Intel 8080 - B2_Page_17

EI
Chapter 3. Instruction Set
IL1 ____________ 0 ___ 0 ___ l ~
Cycles:
States:
4
Flags: none
NOTE
The 8085 TRAP interrupt cannot be disabled. This special interrupt is
intended for serious problems that must be serviced regardless of the
interrupt flag such
as power failure or bus error. However, no interrupt
including
TRAP can interrupt the execution of the 01 or EI instruction.
ENABLE INTERRUPTS
The EI instruction enables the interrupt system following execution of the next program instruction. Enabling
the interrupt system
is delayed one instruction to allow interrupt subroutines to return to the main program
before
2 subsequent interrupt is acknowledged.
In applications that use interrupts, the interrupt system is usually disabled only when the processor accepts an
interrupt or when
a code sequence must not be interrupted. You can disable the interrupt system by including
a
01 instruction at the beginning of the code sequence. Because you cannot predict the occurrence of an
interrupt, include an
EI instruction at the end of the code sequence.
Op co de Operand
EI
Operands are not permitted with the EI instruction.
Example:
Cycles:
States:
4
Flags: none
NOTE
The 8085 TRAP interrupt cannot be disabled. Thi'i special interrupt i'i
intended for serious problems that must be serviced regard Ie,., of the
interrupt flag such
as power failure or bus failure. However, no interrupt
including
TRAP can interrupt the execution of the 01 or EI instruction.
The
EI instruction is frequently used as part of a start-up sequence. When power is first applied, the processor
begins operating
at some indeterminate address. Application of a RESET signal forces the program counter to
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