EI - Z80 instruction reference

eiEnable interrupts 
Sets both interrupt flip-flops, allowing maskable interrupts to occur after the next instruction.
1 byte · 4 cycles
Used in Interrupts: im, ei, di and haltZ80 assembly
 
SZHP/VNC ––––––
Changes no flag
Forms  ei  
Sets both interrupt flip-flops, thus allowing maskable interrupts to occur. An interrupt will not occur until after the immediately following instruction.
         .org 0x8000
        ei
        halt