sei

An opcode for the 6502 processor. This is a so-called flag operation.
A SEI operations sets the interrupt flag. When this happens, the processor goes into 'interrupt mode', and jumps to a specified location in memory to cary out its interrupt cycle. A CLI operation ends this.

Back to the 6502 opcodes metanode

A 6502 instruction that sets the I (interrupt disable) flag, preventing IRQs from interrupting the code.

• Function: 1 => I
• Updates flags: . . . . . I . .
• Opcode number: impl $78

Caution: Do not confuse this with the sti instruction of x86, which does the exact opposite.

As opposed to: CLI
See also: 6502 instructions | 6502 addressing modes