Commodore Plus 4 - Page 304

292 Machine Language on the Commodore Plus/4
i
f
1
c
a
n
t
tvt
IN
y
b
D
1
e
7
8
9
A
B
C
D
E
F
0
BVS
BCC
LDY
Imm
BCS
CPY
Imm
BNE
CPX
Imm
BEQ
/
ADC
0,Y
STA
(,X)
STA
0,Y
LDA
(,X)
LDA
0,Y
CMP
(,X)
CMP
0,Y
SBC
(,X)
SBC
0,Y
2 4
STY
ZP
STY
ZP,X
LDX LDY
Imm ZP
LDY
ZP,X
CPY
ZP
CPX
ZP
Least Significant
5 6 8
ADC
ZP,X
STA
ZP
STA
ZP,X
LDA
ZP
LDA
ZP,X
CMP
ZP
CMP
ZP,X
SBC
ZP
SBC
ZP,X
ROR
ZP,X
STX
ZP
STX
ZP,Y
LDX
ZP
LDX
ZP,Y
DEC
ZP
DEC
ZP,X
INC
ZP
INC
ZP,X
SEI
DEY
TYA
TAY
CLV
INY
CLD
INX
SED
Nybble
9 A
ADC
Ab,Y
STA
Ab,Y
LDA
Imm
LDA
Ab,Y
CMP
Imm
CMP
Ab,Y
SBC
Imm
SBC
Ab,Y
TXA
TXS
TAX
TSX
DEX
NOP
C
STY
Abs
LDY
Abs
LDY
Ab,X
CPY
Abs
CPX
Abs
D
ADC
Ab,X
STA
Abs
STA
Ab,X
LDA
Abs
LDA
Ab,X
CMP
Abs
CMP
Ab,X
SBC
Abs
SBC
Ab,X
E
ROR
Ab,X
STX
Abs
LDX
Abs
LDX
Ab,Y
DEC
Abs
DEC
Ab,X
INC
Abs
INC
Ab,X
Decimal Mode
The 6502 is equipped with a decimal adder that can be used to process data stored
in binary coded decimal (BCD). Two decimal digits are stored in a byte by storing
a 0 through 9 in each of the low and high nybbles. When addition is performed, a
result above 9 in the low nybble causes a carry into the high nybble, and a result
above 9 in the high nybble causes a carry set condition. Subtraction performs in
an analogous way.
Decimal mode is enabled by setting the decimal mode flag in the status register
to 1 (see SED). It is disabled by clearing the flag to 0 (see CLD). One of the
initialization steps when the Plus/4 is first turned on (or reset) is to issue a CLD
instruction. To use decimal mode, issue a SED instruction.