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7542 Group
APPENDIX
NOTES ON PROGRAMMING
1. Processor Status Register
(1) Initializing of processor status register
Flags which affect program execution must be initialized after a re-
set.
In particular, it is essential to initialize the T and D flags because
they have an important effect on calculations.
<Reason>
After a reset, the contents of the processor status register (PS) are
undefined except for the I flag which is "1".
Initializing of flags
Main program
Fig. 1 Initialization of processor status register
(2) How to reference the processor status register
To reference the contents of the processor status register (PS), ex-
ecute the PHP instruction once then read the contents of (S+1). If
necessary, execute the PLP instruction to return the PS to its origi-
nal status.
(S)
(S)+1
Fig. 2 Stack memory contents after PHP instruction execution
Rev.3.02
Oct 31, 2006
REJ03B0006-0302
Reset
↓
↓
Stored PS
Page 125 of 134
2. Decimal calculations
(1) Execution of decimal calculations
The ADC and SBC are the only instructions which will yield proper
decimal notation, set the decimal mode flag (D) to "1" with the
SED instruction. After executing the ADC or SBC instruction, ex-
ecute another instruction before executing the SEC, CLC, or CLD
instruction.
(2) Notes on status flag in decimal mode
When decimal mode is selected, the values of three of the flags in
the status register (the N, V, and Z flags) are invalid after a ADC or
SBC instruction is executed.
The carry flag (C) is set to "1" if a carry is generated as a result of
the calculation, or is cleared to "0" if a borrow is generated. To de-
termine whether a calculation has generated a carry, the C flag
must be initialized to "0" before each calculation. To check for a
borrow, the C flag must be initialized to "1" before each calcula-
tion.
Set D flag to "1"
ADC or SBC instruction
NOP instruction
SEC, CLC, or CLD instruction
Fig. 3 Status flag at decimal calculations
3. JMP instruction
When using the JMP instruction in indirect addressing mode, do
not specify the last address on a page as an indirect address.
4.Multiplication and Division Instructions
(1) The index X mode (T) and the decimal mode (D) flags do not
affect the MUL and DIV instruction.
(2) The execution of these instructions does not change the con-
tents of the processor status register.
↓
↓
↓
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