CHAPTER 3 CPU FUNCTIONS
Cautions 1. Interrupts are not acknowledged for the store instruction for the PRCMD register. This is
because continuous execution of store instructions by the program in steps <3> and <4>
above is assumed. If another instruction is placed between step <3> and <4>, the above
sequence may not be realized when an interrupt is acknowledged for that instruction,
which may cause malfunction.
2. The data written to the PRCMD register is dummy data, but use the same register as the
general-purpose register used for setting data to the special register (step <4>) when
writing to the PRCMD register (step <3>). The same applies to when using a general-
purpose register for addressing.
(2) Command register (PRCMD)
The PRCMD register is an 8-bit register used to prevent data from being written to registers that may have a
large influence on the system, possibly causing the application system to unexpectedly stop, when an
inadvertent program loop occurs. Only the first write operation to the special register following the execution of
a previously executed write operation to the PRCMD register, is valid.
As a result, register values can be overwritten only using a preset sequence, preventing invalid write
operations.
This register can only be written in 8-bit units (if it is read, an undefined value is returned).
After reset: Undefined
7
W
Address: FFFFF1FCH
6
5
4
3
2
1
0
PRCMD
REG7
REG6
REG5
REG4
REG3
REG2
REG1
REG0
(3) System status register (SYS)
This register is allocated with status flags showing the operating state of the entire system.
This register can be read or written in 8-bit or 1-bit units.
After reset: 00H
R/W
0
Address: FFFFF802H
< >
SYS
0
0
0
0
0
0
PRERR
PRERR
Detection of protection error
0
1
Protection error has not occurred
Protection error has occurred
User’s Manual U15862EJ3V0UD
132
NEC [ NEC ]
