Subroutine vs Interrupt Service Routine (ISR):

Feature	Subroutine (Procedure)	Interrupt Service Routine (ISR / Interrupt Handler)
Invocation	Synchronous, explicitly called by program via CALL instruction	Asynchronous, invoked by hardware signal (INTR, NMI) or exception/trap. CPU diverts without program request
Control	Program has control - known location in code flow	Unpredictable timing, depends on external event / fault
Linkage	CALL saves return address (PC) on stack; RET returns	Interrupt logic saves PC and PSW/FLAGS automatically; return via IRET/RETI
Context Saving	Caller may save few registers (convention dependent)	Must save entire processor state (registers, flags) as interrupt can occur between any two instructions
Priority/Nesting	No priority concept, normal nesting via calls	Has priority; higher priority interrupts can preempt lower ISR; requires PIC (8259) arbitration
Synchronization	Blocking / sequential flow	Must be reentrant, short, ends with EOI to PIC; often disables interrupts at entry

Key Idea: Subroutine = software-initiated planned transfer; ISR = hardware-initiated event-driven transfer that transparently suspends current task. Example: CALL SORT vs keyboard ISR at vector 21h invoked on key press.