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(nasm.info)Section B.4.123

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B.4.123. `INT3', `INT1', `ICEBP', `INT01': Breakpoints
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     INT1                          ; F1                   [P6]
     ICEBP                         ; F1                   [P6]
     INT01                         ; F1                   [P6]

     INT3                          ; CC                   [8086]
     INT03                         ; CC                   [8086]

   `INT1' and `INT3' are short one-byte forms of the instructions `INT
1' and `INT 3' (see *Note Section B.4.122::). They perform a similar
function to their longer counterparts, but take up less code space.
They are used as breakpoints by debuggers.

   * `INT1', and its alternative synonyms `INT01' and `ICEBP', is an
     instruction used by in-circuit emulators (ICEs). It is present,
     though not documented, on some processors down to the 286, but is
     only documented for the Pentium Pro. `INT3' is the instruction
     normally used as a breakpoint by debuggers.

   * `INT3', and its synonym `INT03', is not precisely equivalent to
     `INT 3': the short form, since it is designed to be used as a
     breakpoint, bypasses the normal `IOPL' checks in virtual-8086 mode,
     and also does not go through interrupt redirection.