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UM008007-0715 Z80 Instruction Description

Z80 CPU

User Manual

287

RETN

Operation

Return from nonmaskable interrupt

Op Code

RETN

Operands

None.

Description

This instruction is used at the end of a nonmaskable interrupts service routine to restore

the contents of the Program Counter (analogous to the RET instruction). The state of IFF2

is copied back to IFF1 so that maskable interrupts are enabled immediately following the

RETN if they were enabled before the nonmaskable interrupt.

Condition Bits Affected

None.

Example

If the Stack Pointer contains 1000h and the Program Counter contains 1A45h when a

Nonmaskable Interrupt (NMI) signal is received, the CPU ignores the next instruction and

instead restarts, returning to memory address

0066h. The current Program Counter con-

tains

1A45h, which is pushed onto the external stack address of 0FFFh and 0FFEh, high-

order byte first, and

0066h is loaded onto the Program Counter. That address begins an

interrupt service routine that ends with a RETN instruction.

Upon the execution of a RETN instruction, the contents of the former Program Counter

are popped off the external memory stack, low-order first, resulting in the Stack Pointer

again containing

1000h. The program flow continues where it left off with an op code

fetch to address

1A45h, order-byte first, and 0066h is loaded onto the Program Counter.

M Cycles T States 4 MHz E.T.

4 14 (4, 4, 3, 3) 3.50

11 0 01111

01 0 01100

1 2 ... 294 295 296 297 298 299 300 301 302 303 304 ... 325 326

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Zilog Z08470 Bedienungsanleitung Seite 299