7549 datasheet(76/86 Pages) RENESAS

76 / 86 page

Rev.2.02

Mar 31, 2009

Page 74 of 81

REJ03B0202-0202

7549 Group

APPENDIX

Note on Programming

1. Processor Status Register

(1) Initialization of the processor status register

It is required to initialize the processor status register (PS) flags

which affect program execution. It is particularly essential to

initialize the T and D flags because of their effect on

calculations. Initialize these flags at the beginning of the

program.

At a reset, the contents of the processor status register (PS) are

undefined except for the I flag which is “1”.

Fig. 86 Initialization of processor status register flags

(2) How to refer the processor status register

To refer the contents of the processor status register (PS),

execute the PHP instruction once and then read the contents of

(S+1). If necessary, execute the PLP instruction to return the

stored PS to its original status.

Fig 87. Stack memory contents after PHP instruction

execution

2. Decimal Calculations

(1) Instructions for decimal calculations

To perform decimal calculations, set the decimal mode (D) flag

to “1” with the SED instruction and execute the ADC or SBC

instruction. In that case, after the ADC or SBC instruction,

execute another instruction before the SEC, CLC, or CLD

instruction.

Fig 88. Instructions for decimal calculations

(2) Status flag at decimal calculations

When the ADC or SBC instruction is executed in decimal mode

(D flag = “1”), three of the status flags (N, V, and Z) are

disabled.

The carry (C) flag is set to “1” if a carry is generated and is

cleared to “0” if a borrow is generated as a result of a calculation,

so it can be used to determine whether the calculation has

generated a carry or borrow.

Initialize the C flag before each calculation.

3. JMP Instruction

When using the JMP instruction (indirect addressing mode), do

not specify the address where “FF16” is allocated to the low-

order 8 bits as the operand.

4. Multiplication and Division Instructions

(1) The MUL and DIV instructions are not affected by the T

and D flags.

(2) Executing these instructions does not change the contents of

the processor status register.

Reset

Initialize the flags

Main program

Stored PS

(S)

(S) + 1

Set the decimal mode (D) flag to “1”

Execute the ADC or SBC instruction

NOP

Execute the SEC, CLC, or CLD instruction

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