Commodore Plus 4 - Page 279

The 6508 Microprocessor 267
None of the flags is affected. Hence, they may be used to transfer information
between the main program and the subroutine.
Example: This program cycles the two upper left character locations on the screen through
all possible values. They are changed alternately, with a brief wait in between. The
wait routine is a subroutine and is called using the JSR instruction.
. 2000 AS 00 LDX #$00
. 2002 EE 00 OC INC $0000
. 2005 20 12 20 JSR $2012
. 2008 EE 01 OC IUC $0001
. 200B 20 12 20 JSR $2012
. 200E CA DEX
. 200P DO Fl BITE $2002
. 2011 00 BRK
. 2012 A5 A5 LDA $A5
2014 29 08
2016 DO FA
2018 A5 A5
201A 29 08
201C FO FA
201E 60
AJKTD #$08
BITE $2012
LDA $A5
AKTD #$08
BEQ $2018
RTS
.X counts all the possible values.
Next character for the first location.
Call the wait subroutine.
Next character for the second
location.
Call the wait subroutine.
Decrement the counter.
If not done, go back.
Stop processing.
Start of subroutine. Load the jiffy
clock.
Look at bit 3.
Wait until it is off.
Load the value of the jiffy clock.
Look at bit 3.
Wait until it is on.
Return from the subroutine.
LDA—Load Accumulator with Value from Memory
The value currently in the location specified by the operand is placed in the
accumulator.
Operation: M — A
Addressing Mode Syntax
Flags Affected: N, Z
Opcode Bytes Cycles
Immediate
Zero page
Zero page, X
Absolute
Absolute, X
Absolute, Y
(Indirect, X)
(Indirect), Y
LDA #$hh
LDA $hh
LDA $hh,X
LDA $hhhh
LDA $hhhh,X
LDA $hhhh,Y
LDA ($hh,X)
LDA ($hh),Y
A9
A5
B5
AD
BD
B9
Al
Bl
2
2
2
3
3
3
2
2
2
3
4
4
4*
4*
6
5*
* Add 1 when a page boundary is crossed.