Commodore Plus 4 - Page 305

The 6502 Microprocessor 293
Example:
sooo
2001
2002
2004
2006
2007
P8
18
A9
69
D8
00
98
12
SED
CLC
LDA
ADC
CLD
BRK
##98
#$12
Set decimal mode.
Clear the carry for adding.
Load .A with 98 (BCD).
Add 12 (BCD).
Clear decimal mode.
Stop processing.
The result (in .A and the status register) will be 10 (BCD) with a carry.
Note: The zero flag is not valid following a decimal mode operation. Use a
compare to check for zero.
Note: The system interrupt service routine does not clear decimal mode. If it is
set in the main program, it is set during IRQ service unless the IRQ routine clears
decimal mode. Note that clearing decimal mode in the IRQ routine does not
affect its setting in the main program because the status register is restored during
the RTI.
Addressing Modes
Some 6502 instructions must be accompanied by an operand address containing
the data with which the instruction will be performed. There are a number of ways
to specify the operand address. Basically, 6502 addressing modes fall into two
categories, indexed and nonindexed. The nonindexed modes are immediate,
absolute, zero page, relative, and indirect. The indexed modes are absolute
indexed, zero page indexed, indirect indexed, and indexed indirect.
Many instructions do not require an operand. Their addressing mode is
referred to as implied and they require only 1 byte. In the machine-language
monitor, an implied mode instruction appears in this format:
mnemonic
where mnemonic is an instruction mnemonic.
The following instructions are available in implied mode: BRK, CLC, CLD,
CLI, CLV, DEX, DEY, INX, INY, NOP, PHA, PHP, PLA, PLP, RTI, RTS,
SEC, SED, SEI, TAX, TAY, TSX, TXA, TXS, and TYA.
A few instructions may be performed on the data in the accumulator. Their
addressing mode is referred to as accumulator and they require only 1 byte. In the
machine-language monitor, an accumulator mode instruction appears in this
format:
mnemonic
where mnemonic is an instruction mnemonic.
The following instructions are available in accumulator mode: ASL, LSR,
ROL, and ROR.