Rev.2.00
May. 24, 2006
page 74 of 90
REJ03B0028-0200
3826 Group (A version)
(6)
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.
(7)
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.