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Software interrupts of 8085

Short answer

The 8085 software interrupts are RST 0–7, 1-byte instructions that call fixed addresses 0000H, 0008H, … 0038H (vector=8×n). They push PC, jump to ISR, end with RET, and are not maskable. Used for system calls and breakpoints.

What are the software interrupts of 8085?

Software interrupts in the 8085 are the eight restart instructions RST 0 to RST 7. They are 1-byte instructions put in a program, and they act like a CALL to a fixed address. Each is executed in 12 T-states: the PC is pushed on the stack and the program jumps to the vector 8 × n.

InstructionOpcodeVector address
RST 0C7H0000H
RST 1CFH0008H
RST 2D7H0010H
RST 3DFH0018H
RST 4E7H0020H
RST 5EFH0028H
RST 6F7H0030H
RST 7FFH0038H
  • They are not maskable and are not affected by DI/EI, because the CPU itself fetches them.
  • The same opcodes are also used by an external device in response to INTR.
  • RST 0 acts as a reset-like restart at 0000H. RST 1 to RST 7 are used for system calls and debugging breakpoints (RST 7 is often used as a breakpoint).
  • The routine ends with RET.

✓ Answer checked by Claude Opus 5.5 (high effort)

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