Timing difference between async. interrupts and sync exceptions?

[mmcheraghi]

Hi
As I understand, in case of happening interrupts and exceptions, the current instruction is completed first. But the reason that trap_ctrl.pc is filled by pc in case of exception is that the current instruction was unsuccessful and it should be re-executed after trap handling. Is this right?

-- exception program counter: async. interrupt or sync. exception? --
trap_ctrl.pc <= exe_engine.pc2 when (trap_ctrl.cause(trap_ctrl.cause'left) = '1') else
exe_engine.pc;

[mmcheraghi]

So in this architecture, async. interrupt is handled synchronously. Right?

[stnolting]

Just as a general overview:

Synchronous exceptions always occur synchronously to a specific instruction (for example an illegal instruction, a misaligned load/store address, etc.). Hence, the instruction was executed and raised an exception. The exception program counter (mepc CSR) therefore points to the address of this instruction.

Asynchronous exceptions (= interrupts) occur "at some point" - they are not correlated with a specific instruction. In the case of an interrupt, the exception program counter (mepc CSR) points to the next instruction that has not yet been executed.

[...] is that the current instruction was unsuccessful and it should be re-executed after trap handling. Is this right?

That depends on the software. If, for example, the exception was triggered by a user thread in an RTOS (e.g. Zephyr), then the thread is terminated.

In bare metal/machine mode software, this is somewhat more complicated. If the instruction is executed again, the exception occurs again.

So in this architecture, async. interrupt is handled synchronously. Right?

What exactly do you mean by "is handled synchronously"? 🤔

[mmcheraghi]

What exactly do you mean by "is handled synchronously"? 🤔

Thanks, you just clarified it. Being asynchronous refers to the cause of the exception. Obviously CPU is a sync machine and handles these cases (executes trap handler instructions) in a sync manner😅.