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O D I T S Z A P C OF: Overflow flag TF: Trap flag AF: Aux carry
F F F F F F F F F DF: Direction flag SF: Sign flag PF: Parity flag
* * IF: Interrupt flag ZF: Zero flag CF: Carry flag
INT triggers a software interrupt to one of 256 vectors in the first 1,024 bytes of memory. The operand specifies which vector, from 0 to 255. When an interrupt is called, the Flags register is pushed on the stack along with the return address. The IF flag is cleared, which prevents further interrupts (either hardware or software) from being recognized until IF is set again. TF is also cleared.
A special form of the instruction allows calling Interrupt 3 with a single-byte instruction. Debuggers use Interrupt 3 to set breakpoints in code by replacing an instruction with the single-byte opcode for calling Interrupt 3. NASM does not recognize this, and if you want to use INT 3 for some reason (and that instruction form isn't of much use unless you're writing a debugger), you must use a special mnemonic form INT3 rather than INT 3. This is advanced stuff; be careful.
Virtually all your applications of INT will use the other form, which takes an 8-bit immediate numeric value.
Always return from a software interrupt service routine with the IRET instruction. IRET restores the flags that were pushed onto the stack by INT, and in doing so clears IF, allowing further interrupts.
r8 = AL AH BL BH CL CH DL DH r16 = AX BX CX DX BP SP SI DI
sr = CS DS SS ES FS GS r32 = EAX EBX ECX EDX EBP ESP ESI EDI
m8 = 8-bit memory data m16 = 16-bit memory data
m32 = 32-bit memory data i8 = 8-bit immediate data
i16 = 16-bit immediate data i32 = 32-bit immediate data
d8 = 8-bit signed displacement d16 = 16-bit signed displacement
d32 = 32-bit unsigned displacement
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