UPD17P133 Datasheet, PDF (82/289 Pages) NEC – 4-BIT SINGLE-CHIP MICROCONTROLLER

CHAPTER 7 SYSTEM REGISTER (SYSREG)
7.8.2 Handling of System Register Addresses Fixed at 0
In dealing with system register addresses fixed at 0 (refer to Figure 7-2), there are a few points for which caution
should be taken with regard to device, emulator and assembler operation.
Items (1), (2) and (3) explain these points.
(1) Concerning device operation
Trying to write data to an address fixed at 0 will not change the value (0) at that address. Any attempt to
read an address fixed at 0 will result in the value 0 being read.
(2) When using a 17K series in-circuit emulator (IE-17K or IE-17K-ET)
An error will be generated if a write instruction attempts to write the value 1 to an address fixed at 0.
Below is an example of the type of instructions that will cause the in-circuit emulator to generate an error.
Example 1. MOV BAMK, #0100B ; Attempts to write the value 1 to bit 3 (an address fixed at 0).
2. MOV IXL, #1111B ;
MOV IXM, #1111B ;
MOV IXH, #0001B ;
ADD IXL, #1
;
ADDC IXM, #0
;
ADDC IXH, #0
;
However, when all valid bits are set to 1 as shown in Example 2, executing the instructions INC AR or INC IX
will not cause an error to be generated by the in-circuit emulator. This is because when all valid bits of the address
register and index register are set to 1, executing the INC instruction causes all bits to be set to 0.
The only time the in-circuit emulator will not generate an error when an attempt is made to write the value 1 to
a bit fixed at 0 is when the address being written to is in the address register.
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