Commodore Plus 4 - Page 298

2000
2001
2004
2005
2006
2009
200A
200B
200E
BA
8E
48
BA
8E
68
BA
8E
00
00
01
02
21
21
21
TSX
STX
PHA
TSX
STX
PLA
TSX
STX
BRK
$2100
#2101
$2102
mmtmp^*mmB^0 ^mm^m' mm ^mr m^ M^pvw^a —
Transfer the initial SP value to .X
Store at $2100.
Push .A onto the
Transfer the new
Store at $2101.
Pull .A from the
Transfer the new
Store at $2101.
Stop processing.
stack.
SP value to .X.
stack.
SP value to .X.
The M (Memory) command may be used to examine $2100-$2102 following
the execution of this example program. See the section on the stack for more
information.
TXA.—Transfer Value in X Register into Acciunulator
The value currently in the X register is placed in the accumulator.
Example:
Operation: X -* A
Addressing Mode Syntax
Flags Affected: N, Z
Opcode Bytes Cycles
Implied
TXA
8A
1
The value in the X register is not altered. If the value transferred is negative, the
N flag is set to 1; otherwise it is cleared to 0. If the value transferred is zero, the Z
flag is set to 1; otherwise it is cleared to 0.
In this example the bits set to 1 in the value of $2100 are counted. This is done in
the X register because the accumulator cannot be incremented. The value
obtained is then transferred to the accumulator using the TXA instruction.
. 2000
. 2002
. 2004
. 2007
. 2008
. 200A
. 200B
. 200C
. 200E
. 200P
A0
A2
AD
4A
90
E8
88
10
8A
00
07
00
00 21
01
F9
LDY
LDX
LDA
LSR
BCC
IWX
DEY
BPL
TXA
BRK
#$07
#$00
$2100
$200B
$2007
.Y keeps track of which bit.
.X counts bits set to 1.
Get the value to count.
Shift the next bit into carry.
If clear, skip the next instruction.
Increment the count of bits set to 1
Decrement which bit.
If not done, go on.
Transfer the count to .A for future
use.
Stop processing.
The value of $2100 may be altered using the M (Memory) command.