Zilog Z08470 - Manual (Page 297)

UM008007-0715 Z80 Instruction Description
Z80 CPU
User Manual
285
RETI
Operation
Return from Interrupt
Op Code
RETI
Operands
None.
Description
This instruction is used at the end of a maskable interrupt service routine to:
• Restore the contents of the Program Counter (analogous to the RET instruction)
• Signal an I/O device that the interrupt routine is completed. The RETI instruction also
facilitates the nesting of interrupts, allowing higher priority devices to temporarily
suspend service of lower priority service routines. However, this instruction does not
enable interrupts that were disabled when the interrupt routine was entered. Before
doing the RETI instruction, the enable interrupt instruction (EI) should be executed to
allow recognition of interrupts after completion of the current service routine.
Condition Bits Affected
None.
Example
Assume that there are two interrupting devices, A and B, connected in a daisy-chain con-
figuration, with A having a higher priority than B.
M Cycles T States 4 MHz E.T.
4 14 (4, 4, 3, 3) 3.50
1 1 0 0 1 1 1 1 ED
0 1 0 0 1 1 0 1 4D