a part of the overall SFR area. Ports that are frequently accessed in a program and compare and capture registers of the timer/event counter are mapped in this area, allowing SFRs to be manipulated with a small number of bytes and clocks. When 8-bit immediate data is at 20H to FFH, bit 8 of an effective address is set to 0. When it is at 00H to 1FH, bit 8 is set to 1. Refer to the [Illustration] shown below. [Operand format] Identifier Description saddr Immediate data that indicate label or FE20H to FF1FH saddrp Immediate data that indicate label or FE20H to FF1FH (even address only) [Description example] MOV 0FE30H, A; when transferring value of A register to saddr (FE30H) Operation code 1 111001 0 OP code 0 011000 0 30H (saddr-offset) [Illustration] 15 0 Short direct memory Effective address 1 111111 87 0 7 OP code saddr-offset α When 8-bit immediate data is 20H to FFH, α = 0 When 8-bit immediate data is 00H to 1FH, α = 1 |
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