Commodore Plus 4 - Page 278

266 Machine Language on the Commodore Plus/4
Example:
Without affecting any of the flags, the processing control is passed to the
specified address. This is the only instruction that has an indirect addressing
mode. In this mode, the value at the specified address is used as the low byte of the
destination address, and the value stored in the next location is used as the high
byte of the destination address.
In this program the background color is cycled through every possible value. The
JMP instruction is used to jump to the reset routine. The reset routine restores the
background color to its default value (and returns to BASIC).
2000 A9 00 LDA #$00
2002 8D 15 PF STA $FF15
2005 EE 15 FF IMJ $FF15
2008 FO OE
200A A5 A5
200C 29 02
200E DO FA
2010 A5 A5
2012 29 02
2014 FO FA
2016 DO ED
BEQ $2018
LDA $A5
AND #$02
BETE $200A
LDA $A5
AKTD #$02
BEQ $2010
BETE $2005
2018 4C F6 FF JMP $FFF6
Load .A with a zero.
Store zero in the background color
register.
Increment the background color
register.
If it has cycled around to zero, quit.
Now wait. Load the value in the jiffy
clock.
Look at bit 1.
Wait until it is off.
Load the value of the jiffy clock.
Look at bit 1.
Wait until it is on.
Done waiting, go back.
Transfer control to the reset
routine.
JSR—Jump to Subroutine
The current value of the program counter (plus 2) is pushed onto the stack for use
as the return address. The address specified with the instruction is transferred to
the program counter, and processing continues with the instruction located at
that address.
Operation: PC + 2\
, (PC+1)-PCL
(PC+ 2)^ PCH
Addressing Mode Syntax
Flags Affected: None
Opcode Bytes Cycles
Absolute
JSR $hhhh
20
The processor saves the return address on the stack. The RTS instruction pulls
these bytes off the stack. A JSR instruction should have a corresponding RTS.