Commodore Plus 4 - Page 296

884 Machine Language on the Commodore Plus/4
Example: This program looks through $8000-$80FF until it finds an occurrence of the
value SEA (the opcode for a NOP). The location of the SEA is then stored at
$2100 using the STY instruction.
2000 A0 00
2002 A9 EA
2004 D9 00 80
2007 PO 03
2009 C8
200A DO P8
200C 8C 00 21
200P 00
LDY #$00
LDA #$EA
CMP I8000.Y
BEQ $2000
INY
BNE $2004
STY $2100
BRK
Start .Y at zero.
Load .A with the value to look for.
Compare with the next byte.
If equal, then done.
Increment to the next byte.
If there is more to search, then go on.
Store the location in $2100.
Stop processing.
TAX—Transfer Value in Accumulator into X Register
The value currently in the accumulator is placed in the X register.
Operation: A -* X
Addressing Mode
Syntax
Flags Affected: N, Z
Opcode Bytes Cycles
Implied
TAX
AA
1
Example:
The value in the accumulator 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 program a value for the X register is gotten from a location specified by an
indirect index. Because the indirect indexed addressing mode is not available for
the LDX instruction, the value is loaded into the accumulator and then trans
ferred to the X register with the TAX instruction.
2000 A9 00
2002 85 D8
2004 A9 21
2006 85 D9
2008 AO 00
200A Bl D8
200C AA
200D BD 00 21
2010 8D 00 OC
2013 00
LDA #$00
STA $D8
LDA #$21
STA $D9
LDY #$00
LDA ($D8),Y
TAX
LDA $2100,X
STA $0000
BRK
Load .A with a zero.
Store in the low byte of the address.
Load .A with a $21.
Store in the high byte of the address.
Load .Y with a zero.
Get the index for the data retrieval.
Transfer the index to .X.
Load .A with the data byte.
Store on the screen.
Stop processing.