Commodore Plus 4 - Page 290

278 Machine Language on the Commodore Plus/4
RTS—Return from a Subroutine
The value of the PC is pulled from the stack, and processing continues from that
point.
Operation: PC t
PC+1-+ PC
Addressing Mode Syntax
Flags Affected: None
Opcode Bytes Cycles
Implied
RTS
60
1
Example:
This instruction is used to end the processing of a subroutine. The return
address is pulled from the stack, incremented, and stored in the PC. None of the
flags is affected. Hence, the flags may be used to communicate between the main
program and the subroutine.
This program cycles the two upper left character locations on the screen through
all possible values. They are changed alternately, with a brief wait between. The
wait routine is a subroutine that is terminated by an RTS instruction.
2000 A2 00 LDX #$00
2002 EE 00 OC INC $0000
2005 20 12 20 JSR $2012
2008 EE 01 OC INC $0001
200B 20 12 20 JSR $2012
200E CA DEX
200P DO PI BNE $2002
2011 00 BRK
2012 A5 A5 LDA $A5
2014 29 08
2016 DO PA
2018 A5 A5
201A 29 08
201C PO PA
201E 60
AUD #$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. j
Call the wait'subroutine.
Decrement the counter.
If not done, go back.
Stop processing.
Start of the 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.
SBC—Subtract Memoiy from Accumulator with Carry
The operand is subtracted from the value in the accumulator minus the inverse of
the carry, and the result is placed in the accumulator. Normally the carry is set
(see SEC) prior to a subtraction. When in decimal mode, the Z flag is not valid;
check the accumulator for a zero result.