M38261ECFS datasheet(72/91 Pages) RENESAS

72 / 91 page

Rev.1.00

Sep 06, 2006

page 72 of 88

REJ03B0181-0100

3826 Group (One Time PROM version)

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Read-modify-write instruction

Do not execute a read-modify-write instruction to the read invalid address (memory and SFR).

The read-modify-write instruction operates in the following sequence: read one-byte of data from

memory, modify the data, write the data back to original memory. The following instructions are

classified as the read-modify-write instructions in the 740 Family.

•Bit management instructions: CLB, SEB

•Shift and rotate instructions: ASL, LSR, ROL, ROR, RRF

•Add and subtract instructions: DEC, INC

•Logical operation instructions (1’s complement): COM

Add and subtract/logical operation instructions (ADC, SBC, AND, EOR, and ORA) when T flag =

“1”

operate in the way as the read-modify-write instruction. Do not execute the read invalid

memory and SFR.

[Reason]

When the read-modify-write instruction is executed to read invalid memory and SFR, the instruction

may cause the following consequence: the instruction reads unspecified data from the memory

due to the read invalid condition. Then the instruction modifies this unspecified data and writes the

data to the memory. The result will be random data written to the memory or some unexpected

event.

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Instruction execution time

Each instruction execution time is obtained from the cycle time of system clock φ multiplied by the

number of instruction cycles listed in the machine instruction table. Note that the cycle time of

system clock φ is defined by the system clock division ratio selection bit and the system clock

selection bit.

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