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CHAPTER 21 ROM CORRECTION FUNCTION
21.4
Operation of the ROM Correction Function
If the program counter specifies the same address as that in program address detection
register (PADR), the INT9 instruction is executed. The ROM correction function can be
done by processing the INT9 instruction routine.
■ Operation of the ROM Correction Function
An instruction code to be read by the CPU is replaced forcibly with an INT9 instruction code (01
the corresponding address is equal to the value set in an address detection register. Therefore, the CPU
executes the INT9 instruction when executing the set instruction.
A program patch application function can be implemented by processing with the INT #9 interrupt routine.
There are two address detection registers, of which each is provided with an interrupt enable bit and
interrupt flag. When the address is equal to the value set in the address detection register, and the interrupt
enable bit is "1", assume the following: the interrupt flag is set to "1", and the instruction code to be read
by the CPU is replaced forcibly with the INT9 instruction code. The interrupt flag is cleared to "0" by
writing "0" to it using an instruction.
Note:
The address match detection function fails if an address later than the first byte of the instruction is
set in the address detection register. The value in the set address is replaced with "01H"so a wrong
instruction is executed or an invalid address is accessed. Before changing the value set in the
address detection register, set the interrupt enable bit to "0". If data is written while the interrupt
enable bit is "1", the address may be wrongly detected during writing, causing a malfunction.
578
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