What's the main different between these two restart instructions? On the first case they are 1 byte call instruction and next case, they are vectored interrupts but definitely they are also call instructions. Then why RST (0-7) can't be used as interrupts? What's the underlying difference between them?

I think I am not able to build the concept of RST instructions and that's why asking this silly questions. Thank you so much for helping me.


There are four kinds of interrupts on the 8085. The first three have dedicated interrupt pins: 5.5, 6.5, and 7.5. So when an interrupt comes in, the processor can immediately go to the proper interrupt handler, by internally generating a RST x.5 instruction where x=5,6 or 7 as appropriate.

Then there is a shared interrupt pin, INTR. In order for external devices to share this one pin, but at the same time have separate interrupt handlers for each one, there is mechanism whereby the external device places a RST instruction on the bus, along with an interrupt # 0-7. Then it can vector to the appropriate routine.

So that is the main difference between the two: for RST 5.5 through 7.5, the processor generates the RST instruction, and for interrupts 0-7 the external hardware does.

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  • \$\begingroup\$ Understood. Thank you so much. Better explained than my book \$\endgroup\$ – Demietra95 Jun 1 '15 at 19:34

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