Commodore Plus 4 - Page 289

The 6502 Microprocessor 277
RTI—Return from an Interrupt
The values of the SR and PC are pulled from the stack, and processing continues.
Flags Affected: All
Example:
Operation: SR t
PC\
PC+ 1^ PC
Addressing Mode Syntax
Implied
RTI
Opcode Bytes Cycles
40 16
This instruction is used to end the processing of an interrupt. The status
register is restored from the stack. Because an interrupt can occur only when the I
flag is clear, this status register value will have the I flag cleared. The program
counter is also pulled from the stack and incremented, and processing continues
at that point. No other internal registers are affected. See the section on interrupts
for more information.
This example alters the IRQ vector. The normal interrupt processing is sus
pended. The substitute service routine simply increments the border color. The
program must be halted by pressing the reset button on the right side of the
computer.
2000 78
2001 A9 OP
SEI
LDA #$0F
2003 8D 14 03 STA $0314
2006 A9 20 LDA #$20
2008 8D 15 03 STA $0315
200B 58 CLI
200C 40 OC 20 JMP $2000
200P EE 19 PP IMTC $FP19
2012 68 PLA
2013
2014
2015
2016
2017
A8
68
AA
68
40
TAY
PLA
TAX
PLA
RTI
Disable maskable interrupts.
Get the low byte of the address of the
new IRQ processor.
Store in the vector.
Get the high byte of the address of
the new IRQ processor.
Store in the vector.
Reenable maskable interrupts.
Infinite loop.
Increment the border color.
Retrieve the value of .Y from the
stack.
Restore .Y to the preinterrupt value.
Retrieve the value of .X from the
stack.
Restore .X to the preinterrupt value.
Restore the value of .A from the stack.
Return from the interrupt.